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TOMOYO Linux Cross Reference
Linux/kernel/rcu/tree_plugin.h

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  1 /*
  2  * Read-Copy Update mechanism for mutual exclusion (tree-based version)
  3  * Internal non-public definitions that provide either classic
  4  * or preemptible semantics.
  5  *
  6  * This program is free software; you can redistribute it and/or modify
  7  * it under the terms of the GNU General Public License as published by
  8  * the Free Software Foundation; either version 2 of the License, or
  9  * (at your option) any later version.
 10  *
 11  * This program is distributed in the hope that it will be useful,
 12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
 13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 14  * GNU General Public License for more details.
 15  *
 16  * You should have received a copy of the GNU General Public License
 17  * along with this program; if not, you can access it online at
 18  * http://www.gnu.org/licenses/gpl-2.0.html.
 19  *
 20  * Copyright Red Hat, 2009
 21  * Copyright IBM Corporation, 2009
 22  *
 23  * Author: Ingo Molnar <mingo@elte.hu>
 24  *         Paul E. McKenney <paulmck@linux.vnet.ibm.com>
 25  */
 26 
 27 #include <linux/delay.h>
 28 #include <linux/gfp.h>
 29 #include <linux/oom.h>
 30 #include <linux/sched/debug.h>
 31 #include <linux/smpboot.h>
 32 #include <linux/sched/isolation.h>
 33 #include <uapi/linux/sched/types.h>
 34 #include "../time/tick-internal.h"
 35 
 36 #ifdef CONFIG_RCU_BOOST
 37 
 38 #include "../locking/rtmutex_common.h"
 39 
 40 /*
 41  * Control variables for per-CPU and per-rcu_node kthreads.
 42  */
 43 static DEFINE_PER_CPU(struct task_struct *, rcu_cpu_kthread_task);
 44 DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
 45 DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_loops);
 46 DEFINE_PER_CPU(char, rcu_cpu_has_work);
 47 
 48 #else /* #ifdef CONFIG_RCU_BOOST */
 49 
 50 /*
 51  * Some architectures do not define rt_mutexes, but if !CONFIG_RCU_BOOST,
 52  * all uses are in dead code.  Provide a definition to keep the compiler
 53  * happy, but add WARN_ON_ONCE() to complain if used in the wrong place.
 54  * This probably needs to be excluded from -rt builds.
 55  */
 56 #define rt_mutex_owner(a) ({ WARN_ON_ONCE(1); NULL; })
 57 #define rt_mutex_futex_unlock(x) WARN_ON_ONCE(1)
 58 
 59 #endif /* #else #ifdef CONFIG_RCU_BOOST */
 60 
 61 #ifdef CONFIG_RCU_NOCB_CPU
 62 static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */
 63 static bool __read_mostly rcu_nocb_poll;    /* Offload kthread are to poll. */
 64 #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
 65 
 66 /*
 67  * Check the RCU kernel configuration parameters and print informative
 68  * messages about anything out of the ordinary.
 69  */
 70 static void __init rcu_bootup_announce_oddness(void)
 71 {
 72         if (IS_ENABLED(CONFIG_RCU_TRACE))
 73                 pr_info("\tRCU event tracing is enabled.\n");
 74         if ((IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 64) ||
 75             (!IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 32))
 76                 pr_info("\tCONFIG_RCU_FANOUT set to non-default value of %d.\n",
 77                         RCU_FANOUT);
 78         if (rcu_fanout_exact)
 79                 pr_info("\tHierarchical RCU autobalancing is disabled.\n");
 80         if (IS_ENABLED(CONFIG_RCU_FAST_NO_HZ))
 81                 pr_info("\tRCU dyntick-idle grace-period acceleration is enabled.\n");
 82         if (IS_ENABLED(CONFIG_PROVE_RCU))
 83                 pr_info("\tRCU lockdep checking is enabled.\n");
 84         if (RCU_NUM_LVLS >= 4)
 85                 pr_info("\tFour(or more)-level hierarchy is enabled.\n");
 86         if (RCU_FANOUT_LEAF != 16)
 87                 pr_info("\tBuild-time adjustment of leaf fanout to %d.\n",
 88                         RCU_FANOUT_LEAF);
 89         if (rcu_fanout_leaf != RCU_FANOUT_LEAF)
 90                 pr_info("\tBoot-time adjustment of leaf fanout to %d.\n",
 91                         rcu_fanout_leaf);
 92         if (nr_cpu_ids != NR_CPUS)
 93                 pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%u.\n", NR_CPUS, nr_cpu_ids);
 94 #ifdef CONFIG_RCU_BOOST
 95         pr_info("\tRCU priority boosting: priority %d delay %d ms.\n",
 96                 kthread_prio, CONFIG_RCU_BOOST_DELAY);
 97 #endif
 98         if (blimit != DEFAULT_RCU_BLIMIT)
 99                 pr_info("\tBoot-time adjustment of callback invocation limit to %ld.\n", blimit);
100         if (qhimark != DEFAULT_RCU_QHIMARK)
101                 pr_info("\tBoot-time adjustment of callback high-water mark to %ld.\n", qhimark);
102         if (qlowmark != DEFAULT_RCU_QLOMARK)
103                 pr_info("\tBoot-time adjustment of callback low-water mark to %ld.\n", qlowmark);
104         if (jiffies_till_first_fqs != ULONG_MAX)
105                 pr_info("\tBoot-time adjustment of first FQS scan delay to %ld jiffies.\n", jiffies_till_first_fqs);
106         if (jiffies_till_next_fqs != ULONG_MAX)
107                 pr_info("\tBoot-time adjustment of subsequent FQS scan delay to %ld jiffies.\n", jiffies_till_next_fqs);
108         if (jiffies_till_sched_qs != ULONG_MAX)
109                 pr_info("\tBoot-time adjustment of scheduler-enlistment delay to %ld jiffies.\n", jiffies_till_sched_qs);
110         if (rcu_kick_kthreads)
111                 pr_info("\tKick kthreads if too-long grace period.\n");
112         if (IS_ENABLED(CONFIG_DEBUG_OBJECTS_RCU_HEAD))
113                 pr_info("\tRCU callback double-/use-after-free debug enabled.\n");
114         if (gp_preinit_delay)
115                 pr_info("\tRCU debug GP pre-init slowdown %d jiffies.\n", gp_preinit_delay);
116         if (gp_init_delay)
117                 pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_init_delay);
118         if (gp_cleanup_delay)
119                 pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_cleanup_delay);
120         if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
121                 pr_info("\tRCU debug extended QS entry/exit.\n");
122         rcupdate_announce_bootup_oddness();
123 }
124 
125 #ifdef CONFIG_PREEMPT_RCU
126 
127 static void rcu_report_exp_rnp(struct rcu_node *rnp, bool wake);
128 static void rcu_read_unlock_special(struct task_struct *t);
129 
130 /*
131  * Tell them what RCU they are running.
132  */
133 static void __init rcu_bootup_announce(void)
134 {
135         pr_info("Preemptible hierarchical RCU implementation.\n");
136         rcu_bootup_announce_oddness();
137 }
138 
139 /* Flags for rcu_preempt_ctxt_queue() decision table. */
140 #define RCU_GP_TASKS    0x8
141 #define RCU_EXP_TASKS   0x4
142 #define RCU_GP_BLKD     0x2
143 #define RCU_EXP_BLKD    0x1
144 
145 /*
146  * Queues a task preempted within an RCU-preempt read-side critical
147  * section into the appropriate location within the ->blkd_tasks list,
148  * depending on the states of any ongoing normal and expedited grace
149  * periods.  The ->gp_tasks pointer indicates which element the normal
150  * grace period is waiting on (NULL if none), and the ->exp_tasks pointer
151  * indicates which element the expedited grace period is waiting on (again,
152  * NULL if none).  If a grace period is waiting on a given element in the
153  * ->blkd_tasks list, it also waits on all subsequent elements.  Thus,
154  * adding a task to the tail of the list blocks any grace period that is
155  * already waiting on one of the elements.  In contrast, adding a task
156  * to the head of the list won't block any grace period that is already
157  * waiting on one of the elements.
158  *
159  * This queuing is imprecise, and can sometimes make an ongoing grace
160  * period wait for a task that is not strictly speaking blocking it.
161  * Given the choice, we needlessly block a normal grace period rather than
162  * blocking an expedited grace period.
163  *
164  * Note that an endless sequence of expedited grace periods still cannot
165  * indefinitely postpone a normal grace period.  Eventually, all of the
166  * fixed number of preempted tasks blocking the normal grace period that are
167  * not also blocking the expedited grace period will resume and complete
168  * their RCU read-side critical sections.  At that point, the ->gp_tasks
169  * pointer will equal the ->exp_tasks pointer, at which point the end of
170  * the corresponding expedited grace period will also be the end of the
171  * normal grace period.
172  */
173 static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
174         __releases(rnp->lock) /* But leaves rrupts disabled. */
175 {
176         int blkd_state = (rnp->gp_tasks ? RCU_GP_TASKS : 0) +
177                          (rnp->exp_tasks ? RCU_EXP_TASKS : 0) +
178                          (rnp->qsmask & rdp->grpmask ? RCU_GP_BLKD : 0) +
179                          (rnp->expmask & rdp->grpmask ? RCU_EXP_BLKD : 0);
180         struct task_struct *t = current;
181 
182         raw_lockdep_assert_held_rcu_node(rnp);
183         WARN_ON_ONCE(rdp->mynode != rnp);
184         WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
185         /* RCU better not be waiting on newly onlined CPUs! */
186         WARN_ON_ONCE(rnp->qsmaskinitnext & ~rnp->qsmaskinit & rnp->qsmask &
187                      rdp->grpmask);
188 
189         /*
190          * Decide where to queue the newly blocked task.  In theory,
191          * this could be an if-statement.  In practice, when I tried
192          * that, it was quite messy.
193          */
194         switch (blkd_state) {
195         case 0:
196         case                RCU_EXP_TASKS:
197         case                RCU_EXP_TASKS + RCU_GP_BLKD:
198         case RCU_GP_TASKS:
199         case RCU_GP_TASKS + RCU_EXP_TASKS:
200 
201                 /*
202                  * Blocking neither GP, or first task blocking the normal
203                  * GP but not blocking the already-waiting expedited GP.
204                  * Queue at the head of the list to avoid unnecessarily
205                  * blocking the already-waiting GPs.
206                  */
207                 list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
208                 break;
209 
210         case                                              RCU_EXP_BLKD:
211         case                                RCU_GP_BLKD:
212         case                                RCU_GP_BLKD + RCU_EXP_BLKD:
213         case RCU_GP_TASKS +                               RCU_EXP_BLKD:
214         case RCU_GP_TASKS +                 RCU_GP_BLKD + RCU_EXP_BLKD:
215         case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
216 
217                 /*
218                  * First task arriving that blocks either GP, or first task
219                  * arriving that blocks the expedited GP (with the normal
220                  * GP already waiting), or a task arriving that blocks
221                  * both GPs with both GPs already waiting.  Queue at the
222                  * tail of the list to avoid any GP waiting on any of the
223                  * already queued tasks that are not blocking it.
224                  */
225                 list_add_tail(&t->rcu_node_entry, &rnp->blkd_tasks);
226                 break;
227 
228         case                RCU_EXP_TASKS +               RCU_EXP_BLKD:
229         case                RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
230         case RCU_GP_TASKS + RCU_EXP_TASKS +               RCU_EXP_BLKD:
231 
232                 /*
233                  * Second or subsequent task blocking the expedited GP.
234                  * The task either does not block the normal GP, or is the
235                  * first task blocking the normal GP.  Queue just after
236                  * the first task blocking the expedited GP.
237                  */
238                 list_add(&t->rcu_node_entry, rnp->exp_tasks);
239                 break;
240 
241         case RCU_GP_TASKS +                 RCU_GP_BLKD:
242         case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD:
243 
244                 /*
245                  * Second or subsequent task blocking the normal GP.
246                  * The task does not block the expedited GP. Queue just
247                  * after the first task blocking the normal GP.
248                  */
249                 list_add(&t->rcu_node_entry, rnp->gp_tasks);
250                 break;
251 
252         default:
253 
254                 /* Yet another exercise in excessive paranoia. */
255                 WARN_ON_ONCE(1);
256                 break;
257         }
258 
259         /*
260          * We have now queued the task.  If it was the first one to
261          * block either grace period, update the ->gp_tasks and/or
262          * ->exp_tasks pointers, respectively, to reference the newly
263          * blocked tasks.
264          */
265         if (!rnp->gp_tasks && (blkd_state & RCU_GP_BLKD)) {
266                 rnp->gp_tasks = &t->rcu_node_entry;
267                 WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq);
268         }
269         if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))
270                 rnp->exp_tasks = &t->rcu_node_entry;
271         WARN_ON_ONCE(!(blkd_state & RCU_GP_BLKD) !=
272                      !(rnp->qsmask & rdp->grpmask));
273         WARN_ON_ONCE(!(blkd_state & RCU_EXP_BLKD) !=
274                      !(rnp->expmask & rdp->grpmask));
275         raw_spin_unlock_rcu_node(rnp); /* interrupts remain disabled. */
276 
277         /*
278          * Report the quiescent state for the expedited GP.  This expedited
279          * GP should not be able to end until we report, so there should be
280          * no need to check for a subsequent expedited GP.  (Though we are
281          * still in a quiescent state in any case.)
282          */
283         if (blkd_state & RCU_EXP_BLKD && rdp->deferred_qs)
284                 rcu_report_exp_rdp(rdp);
285         else
286                 WARN_ON_ONCE(rdp->deferred_qs);
287 }
288 
289 /*
290  * Record a preemptible-RCU quiescent state for the specified CPU.
291  * Note that this does not necessarily mean that the task currently running
292  * on the CPU is in a quiescent state:  Instead, it means that the current
293  * grace period need not wait on any RCU read-side critical section that
294  * starts later on this CPU.  It also means that if the current task is
295  * in an RCU read-side critical section, it has already added itself to
296  * some leaf rcu_node structure's ->blkd_tasks list.  In addition to the
297  * current task, there might be any number of other tasks blocked while
298  * in an RCU read-side critical section.
299  *
300  * Callers to this function must disable preemption.
301  */
302 static void rcu_qs(void)
303 {
304         RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!\n");
305         if (__this_cpu_read(rcu_data.cpu_no_qs.s)) {
306                 trace_rcu_grace_period(TPS("rcu_preempt"),
307                                        __this_cpu_read(rcu_data.gp_seq),
308                                        TPS("cpuqs"));
309                 __this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
310                 barrier(); /* Coordinate with rcu_flavor_check_callbacks(). */
311                 current->rcu_read_unlock_special.b.need_qs = false;
312         }
313 }
314 
315 /*
316  * We have entered the scheduler, and the current task might soon be
317  * context-switched away from.  If this task is in an RCU read-side
318  * critical section, we will no longer be able to rely on the CPU to
319  * record that fact, so we enqueue the task on the blkd_tasks list.
320  * The task will dequeue itself when it exits the outermost enclosing
321  * RCU read-side critical section.  Therefore, the current grace period
322  * cannot be permitted to complete until the blkd_tasks list entries
323  * predating the current grace period drain, in other words, until
324  * rnp->gp_tasks becomes NULL.
325  *
326  * Caller must disable interrupts.
327  */
328 void rcu_note_context_switch(bool preempt)
329 {
330         struct task_struct *t = current;
331         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
332         struct rcu_node *rnp;
333 
334         barrier(); /* Avoid RCU read-side critical sections leaking down. */
335         trace_rcu_utilization(TPS("Start context switch"));
336         lockdep_assert_irqs_disabled();
337         WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0);
338         if (t->rcu_read_lock_nesting > 0 &&
339             !t->rcu_read_unlock_special.b.blocked) {
340 
341                 /* Possibly blocking in an RCU read-side critical section. */
342                 rnp = rdp->mynode;
343                 raw_spin_lock_rcu_node(rnp);
344                 t->rcu_read_unlock_special.b.blocked = true;
345                 t->rcu_blocked_node = rnp;
346 
347                 /*
348                  * Verify the CPU's sanity, trace the preemption, and
349                  * then queue the task as required based on the states
350                  * of any ongoing and expedited grace periods.
351                  */
352                 WARN_ON_ONCE((rdp->grpmask & rcu_rnp_online_cpus(rnp)) == 0);
353                 WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
354                 trace_rcu_preempt_task(rcu_state.name,
355                                        t->pid,
356                                        (rnp->qsmask & rdp->grpmask)
357                                        ? rnp->gp_seq
358                                        : rcu_seq_snap(&rnp->gp_seq));
359                 rcu_preempt_ctxt_queue(rnp, rdp);
360         } else if (t->rcu_read_lock_nesting < 0 &&
361                    t->rcu_read_unlock_special.s) {
362 
363                 /*
364                  * Complete exit from RCU read-side critical section on
365                  * behalf of preempted instance of __rcu_read_unlock().
366                  */
367                 rcu_read_unlock_special(t);
368                 rcu_preempt_deferred_qs(t);
369         } else {
370                 rcu_preempt_deferred_qs(t);
371         }
372 
373         /*
374          * Either we were not in an RCU read-side critical section to
375          * begin with, or we have now recorded that critical section
376          * globally.  Either way, we can now note a quiescent state
377          * for this CPU.  Again, if we were in an RCU read-side critical
378          * section, and if that critical section was blocking the current
379          * grace period, then the fact that the task has been enqueued
380          * means that we continue to block the current grace period.
381          */
382         rcu_qs();
383         if (rdp->deferred_qs)
384                 rcu_report_exp_rdp(rdp);
385         trace_rcu_utilization(TPS("End context switch"));
386         barrier(); /* Avoid RCU read-side critical sections leaking up. */
387 }
388 EXPORT_SYMBOL_GPL(rcu_note_context_switch);
389 
390 /*
391  * Check for preempted RCU readers blocking the current grace period
392  * for the specified rcu_node structure.  If the caller needs a reliable
393  * answer, it must hold the rcu_node's ->lock.
394  */
395 static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
396 {
397         return rnp->gp_tasks != NULL;
398 }
399 
400 /* Bias and limit values for ->rcu_read_lock_nesting. */
401 #define RCU_NEST_BIAS INT_MAX
402 #define RCU_NEST_NMAX (-INT_MAX / 2)
403 #define RCU_NEST_PMAX (INT_MAX / 2)
404 
405 /*
406  * Preemptible RCU implementation for rcu_read_lock().
407  * Just increment ->rcu_read_lock_nesting, shared state will be updated
408  * if we block.
409  */
410 void __rcu_read_lock(void)
411 {
412         current->rcu_read_lock_nesting++;
413         if (IS_ENABLED(CONFIG_PROVE_LOCKING))
414                 WARN_ON_ONCE(current->rcu_read_lock_nesting > RCU_NEST_PMAX);
415         barrier();  /* critical section after entry code. */
416 }
417 EXPORT_SYMBOL_GPL(__rcu_read_lock);
418 
419 /*
420  * Preemptible RCU implementation for rcu_read_unlock().
421  * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
422  * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
423  * invoke rcu_read_unlock_special() to clean up after a context switch
424  * in an RCU read-side critical section and other special cases.
425  */
426 void __rcu_read_unlock(void)
427 {
428         struct task_struct *t = current;
429 
430         if (t->rcu_read_lock_nesting != 1) {
431                 --t->rcu_read_lock_nesting;
432         } else {
433                 barrier();  /* critical section before exit code. */
434                 t->rcu_read_lock_nesting = -RCU_NEST_BIAS;
435                 barrier();  /* assign before ->rcu_read_unlock_special load */
436                 if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
437                         rcu_read_unlock_special(t);
438                 barrier();  /* ->rcu_read_unlock_special load before assign */
439                 t->rcu_read_lock_nesting = 0;
440         }
441         if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
442                 int rrln = t->rcu_read_lock_nesting;
443 
444                 WARN_ON_ONCE(rrln < 0 && rrln > RCU_NEST_NMAX);
445         }
446 }
447 EXPORT_SYMBOL_GPL(__rcu_read_unlock);
448 
449 /*
450  * Advance a ->blkd_tasks-list pointer to the next entry, instead
451  * returning NULL if at the end of the list.
452  */
453 static struct list_head *rcu_next_node_entry(struct task_struct *t,
454                                              struct rcu_node *rnp)
455 {
456         struct list_head *np;
457 
458         np = t->rcu_node_entry.next;
459         if (np == &rnp->blkd_tasks)
460                 np = NULL;
461         return np;
462 }
463 
464 /*
465  * Return true if the specified rcu_node structure has tasks that were
466  * preempted within an RCU read-side critical section.
467  */
468 static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
469 {
470         return !list_empty(&rnp->blkd_tasks);
471 }
472 
473 /*
474  * Report deferred quiescent states.  The deferral time can
475  * be quite short, for example, in the case of the call from
476  * rcu_read_unlock_special().
477  */
478 static void
479 rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
480 {
481         bool empty_exp;
482         bool empty_norm;
483         bool empty_exp_now;
484         struct list_head *np;
485         bool drop_boost_mutex = false;
486         struct rcu_data *rdp;
487         struct rcu_node *rnp;
488         union rcu_special special;
489 
490         /*
491          * If RCU core is waiting for this CPU to exit its critical section,
492          * report the fact that it has exited.  Because irqs are disabled,
493          * t->rcu_read_unlock_special cannot change.
494          */
495         special = t->rcu_read_unlock_special;
496         rdp = this_cpu_ptr(&rcu_data);
497         if (!special.s && !rdp->deferred_qs) {
498                 local_irq_restore(flags);
499                 return;
500         }
501         if (special.b.need_qs) {
502                 rcu_qs();
503                 t->rcu_read_unlock_special.b.need_qs = false;
504                 if (!t->rcu_read_unlock_special.s && !rdp->deferred_qs) {
505                         local_irq_restore(flags);
506                         return;
507                 }
508         }
509 
510         /*
511          * Respond to a request by an expedited grace period for a
512          * quiescent state from this CPU.  Note that requests from
513          * tasks are handled when removing the task from the
514          * blocked-tasks list below.
515          */
516         if (rdp->deferred_qs) {
517                 rcu_report_exp_rdp(rdp);
518                 if (!t->rcu_read_unlock_special.s) {
519                         local_irq_restore(flags);
520                         return;
521                 }
522         }
523 
524         /* Clean up if blocked during RCU read-side critical section. */
525         if (special.b.blocked) {
526                 t->rcu_read_unlock_special.b.blocked = false;
527 
528                 /*
529                  * Remove this task from the list it blocked on.  The task
530                  * now remains queued on the rcu_node corresponding to the
531                  * CPU it first blocked on, so there is no longer any need
532                  * to loop.  Retain a WARN_ON_ONCE() out of sheer paranoia.
533                  */
534                 rnp = t->rcu_blocked_node;
535                 raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
536                 WARN_ON_ONCE(rnp != t->rcu_blocked_node);
537                 WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
538                 empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
539                 WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
540                              (!empty_norm || rnp->qsmask));
541                 empty_exp = sync_rcu_preempt_exp_done(rnp);
542                 smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
543                 np = rcu_next_node_entry(t, rnp);
544                 list_del_init(&t->rcu_node_entry);
545                 t->rcu_blocked_node = NULL;
546                 trace_rcu_unlock_preempted_task(TPS("rcu_preempt"),
547                                                 rnp->gp_seq, t->pid);
548                 if (&t->rcu_node_entry == rnp->gp_tasks)
549                         rnp->gp_tasks = np;
550                 if (&t->rcu_node_entry == rnp->exp_tasks)
551                         rnp->exp_tasks = np;
552                 if (IS_ENABLED(CONFIG_RCU_BOOST)) {
553                         /* Snapshot ->boost_mtx ownership w/rnp->lock held. */
554                         drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx) == t;
555                         if (&t->rcu_node_entry == rnp->boost_tasks)
556                                 rnp->boost_tasks = np;
557                 }
558 
559                 /*
560                  * If this was the last task on the current list, and if
561                  * we aren't waiting on any CPUs, report the quiescent state.
562                  * Note that rcu_report_unblock_qs_rnp() releases rnp->lock,
563                  * so we must take a snapshot of the expedited state.
564                  */
565                 empty_exp_now = sync_rcu_preempt_exp_done(rnp);
566                 if (!empty_norm && !rcu_preempt_blocked_readers_cgp(rnp)) {
567                         trace_rcu_quiescent_state_report(TPS("preempt_rcu"),
568                                                          rnp->gp_seq,
569                                                          0, rnp->qsmask,
570                                                          rnp->level,
571                                                          rnp->grplo,
572                                                          rnp->grphi,
573                                                          !!rnp->gp_tasks);
574                         rcu_report_unblock_qs_rnp(rnp, flags);
575                 } else {
576                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
577                 }
578 
579                 /* Unboost if we were boosted. */
580                 if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
581                         rt_mutex_futex_unlock(&rnp->boost_mtx);
582 
583                 /*
584                  * If this was the last task on the expedited lists,
585                  * then we need to report up the rcu_node hierarchy.
586                  */
587                 if (!empty_exp && empty_exp_now)
588                         rcu_report_exp_rnp(rnp, true);
589         } else {
590                 local_irq_restore(flags);
591         }
592 }
593 
594 /*
595  * Is a deferred quiescent-state pending, and are we also not in
596  * an RCU read-side critical section?  It is the caller's responsibility
597  * to ensure it is otherwise safe to report any deferred quiescent
598  * states.  The reason for this is that it is safe to report a
599  * quiescent state during context switch even though preemption
600  * is disabled.  This function cannot be expected to understand these
601  * nuances, so the caller must handle them.
602  */
603 static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
604 {
605         return (__this_cpu_read(rcu_data.deferred_qs) ||
606                 READ_ONCE(t->rcu_read_unlock_special.s)) &&
607                t->rcu_read_lock_nesting <= 0;
608 }
609 
610 /*
611  * Report a deferred quiescent state if needed and safe to do so.
612  * As with rcu_preempt_need_deferred_qs(), "safe" involves only
613  * not being in an RCU read-side critical section.  The caller must
614  * evaluate safety in terms of interrupt, softirq, and preemption
615  * disabling.
616  */
617 static void rcu_preempt_deferred_qs(struct task_struct *t)
618 {
619         unsigned long flags;
620         bool couldrecurse = t->rcu_read_lock_nesting >= 0;
621 
622         if (!rcu_preempt_need_deferred_qs(t))
623                 return;
624         if (couldrecurse)
625                 t->rcu_read_lock_nesting -= RCU_NEST_BIAS;
626         local_irq_save(flags);
627         rcu_preempt_deferred_qs_irqrestore(t, flags);
628         if (couldrecurse)
629                 t->rcu_read_lock_nesting += RCU_NEST_BIAS;
630 }
631 
632 /*
633  * Handle special cases during rcu_read_unlock(), such as needing to
634  * notify RCU core processing or task having blocked during the RCU
635  * read-side critical section.
636  */
637 static void rcu_read_unlock_special(struct task_struct *t)
638 {
639         unsigned long flags;
640         bool preempt_bh_were_disabled =
641                         !!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
642         bool irqs_were_disabled;
643 
644         /* NMI handlers cannot block and cannot safely manipulate state. */
645         if (in_nmi())
646                 return;
647 
648         local_irq_save(flags);
649         irqs_were_disabled = irqs_disabled_flags(flags);
650         if (preempt_bh_were_disabled || irqs_were_disabled) {
651                 WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
652                 /* Need to defer quiescent state until everything is enabled. */
653                 if (irqs_were_disabled) {
654                         /* Enabling irqs does not reschedule, so... */
655                         raise_softirq_irqoff(RCU_SOFTIRQ);
656                 } else {
657                         /* Enabling BH or preempt does reschedule, so... */
658                         set_tsk_need_resched(current);
659                         set_preempt_need_resched();
660                 }
661                 local_irq_restore(flags);
662                 return;
663         }
664         WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
665         rcu_preempt_deferred_qs_irqrestore(t, flags);
666 }
667 
668 /*
669  * Dump detailed information for all tasks blocking the current RCU
670  * grace period on the specified rcu_node structure.
671  */
672 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
673 {
674         unsigned long flags;
675         struct task_struct *t;
676 
677         raw_spin_lock_irqsave_rcu_node(rnp, flags);
678         if (!rcu_preempt_blocked_readers_cgp(rnp)) {
679                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
680                 return;
681         }
682         t = list_entry(rnp->gp_tasks->prev,
683                        struct task_struct, rcu_node_entry);
684         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
685                 /*
686                  * We could be printing a lot while holding a spinlock.
687                  * Avoid triggering hard lockup.
688                  */
689                 touch_nmi_watchdog();
690                 sched_show_task(t);
691         }
692         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
693 }
694 
695 /*
696  * Dump detailed information for all tasks blocking the current RCU
697  * grace period.
698  */
699 static void rcu_print_detail_task_stall(void)
700 {
701         struct rcu_node *rnp = rcu_get_root();
702 
703         rcu_print_detail_task_stall_rnp(rnp);
704         rcu_for_each_leaf_node(rnp)
705                 rcu_print_detail_task_stall_rnp(rnp);
706 }
707 
708 static void rcu_print_task_stall_begin(struct rcu_node *rnp)
709 {
710         pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
711                rnp->level, rnp->grplo, rnp->grphi);
712 }
713 
714 static void rcu_print_task_stall_end(void)
715 {
716         pr_cont("\n");
717 }
718 
719 /*
720  * Scan the current list of tasks blocked within RCU read-side critical
721  * sections, printing out the tid of each.
722  */
723 static int rcu_print_task_stall(struct rcu_node *rnp)
724 {
725         struct task_struct *t;
726         int ndetected = 0;
727 
728         if (!rcu_preempt_blocked_readers_cgp(rnp))
729                 return 0;
730         rcu_print_task_stall_begin(rnp);
731         t = list_entry(rnp->gp_tasks->prev,
732                        struct task_struct, rcu_node_entry);
733         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
734                 pr_cont(" P%d", t->pid);
735                 ndetected++;
736         }
737         rcu_print_task_stall_end();
738         return ndetected;
739 }
740 
741 /*
742  * Scan the current list of tasks blocked within RCU read-side critical
743  * sections, printing out the tid of each that is blocking the current
744  * expedited grace period.
745  */
746 static int rcu_print_task_exp_stall(struct rcu_node *rnp)
747 {
748         struct task_struct *t;
749         int ndetected = 0;
750 
751         if (!rnp->exp_tasks)
752                 return 0;
753         t = list_entry(rnp->exp_tasks->prev,
754                        struct task_struct, rcu_node_entry);
755         list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
756                 pr_cont(" P%d", t->pid);
757                 ndetected++;
758         }
759         return ndetected;
760 }
761 
762 /*
763  * Check that the list of blocked tasks for the newly completed grace
764  * period is in fact empty.  It is a serious bug to complete a grace
765  * period that still has RCU readers blocked!  This function must be
766  * invoked -before- updating this rnp's ->gp_seq, and the rnp's ->lock
767  * must be held by the caller.
768  *
769  * Also, if there are blocked tasks on the list, they automatically
770  * block the newly created grace period, so set up ->gp_tasks accordingly.
771  */
772 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
773 {
774         struct task_struct *t;
775 
776         RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
777         if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
778                 dump_blkd_tasks(rnp, 10);
779         if (rcu_preempt_has_tasks(rnp) &&
780             (rnp->qsmaskinit || rnp->wait_blkd_tasks)) {
781                 rnp->gp_tasks = rnp->blkd_tasks.next;
782                 t = container_of(rnp->gp_tasks, struct task_struct,
783                                  rcu_node_entry);
784                 trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
785                                                 rnp->gp_seq, t->pid);
786         }
787         WARN_ON_ONCE(rnp->qsmask);
788 }
789 
790 /*
791  * Check for a quiescent state from the current CPU.  When a task blocks,
792  * the task is recorded in the corresponding CPU's rcu_node structure,
793  * which is checked elsewhere.
794  *
795  * Caller must disable hard irqs.
796  */
797 static void rcu_flavor_check_callbacks(int user)
798 {
799         struct task_struct *t = current;
800 
801         if (user || rcu_is_cpu_rrupt_from_idle()) {
802                 rcu_note_voluntary_context_switch(current);
803         }
804         if (t->rcu_read_lock_nesting > 0 ||
805             (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
806                 /* No QS, force context switch if deferred. */
807                 if (rcu_preempt_need_deferred_qs(t)) {
808                         set_tsk_need_resched(t);
809                         set_preempt_need_resched();
810                 }
811         } else if (rcu_preempt_need_deferred_qs(t)) {
812                 rcu_preempt_deferred_qs(t); /* Report deferred QS. */
813                 return;
814         } else if (!t->rcu_read_lock_nesting) {
815                 rcu_qs(); /* Report immediate QS. */
816                 return;
817         }
818 
819         /* If GP is oldish, ask for help from rcu_read_unlock_special(). */
820         if (t->rcu_read_lock_nesting > 0 &&
821             __this_cpu_read(rcu_data.core_needs_qs) &&
822             __this_cpu_read(rcu_data.cpu_no_qs.b.norm) &&
823             !t->rcu_read_unlock_special.b.need_qs &&
824             time_after(jiffies, rcu_state.gp_start + HZ))
825                 t->rcu_read_unlock_special.b.need_qs = true;
826 }
827 
828 /**
829  * synchronize_rcu - wait until a grace period has elapsed.
830  *
831  * Control will return to the caller some time after a full grace
832  * period has elapsed, in other words after all currently executing RCU
833  * read-side critical sections have completed.  Note, however, that
834  * upon return from synchronize_rcu(), the caller might well be executing
835  * concurrently with new RCU read-side critical sections that began while
836  * synchronize_rcu() was waiting.  RCU read-side critical sections are
837  * delimited by rcu_read_lock() and rcu_read_unlock(), and may be nested.
838  * In addition, regions of code across which interrupts, preemption, or
839  * softirqs have been disabled also serve as RCU read-side critical
840  * sections.  This includes hardware interrupt handlers, softirq handlers,
841  * and NMI handlers.
842  *
843  * Note that this guarantee implies further memory-ordering guarantees.
844  * On systems with more than one CPU, when synchronize_rcu() returns,
845  * each CPU is guaranteed to have executed a full memory barrier since
846  * the end of its last RCU read-side critical section whose beginning
847  * preceded the call to synchronize_rcu().  In addition, each CPU having
848  * an RCU read-side critical section that extends beyond the return from
849  * synchronize_rcu() is guaranteed to have executed a full memory barrier
850  * after the beginning of synchronize_rcu() and before the beginning of
851  * that RCU read-side critical section.  Note that these guarantees include
852  * CPUs that are offline, idle, or executing in user mode, as well as CPUs
853  * that are executing in the kernel.
854  *
855  * Furthermore, if CPU A invoked synchronize_rcu(), which returned
856  * to its caller on CPU B, then both CPU A and CPU B are guaranteed
857  * to have executed a full memory barrier during the execution of
858  * synchronize_rcu() -- even if CPU A and CPU B are the same CPU (but
859  * again only if the system has more than one CPU).
860  */
861 void synchronize_rcu(void)
862 {
863         RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
864                          lock_is_held(&rcu_lock_map) ||
865                          lock_is_held(&rcu_sched_lock_map),
866                          "Illegal synchronize_rcu() in RCU read-side critical section");
867         if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
868                 return;
869         if (rcu_gp_is_expedited())
870                 synchronize_rcu_expedited();
871         else
872                 wait_rcu_gp(call_rcu);
873 }
874 EXPORT_SYMBOL_GPL(synchronize_rcu);
875 
876 /*
877  * Check for a task exiting while in a preemptible-RCU read-side
878  * critical section, clean up if so.  No need to issue warnings,
879  * as debug_check_no_locks_held() already does this if lockdep
880  * is enabled.
881  */
882 void exit_rcu(void)
883 {
884         struct task_struct *t = current;
885 
886         if (likely(list_empty(&current->rcu_node_entry)))
887                 return;
888         t->rcu_read_lock_nesting = 1;
889         barrier();
890         t->rcu_read_unlock_special.b.blocked = true;
891         __rcu_read_unlock();
892         rcu_preempt_deferred_qs(current);
893 }
894 
895 /*
896  * Dump the blocked-tasks state, but limit the list dump to the
897  * specified number of elements.
898  */
899 static void
900 dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
901 {
902         int cpu;
903         int i;
904         struct list_head *lhp;
905         bool onl;
906         struct rcu_data *rdp;
907         struct rcu_node *rnp1;
908 
909         raw_lockdep_assert_held_rcu_node(rnp);
910         pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
911                 __func__, rnp->grplo, rnp->grphi, rnp->level,
912                 (long)rnp->gp_seq, (long)rnp->completedqs);
913         for (rnp1 = rnp; rnp1; rnp1 = rnp1->parent)
914                 pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx\n",
915                         __func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext);
916         pr_info("%s: ->gp_tasks %p ->boost_tasks %p ->exp_tasks %p\n",
917                 __func__, rnp->gp_tasks, rnp->boost_tasks, rnp->exp_tasks);
918         pr_info("%s: ->blkd_tasks", __func__);
919         i = 0;
920         list_for_each(lhp, &rnp->blkd_tasks) {
921                 pr_cont(" %p", lhp);
922                 if (++i >= 10)
923                         break;
924         }
925         pr_cont("\n");
926         for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++) {
927                 rdp = per_cpu_ptr(&rcu_data, cpu);
928                 onl = !!(rdp->grpmask & rcu_rnp_online_cpus(rnp));
929                 pr_info("\t%d: %c online: %ld(%d) offline: %ld(%d)\n",
930                         cpu, ".o"[onl],
931                         (long)rdp->rcu_onl_gp_seq, rdp->rcu_onl_gp_flags,
932                         (long)rdp->rcu_ofl_gp_seq, rdp->rcu_ofl_gp_flags);
933         }
934 }
935 
936 #else /* #ifdef CONFIG_PREEMPT_RCU */
937 
938 /*
939  * Tell them what RCU they are running.
940  */
941 static void __init rcu_bootup_announce(void)
942 {
943         pr_info("Hierarchical RCU implementation.\n");
944         rcu_bootup_announce_oddness();
945 }
946 
947 /*
948  * Note a quiescent state for PREEMPT=n.  Because we do not need to know
949  * how many quiescent states passed, just if there was at least one since
950  * the start of the grace period, this just sets a flag.  The caller must
951  * have disabled preemption.
952  */
953 static void rcu_qs(void)
954 {
955         RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!");
956         if (!__this_cpu_read(rcu_data.cpu_no_qs.s))
957                 return;
958         trace_rcu_grace_period(TPS("rcu_sched"),
959                                __this_cpu_read(rcu_data.gp_seq), TPS("cpuqs"));
960         __this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
961         if (!__this_cpu_read(rcu_data.cpu_no_qs.b.exp))
962                 return;
963         __this_cpu_write(rcu_data.cpu_no_qs.b.exp, false);
964         rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
965 }
966 
967 /*
968  * Register an urgently needed quiescent state.  If there is an
969  * emergency, invoke rcu_momentary_dyntick_idle() to do a heavy-weight
970  * dyntick-idle quiescent state visible to other CPUs, which will in
971  * some cases serve for expedited as well as normal grace periods.
972  * Either way, register a lightweight quiescent state.
973  *
974  * The barrier() calls are redundant in the common case when this is
975  * called externally, but just in case this is called from within this
976  * file.
977  *
978  */
979 void rcu_all_qs(void)
980 {
981         unsigned long flags;
982 
983         if (!raw_cpu_read(rcu_data.rcu_urgent_qs))
984                 return;
985         preempt_disable();
986         /* Load rcu_urgent_qs before other flags. */
987         if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) {
988                 preempt_enable();
989                 return;
990         }
991         this_cpu_write(rcu_data.rcu_urgent_qs, false);
992         barrier(); /* Avoid RCU read-side critical sections leaking down. */
993         if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs))) {
994                 local_irq_save(flags);
995                 rcu_momentary_dyntick_idle();
996                 local_irq_restore(flags);
997         }
998         rcu_qs();
999         barrier(); /* Avoid RCU read-side critical sections leaking up. */
1000         preempt_enable();
1001 }
1002 EXPORT_SYMBOL_GPL(rcu_all_qs);
1003 
1004 /*
1005  * Note a PREEMPT=n context switch.  The caller must have disabled interrupts.
1006  */
1007 void rcu_note_context_switch(bool preempt)
1008 {
1009         barrier(); /* Avoid RCU read-side critical sections leaking down. */
1010         trace_rcu_utilization(TPS("Start context switch"));
1011         rcu_qs();
1012         /* Load rcu_urgent_qs before other flags. */
1013         if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs)))
1014                 goto out;
1015         this_cpu_write(rcu_data.rcu_urgent_qs, false);
1016         if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs)))
1017                 rcu_momentary_dyntick_idle();
1018         if (!preempt)
1019                 rcu_tasks_qs(current);
1020 out:
1021         trace_rcu_utilization(TPS("End context switch"));
1022         barrier(); /* Avoid RCU read-side critical sections leaking up. */
1023 }
1024 EXPORT_SYMBOL_GPL(rcu_note_context_switch);
1025 
1026 /*
1027  * Because preemptible RCU does not exist, there are never any preempted
1028  * RCU readers.
1029  */
1030 static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
1031 {
1032         return 0;
1033 }
1034 
1035 /*
1036  * Because there is no preemptible RCU, there can be no readers blocked.
1037  */
1038 static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
1039 {
1040         return false;
1041 }
1042 
1043 /*
1044  * Because there is no preemptible RCU, there can be no deferred quiescent
1045  * states.
1046  */
1047 static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
1048 {
1049         return false;
1050 }
1051 static void rcu_preempt_deferred_qs(struct task_struct *t) { }
1052 
1053 /*
1054  * Because preemptible RCU does not exist, we never have to check for
1055  * tasks blocked within RCU read-side critical sections.
1056  */
1057 static void rcu_print_detail_task_stall(void)
1058 {
1059 }
1060 
1061 /*
1062  * Because preemptible RCU does not exist, we never have to check for
1063  * tasks blocked within RCU read-side critical sections.
1064  */
1065 static int rcu_print_task_stall(struct rcu_node *rnp)
1066 {
1067         return 0;
1068 }
1069 
1070 /*
1071  * Because preemptible RCU does not exist, we never have to check for
1072  * tasks blocked within RCU read-side critical sections that are
1073  * blocking the current expedited grace period.
1074  */
1075 static int rcu_print_task_exp_stall(struct rcu_node *rnp)
1076 {
1077         return 0;
1078 }
1079 
1080 /*
1081  * Because there is no preemptible RCU, there can be no readers blocked,
1082  * so there is no need to check for blocked tasks.  So check only for
1083  * bogus qsmask values.
1084  */
1085 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
1086 {
1087         WARN_ON_ONCE(rnp->qsmask);
1088 }
1089 
1090 /*
1091  * Check to see if this CPU is in a non-context-switch quiescent state
1092  * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
1093  * Also schedule RCU core processing.
1094  *
1095  * This function must be called from hardirq context.  It is normally
1096  * invoked from the scheduling-clock interrupt.
1097  */
1098 static void rcu_flavor_check_callbacks(int user)
1099 {
1100         if (user || rcu_is_cpu_rrupt_from_idle()) {
1101 
1102                 /*
1103                  * Get here if this CPU took its interrupt from user
1104                  * mode or from the idle loop, and if this is not a
1105                  * nested interrupt.  In this case, the CPU is in
1106                  * a quiescent state, so note it.
1107                  *
1108                  * No memory barrier is required here because rcu_qs()
1109                  * references only CPU-local variables that other CPUs
1110                  * neither access nor modify, at least not while the
1111                  * corresponding CPU is online.
1112                  */
1113 
1114                 rcu_qs();
1115         }
1116 }
1117 
1118 /* PREEMPT=n implementation of synchronize_rcu(). */
1119 void synchronize_rcu(void)
1120 {
1121         RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
1122                          lock_is_held(&rcu_lock_map) ||
1123                          lock_is_held(&rcu_sched_lock_map),
1124                          "Illegal synchronize_rcu() in RCU read-side critical section");
1125         if (rcu_blocking_is_gp())
1126                 return;
1127         if (rcu_gp_is_expedited())
1128                 synchronize_rcu_expedited();
1129         else
1130                 wait_rcu_gp(call_rcu);
1131 }
1132 EXPORT_SYMBOL_GPL(synchronize_rcu);
1133 
1134 /*
1135  * Because preemptible RCU does not exist, tasks cannot possibly exit
1136  * while in preemptible RCU read-side critical sections.
1137  */
1138 void exit_rcu(void)
1139 {
1140 }
1141 
1142 /*
1143  * Dump the guaranteed-empty blocked-tasks state.  Trust but verify.
1144  */
1145 static void
1146 dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
1147 {
1148         WARN_ON_ONCE(!list_empty(&rnp->blkd_tasks));
1149 }
1150 
1151 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
1152 
1153 #ifdef CONFIG_RCU_BOOST
1154 
1155 static void rcu_wake_cond(struct task_struct *t, int status)
1156 {
1157         /*
1158          * If the thread is yielding, only wake it when this
1159          * is invoked from idle
1160          */
1161         if (status != RCU_KTHREAD_YIELDING || is_idle_task(current))
1162                 wake_up_process(t);
1163 }
1164 
1165 /*
1166  * Carry out RCU priority boosting on the task indicated by ->exp_tasks
1167  * or ->boost_tasks, advancing the pointer to the next task in the
1168  * ->blkd_tasks list.
1169  *
1170  * Note that irqs must be enabled: boosting the task can block.
1171  * Returns 1 if there are more tasks needing to be boosted.
1172  */
1173 static int rcu_boost(struct rcu_node *rnp)
1174 {
1175         unsigned long flags;
1176         struct task_struct *t;
1177         struct list_head *tb;
1178 
1179         if (READ_ONCE(rnp->exp_tasks) == NULL &&
1180             READ_ONCE(rnp->boost_tasks) == NULL)
1181                 return 0;  /* Nothing left to boost. */
1182 
1183         raw_spin_lock_irqsave_rcu_node(rnp, flags);
1184 
1185         /*
1186          * Recheck under the lock: all tasks in need of boosting
1187          * might exit their RCU read-side critical sections on their own.
1188          */
1189         if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {
1190                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1191                 return 0;
1192         }
1193 
1194         /*
1195          * Preferentially boost tasks blocking expedited grace periods.
1196          * This cannot starve the normal grace periods because a second
1197          * expedited grace period must boost all blocked tasks, including
1198          * those blocking the pre-existing normal grace period.
1199          */
1200         if (rnp->exp_tasks != NULL)
1201                 tb = rnp->exp_tasks;
1202         else
1203                 tb = rnp->boost_tasks;
1204 
1205         /*
1206          * We boost task t by manufacturing an rt_mutex that appears to
1207          * be held by task t.  We leave a pointer to that rt_mutex where
1208          * task t can find it, and task t will release the mutex when it
1209          * exits its outermost RCU read-side critical section.  Then
1210          * simply acquiring this artificial rt_mutex will boost task
1211          * t's priority.  (Thanks to tglx for suggesting this approach!)
1212          *
1213          * Note that task t must acquire rnp->lock to remove itself from
1214          * the ->blkd_tasks list, which it will do from exit() if from
1215          * nowhere else.  We therefore are guaranteed that task t will
1216          * stay around at least until we drop rnp->lock.  Note that
1217          * rnp->lock also resolves races between our priority boosting
1218          * and task t's exiting its outermost RCU read-side critical
1219          * section.
1220          */
1221         t = container_of(tb, struct task_struct, rcu_node_entry);
1222         rt_mutex_init_proxy_locked(&rnp->boost_mtx, t);
1223         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1224         /* Lock only for side effect: boosts task t's priority. */
1225         rt_mutex_lock(&rnp->boost_mtx);
1226         rt_mutex_unlock(&rnp->boost_mtx);  /* Then keep lockdep happy. */
1227 
1228         return READ_ONCE(rnp->exp_tasks) != NULL ||
1229                READ_ONCE(rnp->boost_tasks) != NULL;
1230 }
1231 
1232 /*
1233  * Priority-boosting kthread, one per leaf rcu_node.
1234  */
1235 static int rcu_boost_kthread(void *arg)
1236 {
1237         struct rcu_node *rnp = (struct rcu_node *)arg;
1238         int spincnt = 0;
1239         int more2boost;
1240 
1241         trace_rcu_utilization(TPS("Start boost kthread@init"));
1242         for (;;) {
1243                 rnp->boost_kthread_status = RCU_KTHREAD_WAITING;
1244                 trace_rcu_utilization(TPS("End boost kthread@rcu_wait"));
1245                 rcu_wait(rnp->boost_tasks || rnp->exp_tasks);
1246                 trace_rcu_utilization(TPS("Start boost kthread@rcu_wait"));
1247                 rnp->boost_kthread_status = RCU_KTHREAD_RUNNING;
1248                 more2boost = rcu_boost(rnp);
1249                 if (more2boost)
1250                         spincnt++;
1251                 else
1252                         spincnt = 0;
1253                 if (spincnt > 10) {
1254                         rnp->boost_kthread_status = RCU_KTHREAD_YIELDING;
1255                         trace_rcu_utilization(TPS("End boost kthread@rcu_yield"));
1256                         schedule_timeout_interruptible(2);
1257                         trace_rcu_utilization(TPS("Start boost kthread@rcu_yield"));
1258                         spincnt = 0;
1259                 }
1260         }
1261         /* NOTREACHED */
1262         trace_rcu_utilization(TPS("End boost kthread@notreached"));
1263         return 0;
1264 }
1265 
1266 /*
1267  * Check to see if it is time to start boosting RCU readers that are
1268  * blocking the current grace period, and, if so, tell the per-rcu_node
1269  * kthread to start boosting them.  If there is an expedited grace
1270  * period in progress, it is always time to boost.
1271  *
1272  * The caller must hold rnp->lock, which this function releases.
1273  * The ->boost_kthread_task is immortal, so we don't need to worry
1274  * about it going away.
1275  */
1276 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
1277         __releases(rnp->lock)
1278 {
1279         struct task_struct *t;
1280 
1281         raw_lockdep_assert_held_rcu_node(rnp);
1282         if (!rcu_preempt_blocked_readers_cgp(rnp) && rnp->exp_tasks == NULL) {
1283                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1284                 return;
1285         }
1286         if (rnp->exp_tasks != NULL ||
1287             (rnp->gp_tasks != NULL &&
1288              rnp->boost_tasks == NULL &&
1289              rnp->qsmask == 0 &&
1290              ULONG_CMP_GE(jiffies, rnp->boost_time))) {
1291                 if (rnp->exp_tasks == NULL)
1292                         rnp->boost_tasks = rnp->gp_tasks;
1293                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1294                 t = rnp->boost_kthread_task;
1295                 if (t)
1296                         rcu_wake_cond(t, rnp->boost_kthread_status);
1297         } else {
1298                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1299         }
1300 }
1301 
1302 /*
1303  * Wake up the per-CPU kthread to invoke RCU callbacks.
1304  */
1305 static void invoke_rcu_callbacks_kthread(void)
1306 {
1307         unsigned long flags;
1308 
1309         local_irq_save(flags);
1310         __this_cpu_write(rcu_cpu_has_work, 1);
1311         if (__this_cpu_read(rcu_cpu_kthread_task) != NULL &&
1312             current != __this_cpu_read(rcu_cpu_kthread_task)) {
1313                 rcu_wake_cond(__this_cpu_read(rcu_cpu_kthread_task),
1314                               __this_cpu_read(rcu_cpu_kthread_status));
1315         }
1316         local_irq_restore(flags);
1317 }
1318 
1319 /*
1320  * Is the current CPU running the RCU-callbacks kthread?
1321  * Caller must have preemption disabled.
1322  */
1323 static bool rcu_is_callbacks_kthread(void)
1324 {
1325         return __this_cpu_read(rcu_cpu_kthread_task) == current;
1326 }
1327 
1328 #define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
1329 
1330 /*
1331  * Do priority-boost accounting for the start of a new grace period.
1332  */
1333 static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
1334 {
1335         rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
1336 }
1337 
1338 /*
1339  * Create an RCU-boost kthread for the specified node if one does not
1340  * already exist.  We only create this kthread for preemptible RCU.
1341  * Returns zero if all is well, a negated errno otherwise.
1342  */
1343 static int rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
1344 {
1345         int rnp_index = rnp - rcu_get_root();
1346         unsigned long flags;
1347         struct sched_param sp;
1348         struct task_struct *t;
1349 
1350         if (!IS_ENABLED(CONFIG_PREEMPT_RCU))
1351                 return 0;
1352 
1353         if (!rcu_scheduler_fully_active || rcu_rnp_online_cpus(rnp) == 0)
1354                 return 0;
1355 
1356         rcu_state.boost = 1;
1357         if (rnp->boost_kthread_task != NULL)
1358                 return 0;
1359         t = kthread_create(rcu_boost_kthread, (void *)rnp,
1360                            "rcub/%d", rnp_index);
1361         if (IS_ERR(t))
1362                 return PTR_ERR(t);
1363         raw_spin_lock_irqsave_rcu_node(rnp, flags);
1364         rnp->boost_kthread_task = t;
1365         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1366         sp.sched_priority = kthread_prio;
1367         sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
1368         wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
1369         return 0;
1370 }
1371 
1372 static void rcu_kthread_do_work(void)
1373 {
1374         rcu_do_batch(this_cpu_ptr(&rcu_data));
1375 }
1376 
1377 static void rcu_cpu_kthread_setup(unsigned int cpu)
1378 {
1379         struct sched_param sp;
1380 
1381         sp.sched_priority = kthread_prio;
1382         sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
1383 }
1384 
1385 static void rcu_cpu_kthread_park(unsigned int cpu)
1386 {
1387         per_cpu(rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
1388 }
1389 
1390 static int rcu_cpu_kthread_should_run(unsigned int cpu)
1391 {
1392         return __this_cpu_read(rcu_cpu_has_work);
1393 }
1394 
1395 /*
1396  * Per-CPU kernel thread that invokes RCU callbacks.  This replaces
1397  * the RCU softirq used in configurations of RCU that do not support RCU
1398  * priority boosting.
1399  */
1400 static void rcu_cpu_kthread(unsigned int cpu)
1401 {
1402         unsigned int *statusp = this_cpu_ptr(&rcu_cpu_kthread_status);
1403         char work, *workp = this_cpu_ptr(&rcu_cpu_has_work);
1404         int spincnt;
1405 
1406         for (spincnt = 0; spincnt < 10; spincnt++) {
1407                 trace_rcu_utilization(TPS("Start CPU kthread@rcu_wait"));
1408                 local_bh_disable();
1409                 *statusp = RCU_KTHREAD_RUNNING;
1410                 this_cpu_inc(rcu_cpu_kthread_loops);
1411                 local_irq_disable();
1412                 work = *workp;
1413                 *workp = 0;
1414                 local_irq_enable();
1415                 if (work)
1416                         rcu_kthread_do_work();
1417                 local_bh_enable();
1418                 if (*workp == 0) {
1419                         trace_rcu_utilization(TPS("End CPU kthread@rcu_wait"));
1420                         *statusp = RCU_KTHREAD_WAITING;
1421                         return;
1422                 }
1423         }
1424         *statusp = RCU_KTHREAD_YIELDING;
1425         trace_rcu_utilization(TPS("Start CPU kthread@rcu_yield"));
1426         schedule_timeout_interruptible(2);
1427         trace_rcu_utilization(TPS("End CPU kthread@rcu_yield"));
1428         *statusp = RCU_KTHREAD_WAITING;
1429 }
1430 
1431 /*
1432  * Set the per-rcu_node kthread's affinity to cover all CPUs that are
1433  * served by the rcu_node in question.  The CPU hotplug lock is still
1434  * held, so the value of rnp->qsmaskinit will be stable.
1435  *
1436  * We don't include outgoingcpu in the affinity set, use -1 if there is
1437  * no outgoing CPU.  If there are no CPUs left in the affinity set,
1438  * this function allows the kthread to execute on any CPU.
1439  */
1440 static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
1441 {
1442         struct task_struct *t = rnp->boost_kthread_task;
1443         unsigned long mask = rcu_rnp_online_cpus(rnp);
1444         cpumask_var_t cm;
1445         int cpu;
1446 
1447         if (!t)
1448                 return;
1449         if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
1450                 return;
1451         for_each_leaf_node_possible_cpu(rnp, cpu)
1452                 if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
1453                     cpu != outgoingcpu)
1454                         cpumask_set_cpu(cpu, cm);
1455         if (cpumask_weight(cm) == 0)
1456                 cpumask_setall(cm);
1457         set_cpus_allowed_ptr(t, cm);
1458         free_cpumask_var(cm);
1459 }
1460 
1461 static struct smp_hotplug_thread rcu_cpu_thread_spec = {
1462         .store                  = &rcu_cpu_kthread_task,
1463         .thread_should_run      = rcu_cpu_kthread_should_run,
1464         .thread_fn              = rcu_cpu_kthread,
1465         .thread_comm            = "rcuc/%u",
1466         .setup                  = rcu_cpu_kthread_setup,
1467         .park                   = rcu_cpu_kthread_park,
1468 };
1469 
1470 /*
1471  * Spawn boost kthreads -- called as soon as the scheduler is running.
1472  */
1473 static void __init rcu_spawn_boost_kthreads(void)
1474 {
1475         struct rcu_node *rnp;
1476         int cpu;
1477 
1478         for_each_possible_cpu(cpu)
1479                 per_cpu(rcu_cpu_has_work, cpu) = 0;
1480         if (WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), "%s: Could not start rcub kthread, OOM is now expected behavior\n", __func__))
1481                 return;
1482         rcu_for_each_leaf_node(rnp)
1483                 (void)rcu_spawn_one_boost_kthread(rnp);
1484 }
1485 
1486 static void rcu_prepare_kthreads(int cpu)
1487 {
1488         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
1489         struct rcu_node *rnp = rdp->mynode;
1490 
1491         /* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
1492         if (rcu_scheduler_fully_active)
1493                 (void)rcu_spawn_one_boost_kthread(rnp);
1494 }
1495 
1496 #else /* #ifdef CONFIG_RCU_BOOST */
1497 
1498 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
1499         __releases(rnp->lock)
1500 {
1501         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1502 }
1503 
1504 static void invoke_rcu_callbacks_kthread(void)
1505 {
1506         WARN_ON_ONCE(1);
1507 }
1508 
1509 static bool rcu_is_callbacks_kthread(void)
1510 {
1511         return false;
1512 }
1513 
1514 static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
1515 {
1516 }
1517 
1518 static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
1519 {
1520 }
1521 
1522 static void __init rcu_spawn_boost_kthreads(void)
1523 {
1524 }
1525 
1526 static void rcu_prepare_kthreads(int cpu)
1527 {
1528 }
1529 
1530 #endif /* #else #ifdef CONFIG_RCU_BOOST */
1531 
1532 #if !defined(CONFIG_RCU_FAST_NO_HZ)
1533 
1534 /*
1535  * Check to see if any future RCU-related work will need to be done
1536  * by the current CPU, even if none need be done immediately, returning
1537  * 1 if so.  This function is part of the RCU implementation; it is -not-
1538  * an exported member of the RCU API.
1539  *
1540  * Because we not have RCU_FAST_NO_HZ, just check whether or not this
1541  * CPU has RCU callbacks queued.
1542  */
1543 int rcu_needs_cpu(u64 basemono, u64 *nextevt)
1544 {
1545         *nextevt = KTIME_MAX;
1546         return rcu_cpu_has_callbacks(NULL);
1547 }
1548 
1549 /*
1550  * Because we do not have RCU_FAST_NO_HZ, don't bother cleaning up
1551  * after it.
1552  */
1553 static void rcu_cleanup_after_idle(void)
1554 {
1555 }
1556 
1557 /*
1558  * Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=n,
1559  * is nothing.
1560  */
1561 static void rcu_prepare_for_idle(void)
1562 {
1563 }
1564 
1565 /*
1566  * Don't bother keeping a running count of the number of RCU callbacks
1567  * posted because CONFIG_RCU_FAST_NO_HZ=n.
1568  */
1569 static void rcu_idle_count_callbacks_posted(void)
1570 {
1571 }
1572 
1573 #else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
1574 
1575 /*
1576  * This code is invoked when a CPU goes idle, at which point we want
1577  * to have the CPU do everything required for RCU so that it can enter
1578  * the energy-efficient dyntick-idle mode.  This is handled by a
1579  * state machine implemented by rcu_prepare_for_idle() below.
1580  *
1581  * The following three proprocessor symbols control this state machine:
1582  *
1583  * RCU_IDLE_GP_DELAY gives the number of jiffies that a CPU is permitted
1584  *      to sleep in dyntick-idle mode with RCU callbacks pending.  This
1585  *      is sized to be roughly one RCU grace period.  Those energy-efficiency
1586  *      benchmarkers who might otherwise be tempted to set this to a large
1587  *      number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
1588  *      system.  And if you are -that- concerned about energy efficiency,
1589  *      just power the system down and be done with it!
1590  * RCU_IDLE_LAZY_GP_DELAY gives the number of jiffies that a CPU is
1591  *      permitted to sleep in dyntick-idle mode with only lazy RCU
1592  *      callbacks pending.  Setting this too high can OOM your system.
1593  *
1594  * The values below work well in practice.  If future workloads require
1595  * adjustment, they can be converted into kernel config parameters, though
1596  * making the state machine smarter might be a better option.
1597  */
1598 #define RCU_IDLE_GP_DELAY 4             /* Roughly one grace period. */
1599 #define RCU_IDLE_LAZY_GP_DELAY (6 * HZ) /* Roughly six seconds. */
1600 
1601 static int rcu_idle_gp_delay = RCU_IDLE_GP_DELAY;
1602 module_param(rcu_idle_gp_delay, int, 0644);
1603 static int rcu_idle_lazy_gp_delay = RCU_IDLE_LAZY_GP_DELAY;
1604 module_param(rcu_idle_lazy_gp_delay, int, 0644);
1605 
1606 /*
1607  * Try to advance callbacks on the current CPU, but only if it has been
1608  * awhile since the last time we did so.  Afterwards, if there are any
1609  * callbacks ready for immediate invocation, return true.
1610  */
1611 static bool __maybe_unused rcu_try_advance_all_cbs(void)
1612 {
1613         bool cbs_ready = false;
1614         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
1615         struct rcu_node *rnp;
1616 
1617         /* Exit early if we advanced recently. */
1618         if (jiffies == rdp->last_advance_all)
1619                 return false;
1620         rdp->last_advance_all = jiffies;
1621 
1622         rnp = rdp->mynode;
1623 
1624         /*
1625          * Don't bother checking unless a grace period has
1626          * completed since we last checked and there are
1627          * callbacks not yet ready to invoke.
1628          */
1629         if ((rcu_seq_completed_gp(rdp->gp_seq,
1630                                   rcu_seq_current(&rnp->gp_seq)) ||
1631              unlikely(READ_ONCE(rdp->gpwrap))) &&
1632             rcu_segcblist_pend_cbs(&rdp->cblist))
1633                 note_gp_changes(rdp);
1634 
1635         if (rcu_segcblist_ready_cbs(&rdp->cblist))
1636                 cbs_ready = true;
1637         return cbs_ready;
1638 }
1639 
1640 /*
1641  * Allow the CPU to enter dyntick-idle mode unless it has callbacks ready
1642  * to invoke.  If the CPU has callbacks, try to advance them.  Tell the
1643  * caller to set the timeout based on whether or not there are non-lazy
1644  * callbacks.
1645  *
1646  * The caller must have disabled interrupts.
1647  */
1648 int rcu_needs_cpu(u64 basemono, u64 *nextevt)
1649 {
1650         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
1651         unsigned long dj;
1652 
1653         lockdep_assert_irqs_disabled();
1654 
1655         /* Snapshot to detect later posting of non-lazy callback. */
1656         rdp->nonlazy_posted_snap = rdp->nonlazy_posted;
1657 
1658         /* If no callbacks, RCU doesn't need the CPU. */
1659         if (!rcu_cpu_has_callbacks(&rdp->all_lazy)) {
1660                 *nextevt = KTIME_MAX;
1661                 return 0;
1662         }
1663 
1664         /* Attempt to advance callbacks. */
1665         if (rcu_try_advance_all_cbs()) {
1666                 /* Some ready to invoke, so initiate later invocation. */
1667                 invoke_rcu_core();
1668                 return 1;
1669         }
1670         rdp->last_accelerate = jiffies;
1671 
1672         /* Request timer delay depending on laziness, and round. */
1673         if (!rdp->all_lazy) {
1674                 dj = round_up(rcu_idle_gp_delay + jiffies,
1675                                rcu_idle_gp_delay) - jiffies;
1676         } else {
1677                 dj = round_jiffies(rcu_idle_lazy_gp_delay + jiffies) - jiffies;
1678         }
1679         *nextevt = basemono + dj * TICK_NSEC;
1680         return 0;
1681 }
1682 
1683 /*
1684  * Prepare a CPU for idle from an RCU perspective.  The first major task
1685  * is to sense whether nohz mode has been enabled or disabled via sysfs.
1686  * The second major task is to check to see if a non-lazy callback has
1687  * arrived at a CPU that previously had only lazy callbacks.  The third
1688  * major task is to accelerate (that is, assign grace-period numbers to)
1689  * any recently arrived callbacks.
1690  *
1691  * The caller must have disabled interrupts.
1692  */
1693 static void rcu_prepare_for_idle(void)
1694 {
1695         bool needwake;
1696         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
1697         struct rcu_node *rnp;
1698         int tne;
1699 
1700         lockdep_assert_irqs_disabled();
1701         if (rcu_is_nocb_cpu(smp_processor_id()))
1702                 return;
1703 
1704         /* Handle nohz enablement switches conservatively. */
1705         tne = READ_ONCE(tick_nohz_active);
1706         if (tne != rdp->tick_nohz_enabled_snap) {
1707                 if (rcu_cpu_has_callbacks(NULL))
1708                         invoke_rcu_core(); /* force nohz to see update. */
1709                 rdp->tick_nohz_enabled_snap = tne;
1710                 return;
1711         }
1712         if (!tne)
1713                 return;
1714 
1715         /*
1716          * If a non-lazy callback arrived at a CPU having only lazy
1717          * callbacks, invoke RCU core for the side-effect of recalculating
1718          * idle duration on re-entry to idle.
1719          */
1720         if (rdp->all_lazy &&
1721             rdp->nonlazy_posted != rdp->nonlazy_posted_snap) {
1722                 rdp->all_lazy = false;
1723                 rdp->nonlazy_posted_snap = rdp->nonlazy_posted;
1724                 invoke_rcu_core();
1725                 return;
1726         }
1727 
1728         /*
1729          * If we have not yet accelerated this jiffy, accelerate all
1730          * callbacks on this CPU.
1731          */
1732         if (rdp->last_accelerate == jiffies)
1733                 return;
1734         rdp->last_accelerate = jiffies;
1735         if (rcu_segcblist_pend_cbs(&rdp->cblist)) {
1736                 rnp = rdp->mynode;
1737                 raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
1738                 needwake = rcu_accelerate_cbs(rnp, rdp);
1739                 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
1740                 if (needwake)
1741                         rcu_gp_kthread_wake();
1742         }
1743 }
1744 
1745 /*
1746  * Clean up for exit from idle.  Attempt to advance callbacks based on
1747  * any grace periods that elapsed while the CPU was idle, and if any
1748  * callbacks are now ready to invoke, initiate invocation.
1749  */
1750 static void rcu_cleanup_after_idle(void)
1751 {
1752         lockdep_assert_irqs_disabled();
1753         if (rcu_is_nocb_cpu(smp_processor_id()))
1754                 return;
1755         if (rcu_try_advance_all_cbs())
1756                 invoke_rcu_core();
1757 }
1758 
1759 /*
1760  * Keep a running count of the number of non-lazy callbacks posted
1761  * on this CPU.  This running counter (which is never decremented) allows
1762  * rcu_prepare_for_idle() to detect when something out of the idle loop
1763  * posts a callback, even if an equal number of callbacks are invoked.
1764  * Of course, callbacks should only be posted from within a trace event
1765  * designed to be called from idle or from within RCU_NONIDLE().
1766  */
1767 static void rcu_idle_count_callbacks_posted(void)
1768 {
1769         __this_cpu_add(rcu_data.nonlazy_posted, 1);
1770 }
1771 
1772 #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
1773 
1774 #ifdef CONFIG_RCU_FAST_NO_HZ
1775 
1776 static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
1777 {
1778         struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
1779         unsigned long nlpd = rdp->nonlazy_posted - rdp->nonlazy_posted_snap;
1780 
1781         sprintf(cp, "last_accelerate: %04lx/%04lx, nonlazy_posted: %ld, %c%c",
1782                 rdp->last_accelerate & 0xffff, jiffies & 0xffff,
1783                 ulong2long(nlpd),
1784                 rdp->all_lazy ? 'L' : '.',
1785                 rdp->tick_nohz_enabled_snap ? '.' : 'D');
1786 }
1787 
1788 #else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
1789 
1790 static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
1791 {
1792         *cp = '\0';
1793 }
1794 
1795 #endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */
1796 
1797 /* Initiate the stall-info list. */
1798 static void print_cpu_stall_info_begin(void)
1799 {
1800         pr_cont("\n");
1801 }
1802 
1803 /*
1804  * Print out diagnostic information for the specified stalled CPU.
1805  *
1806  * If the specified CPU is aware of the current RCU grace period, then
1807  * print the number of scheduling clock interrupts the CPU has taken
1808  * during the time that it has been aware.  Otherwise, print the number
1809  * of RCU grace periods that this CPU is ignorant of, for example, "1"
1810  * if the CPU was aware of the previous grace period.
1811  *
1812  * Also print out idle and (if CONFIG_RCU_FAST_NO_HZ) idle-entry info.
1813  */
1814 static void print_cpu_stall_info(int cpu)
1815 {
1816         unsigned long delta;
1817         char fast_no_hz[72];
1818         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
1819         char *ticks_title;
1820         unsigned long ticks_value;
1821 
1822         /*
1823          * We could be printing a lot while holding a spinlock.  Avoid
1824          * triggering hard lockup.
1825          */
1826         touch_nmi_watchdog();
1827 
1828         ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
1829         if (ticks_value) {
1830                 ticks_title = "GPs behind";
1831         } else {
1832                 ticks_title = "ticks this GP";
1833                 ticks_value = rdp->ticks_this_gp;
1834         }
1835         print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
1836         delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
1837         pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s\n",
1838                cpu,
1839                "O."[!!cpu_online(cpu)],
1840                "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
1841                "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
1842                !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
1843                         rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '' :
1844                                 "!."[!delta],
1845                ticks_value, ticks_title,
1846                rcu_dynticks_snap(rdp) & 0xfff,
1847                rdp->dynticks_nesting, rdp->dynticks_nmi_nesting,
1848                rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
1849                READ_ONCE(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
1850                fast_no_hz);
1851 }
1852 
1853 /* Terminate the stall-info list. */
1854 static void print_cpu_stall_info_end(void)
1855 {
1856         pr_err("\t");
1857 }
1858 
1859 /* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
1860 static void zero_cpu_stall_ticks(struct rcu_data *rdp)
1861 {
1862         rdp->ticks_this_gp = 0;
1863         rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
1864         WRITE_ONCE(rdp->last_fqs_resched, jiffies);
1865 }
1866 
1867 #ifdef CONFIG_RCU_NOCB_CPU
1868 
1869 /*
1870  * Offload callback processing from the boot-time-specified set of CPUs
1871  * specified by rcu_nocb_mask.  For each CPU in the set, there is a
1872  * kthread created that pulls the callbacks from the corresponding CPU,
1873  * waits for a grace period to elapse, and invokes the callbacks.
1874  * The no-CBs CPUs do a wake_up() on their kthread when they insert
1875  * a callback into any empty list, unless the rcu_nocb_poll boot parameter
1876  * has been specified, in which case each kthread actively polls its
1877  * CPU.  (Which isn't so great for energy efficiency, but which does
1878  * reduce RCU's overhead on that CPU.)
1879  *
1880  * This is intended to be used in conjunction with Frederic Weisbecker's
1881  * adaptive-idle work, which would seriously reduce OS jitter on CPUs
1882  * running CPU-bound user-mode computations.
1883  *
1884  * Offloading of callback processing could also in theory be used as
1885  * an energy-efficiency measure because CPUs with no RCU callbacks
1886  * queued are more aggressive about entering dyntick-idle mode.
1887  */
1888 
1889 
1890 /* Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters. */
1891 static int __init rcu_nocb_setup(char *str)
1892 {
1893         alloc_bootmem_cpumask_var(&rcu_nocb_mask);
1894         cpulist_parse(str, rcu_nocb_mask);
1895         return 1;
1896 }
1897 __setup("rcu_nocbs=", rcu_nocb_setup);
1898 
1899 static int __init parse_rcu_nocb_poll(char *arg)
1900 {
1901         rcu_nocb_poll = true;
1902         return 0;
1903 }
1904 early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
1905 
1906 /*
1907  * Wake up any no-CBs CPUs' kthreads that were waiting on the just-ended
1908  * grace period.
1909  */
1910 static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
1911 {
1912         swake_up_all(sq);
1913 }
1914 
1915 static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
1916 {
1917         return &rnp->nocb_gp_wq[rcu_seq_ctr(rnp->gp_seq) & 0x1];
1918 }
1919 
1920 static void rcu_init_one_nocb(struct rcu_node *rnp)
1921 {
1922         init_swait_queue_head(&rnp->nocb_gp_wq[0]);
1923         init_swait_queue_head(&rnp->nocb_gp_wq[1]);
1924 }
1925 
1926 /* Is the specified CPU a no-CBs CPU? */
1927 bool rcu_is_nocb_cpu(int cpu)
1928 {
1929         if (cpumask_available(rcu_nocb_mask))
1930                 return cpumask_test_cpu(cpu, rcu_nocb_mask);
1931         return false;
1932 }
1933 
1934 /*
1935  * Kick the leader kthread for this NOCB group.  Caller holds ->nocb_lock
1936  * and this function releases it.
1937  */
1938 static void __wake_nocb_leader(struct rcu_data *rdp, bool force,
1939                                unsigned long flags)
1940         __releases(rdp->nocb_lock)
1941 {
1942         struct rcu_data *rdp_leader = rdp->nocb_leader;
1943 
1944         lockdep_assert_held(&rdp->nocb_lock);
1945         if (!READ_ONCE(rdp_leader->nocb_kthread)) {
1946                 raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
1947                 return;
1948         }
1949         if (rdp_leader->nocb_leader_sleep || force) {
1950                 /* Prior smp_mb__after_atomic() orders against prior enqueue. */
1951                 WRITE_ONCE(rdp_leader->nocb_leader_sleep, false);
1952                 del_timer(&rdp->nocb_timer);
1953                 raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
1954                 smp_mb(); /* ->nocb_leader_sleep before swake_up_one(). */
1955                 swake_up_one(&rdp_leader->nocb_wq);
1956         } else {
1957                 raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
1958         }
1959 }
1960 
1961 /*
1962  * Kick the leader kthread for this NOCB group, but caller has not
1963  * acquired locks.
1964  */
1965 static void wake_nocb_leader(struct rcu_data *rdp, bool force)
1966 {
1967         unsigned long flags;
1968 
1969         raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
1970         __wake_nocb_leader(rdp, force, flags);
1971 }
1972 
1973 /*
1974  * Arrange to wake the leader kthread for this NOCB group at some
1975  * future time when it is safe to do so.
1976  */
1977 static void wake_nocb_leader_defer(struct rcu_data *rdp, int waketype,
1978                                    const char *reason)
1979 {
1980         unsigned long flags;
1981 
1982         raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
1983         if (rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT)
1984                 mod_timer(&rdp->nocb_timer, jiffies + 1);
1985         WRITE_ONCE(rdp->nocb_defer_wakeup, waketype);
1986         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, reason);
1987         raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
1988 }
1989 
1990 /*
1991  * Does the specified CPU need an RCU callback for this invocation
1992  * of rcu_barrier()?
1993  */
1994 static bool rcu_nocb_cpu_needs_barrier(int cpu)
1995 {
1996         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
1997         unsigned long ret;
1998 #ifdef CONFIG_PROVE_RCU
1999         struct rcu_head *rhp;
2000 #endif /* #ifdef CONFIG_PROVE_RCU */
2001 
2002         /*
2003          * Check count of all no-CBs callbacks awaiting invocation.
2004          * There needs to be a barrier before this function is called,
2005          * but associated with a prior determination that no more
2006          * callbacks would be posted.  In the worst case, the first
2007          * barrier in rcu_barrier() suffices (but the caller cannot
2008          * necessarily rely on this, not a substitute for the caller
2009          * getting the concurrency design right!).  There must also be
2010          * a barrier between the following load an posting of a callback
2011          * (if a callback is in fact needed).  This is associated with an
2012          * atomic_inc() in the caller.
2013          */
2014         ret = rcu_get_n_cbs_nocb_cpu(rdp);
2015 
2016 #ifdef CONFIG_PROVE_RCU
2017         rhp = READ_ONCE(rdp->nocb_head);
2018         if (!rhp)
2019                 rhp = READ_ONCE(rdp->nocb_gp_head);
2020         if (!rhp)
2021                 rhp = READ_ONCE(rdp->nocb_follower_head);
2022 
2023         /* Having no rcuo kthread but CBs after scheduler starts is bad! */
2024         if (!READ_ONCE(rdp->nocb_kthread) && rhp &&
2025             rcu_scheduler_fully_active) {
2026                 /* RCU callback enqueued before CPU first came online??? */
2027                 pr_err("RCU: Never-onlined no-CBs CPU %d has CB %p\n",
2028                        cpu, rhp->func);
2029                 WARN_ON_ONCE(1);
2030         }
2031 #endif /* #ifdef CONFIG_PROVE_RCU */
2032 
2033         return !!ret;
2034 }
2035 
2036 /*
2037  * Enqueue the specified string of rcu_head structures onto the specified
2038  * CPU's no-CBs lists.  The CPU is specified by rdp, the head of the
2039  * string by rhp, and the tail of the string by rhtp.  The non-lazy/lazy
2040  * counts are supplied by rhcount and rhcount_lazy.
2041  *
2042  * If warranted, also wake up the kthread servicing this CPUs queues.
2043  */
2044 static void __call_rcu_nocb_enqueue(struct rcu_data *rdp,
2045                                     struct rcu_head *rhp,
2046                                     struct rcu_head **rhtp,
2047                                     int rhcount, int rhcount_lazy,
2048                                     unsigned long flags)
2049 {
2050         int len;
2051         struct rcu_head **old_rhpp;
2052         struct task_struct *t;
2053 
2054         /* Enqueue the callback on the nocb list and update counts. */
2055         atomic_long_add(rhcount, &rdp->nocb_q_count);
2056         /* rcu_barrier() relies on ->nocb_q_count add before xchg. */
2057         old_rhpp = xchg(&rdp->nocb_tail, rhtp);
2058         WRITE_ONCE(*old_rhpp, rhp);
2059         atomic_long_add(rhcount_lazy, &rdp->nocb_q_count_lazy);
2060         smp_mb__after_atomic(); /* Store *old_rhpp before _wake test. */
2061 
2062         /* If we are not being polled and there is a kthread, awaken it ... */
2063         t = READ_ONCE(rdp->nocb_kthread);
2064         if (rcu_nocb_poll || !t) {
2065                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
2066                                     TPS("WakeNotPoll"));
2067                 return;
2068         }
2069         len = rcu_get_n_cbs_nocb_cpu(rdp);
2070         if (old_rhpp == &rdp->nocb_head) {
2071                 if (!irqs_disabled_flags(flags)) {
2072                         /* ... if queue was empty ... */
2073                         wake_nocb_leader(rdp, false);
2074                         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
2075                                             TPS("WakeEmpty"));
2076                 } else {
2077                         wake_nocb_leader_defer(rdp, RCU_NOCB_WAKE,
2078                                                TPS("WakeEmptyIsDeferred"));
2079                 }
2080                 rdp->qlen_last_fqs_check = 0;
2081         } else if (len > rdp->qlen_last_fqs_check + qhimark) {
2082                 /* ... or if many callbacks queued. */
2083                 if (!irqs_disabled_flags(flags)) {
2084                         wake_nocb_leader(rdp, true);
2085                         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
2086                                             TPS("WakeOvf"));
2087                 } else {
2088                         wake_nocb_leader_defer(rdp, RCU_NOCB_WAKE_FORCE,
2089                                                TPS("WakeOvfIsDeferred"));
2090                 }
2091                 rdp->qlen_last_fqs_check = LONG_MAX / 2;
2092         } else {
2093                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
2094         }
2095         return;
2096 }
2097 
2098 /*
2099  * This is a helper for __call_rcu(), which invokes this when the normal
2100  * callback queue is inoperable.  If this is not a no-CBs CPU, this
2101  * function returns failure back to __call_rcu(), which can complain
2102  * appropriately.
2103  *
2104  * Otherwise, this function queues the callback where the corresponding
2105  * "rcuo" kthread can find it.
2106  */
2107 static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
2108                             bool lazy, unsigned long flags)
2109 {
2110 
2111         if (!rcu_is_nocb_cpu(rdp->cpu))
2112                 return false;
2113         __call_rcu_nocb_enqueue(rdp, rhp, &rhp->next, 1, lazy, flags);
2114         if (__is_kfree_rcu_offset((unsigned long)rhp->func))
2115                 trace_rcu_kfree_callback(rcu_state.name, rhp,
2116                                          (unsigned long)rhp->func,
2117                                          -atomic_long_read(&rdp->nocb_q_count_lazy),
2118                                          -rcu_get_n_cbs_nocb_cpu(rdp));
2119         else
2120                 trace_rcu_callback(rcu_state.name, rhp,
2121                                    -atomic_long_read(&rdp->nocb_q_count_lazy),
2122                                    -rcu_get_n_cbs_nocb_cpu(rdp));
2123 
2124         /*
2125          * If called from an extended quiescent state with interrupts
2126          * disabled, invoke the RCU core in order to allow the idle-entry
2127          * deferred-wakeup check to function.
2128          */
2129         if (irqs_disabled_flags(flags) &&
2130             !rcu_is_watching() &&
2131             cpu_online(smp_processor_id()))
2132                 invoke_rcu_core();
2133 
2134         return true;
2135 }
2136 
2137 /*
2138  * Adopt orphaned callbacks on a no-CBs CPU, or return 0 if this is
2139  * not a no-CBs CPU.
2140  */
2141 static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_data *my_rdp,
2142                                                      struct rcu_data *rdp,
2143                                                      unsigned long flags)
2144 {
2145         lockdep_assert_irqs_disabled();
2146         if (!rcu_is_nocb_cpu(smp_processor_id()))
2147                 return false; /* Not NOCBs CPU, caller must migrate CBs. */
2148         __call_rcu_nocb_enqueue(my_rdp, rcu_segcblist_head(&rdp->cblist),
2149                                 rcu_segcblist_tail(&rdp->cblist),
2150                                 rcu_segcblist_n_cbs(&rdp->cblist),
2151                                 rcu_segcblist_n_lazy_cbs(&rdp->cblist), flags);
2152         rcu_segcblist_init(&rdp->cblist);
2153         rcu_segcblist_disable(&rdp->cblist);
2154         return true;
2155 }
2156 
2157 /*
2158  * If necessary, kick off a new grace period, and either way wait
2159  * for a subsequent grace period to complete.
2160  */
2161 static void rcu_nocb_wait_gp(struct rcu_data *rdp)
2162 {
2163         unsigned long c;
2164         bool d;
2165         unsigned long flags;
2166         bool needwake;
2167         struct rcu_node *rnp = rdp->mynode;
2168 
2169         local_irq_save(flags);
2170         c = rcu_seq_snap(&rcu_state.gp_seq);
2171         if (!rdp->gpwrap && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {
2172                 local_irq_restore(flags);
2173         } else {
2174                 raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
2175                 needwake = rcu_start_this_gp(rnp, rdp, c);
2176                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
2177                 if (needwake)
2178                         rcu_gp_kthread_wake();
2179         }
2180 
2181         /*
2182          * Wait for the grace period.  Do so interruptibly to avoid messing
2183          * up the load average.
2184          */
2185         trace_rcu_this_gp(rnp, rdp, c, TPS("StartWait"));
2186         for (;;) {
2187                 swait_event_interruptible_exclusive(
2188                         rnp->nocb_gp_wq[rcu_seq_ctr(c) & 0x1],
2189                         (d = rcu_seq_done(&rnp->gp_seq, c)));
2190                 if (likely(d))
2191                         break;
2192                 WARN_ON(signal_pending(current));
2193                 trace_rcu_this_gp(rnp, rdp, c, TPS("ResumeWait"));
2194         }
2195         trace_rcu_this_gp(rnp, rdp, c, TPS("EndWait"));
2196         smp_mb(); /* Ensure that CB invocation happens after GP end. */
2197 }
2198 
2199 /*
2200  * Leaders come here to wait for additional callbacks to show up.
2201  * This function does not return until callbacks appear.
2202  */
2203 static void nocb_leader_wait(struct rcu_data *my_rdp)
2204 {
2205         bool firsttime = true;
2206         unsigned long flags;
2207         bool gotcbs;
2208         struct rcu_data *rdp;
2209         struct rcu_head **tail;
2210 
2211 wait_again:
2212 
2213         /* Wait for callbacks to appear. */
2214         if (!rcu_nocb_poll) {
2215                 trace_rcu_nocb_wake(rcu_state.name, my_rdp->cpu, TPS("Sleep"));
2216                 swait_event_interruptible_exclusive(my_rdp->nocb_wq,
2217                                 !READ_ONCE(my_rdp->nocb_leader_sleep));
2218                 raw_spin_lock_irqsave(&my_rdp->nocb_lock, flags);
2219                 my_rdp->nocb_leader_sleep = true;
2220                 WRITE_ONCE(my_rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
2221                 del_timer(&my_rdp->nocb_timer);
2222                 raw_spin_unlock_irqrestore(&my_rdp->nocb_lock, flags);
2223         } else if (firsttime) {
2224                 firsttime = false; /* Don't drown trace log with "Poll"! */
2225                 trace_rcu_nocb_wake(rcu_state.name, my_rdp->cpu, TPS("Poll"));
2226         }
2227 
2228         /*
2229          * Each pass through the following loop checks a follower for CBs.
2230          * We are our own first follower.  Any CBs found are moved to
2231          * nocb_gp_head, where they await a grace period.
2232          */
2233         gotcbs = false;
2234         smp_mb(); /* wakeup and _sleep before ->nocb_head reads. */
2235         for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_follower) {
2236                 rdp->nocb_gp_head = READ_ONCE(rdp->nocb_head);
2237                 if (!rdp->nocb_gp_head)
2238                         continue;  /* No CBs here, try next follower. */
2239 
2240                 /* Move callbacks to wait-for-GP list, which is empty. */
2241                 WRITE_ONCE(rdp->nocb_head, NULL);
2242                 rdp->nocb_gp_tail = xchg(&rdp->nocb_tail, &rdp->nocb_head);
2243                 gotcbs = true;
2244         }
2245 
2246         /* No callbacks?  Sleep a bit if polling, and go retry.  */
2247         if (unlikely(!gotcbs)) {
2248                 WARN_ON(signal_pending(current));
2249                 if (rcu_nocb_poll) {
2250                         schedule_timeout_interruptible(1);
2251                 } else {
2252                         trace_rcu_nocb_wake(rcu_state.name, my_rdp->cpu,
2253                                             TPS("WokeEmpty"));
2254                 }
2255                 goto wait_again;
2256         }
2257 
2258         /* Wait for one grace period. */
2259         rcu_nocb_wait_gp(my_rdp);
2260 
2261         /* Each pass through the following loop wakes a follower, if needed. */
2262         for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_follower) {
2263                 if (!rcu_nocb_poll &&
2264                     READ_ONCE(rdp->nocb_head) &&
2265                     READ_ONCE(my_rdp->nocb_leader_sleep)) {
2266                         raw_spin_lock_irqsave(&my_rdp->nocb_lock, flags);
2267                         my_rdp->nocb_leader_sleep = false;/* No need to sleep.*/
2268                         raw_spin_unlock_irqrestore(&my_rdp->nocb_lock, flags);
2269                 }
2270                 if (!rdp->nocb_gp_head)
2271                         continue; /* No CBs, so no need to wake follower. */
2272 
2273                 /* Append callbacks to follower's "done" list. */
2274                 raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
2275                 tail = rdp->nocb_follower_tail;
2276                 rdp->nocb_follower_tail = rdp->nocb_gp_tail;
2277                 *tail = rdp->nocb_gp_head;
2278                 raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
2279                 if (rdp != my_rdp && tail == &rdp->nocb_follower_head) {
2280                         /* List was empty, so wake up the follower.  */
2281                         swake_up_one(&rdp->nocb_wq);
2282                 }
2283         }
2284 
2285         /* If we (the leader) don't have CBs, go wait some more. */
2286         if (!my_rdp->nocb_follower_head)
2287                 goto wait_again;
2288 }
2289 
2290 /*
2291  * Followers come here to wait for additional callbacks to show up.
2292  * This function does not return until callbacks appear.
2293  */
2294 static void nocb_follower_wait(struct rcu_data *rdp)
2295 {
2296         for (;;) {
2297                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FollowerSleep"));
2298                 swait_event_interruptible_exclusive(rdp->nocb_wq,
2299                                          READ_ONCE(rdp->nocb_follower_head));
2300                 if (smp_load_acquire(&rdp->nocb_follower_head)) {
2301                         /* ^^^ Ensure CB invocation follows _head test. */
2302                         return;
2303                 }
2304                 WARN_ON(signal_pending(current));
2305                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
2306         }
2307 }
2308 
2309 /*
2310  * Per-rcu_data kthread, but only for no-CBs CPUs.  Each kthread invokes
2311  * callbacks queued by the corresponding no-CBs CPU, however, there is
2312  * an optional leader-follower relationship so that the grace-period
2313  * kthreads don't have to do quite so many wakeups.
2314  */
2315 static int rcu_nocb_kthread(void *arg)
2316 {
2317         int c, cl;
2318         unsigned long flags;
2319         struct rcu_head *list;
2320         struct rcu_head *next;
2321         struct rcu_head **tail;
2322         struct rcu_data *rdp = arg;
2323 
2324         /* Each pass through this loop invokes one batch of callbacks */
2325         for (;;) {
2326                 /* Wait for callbacks. */
2327                 if (rdp->nocb_leader == rdp)
2328                         nocb_leader_wait(rdp);
2329                 else
2330                         nocb_follower_wait(rdp);
2331 
2332                 /* Pull the ready-to-invoke callbacks onto local list. */
2333                 raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
2334                 list = rdp->nocb_follower_head;
2335                 rdp->nocb_follower_head = NULL;
2336                 tail = rdp->nocb_follower_tail;
2337                 rdp->nocb_follower_tail = &rdp->nocb_follower_head;
2338                 raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
2339                 if (WARN_ON_ONCE(!list))
2340                         continue;
2341                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeNonEmpty"));
2342 
2343                 /* Each pass through the following loop invokes a callback. */
2344                 trace_rcu_batch_start(rcu_state.name,
2345                                       atomic_long_read(&rdp->nocb_q_count_lazy),
2346                                       rcu_get_n_cbs_nocb_cpu(rdp), -1);
2347                 c = cl = 0;
2348                 while (list) {
2349                         next = list->next;
2350                         /* Wait for enqueuing to complete, if needed. */
2351                         while (next == NULL && &list->next != tail) {
2352                                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
2353                                                     TPS("WaitQueue"));
2354                                 schedule_timeout_interruptible(1);
2355                                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
2356                                                     TPS("WokeQueue"));
2357                                 next = list->next;
2358                         }
2359                         debug_rcu_head_unqueue(list);
2360                         local_bh_disable();
2361                         if (__rcu_reclaim(rcu_state.name, list))
2362                                 cl++;
2363                         c++;
2364                         local_bh_enable();
2365                         cond_resched_tasks_rcu_qs();
2366                         list = next;
2367                 }
2368                 trace_rcu_batch_end(rcu_state.name, c, !!list, 0, 0, 1);
2369                 smp_mb__before_atomic();  /* _add after CB invocation. */
2370                 atomic_long_add(-c, &rdp->nocb_q_count);
2371                 atomic_long_add(-cl, &rdp->nocb_q_count_lazy);
2372         }
2373         return 0;
2374 }
2375 
2376 /* Is a deferred wakeup of rcu_nocb_kthread() required? */
2377 static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp)
2378 {
2379         return READ_ONCE(rdp->nocb_defer_wakeup);
2380 }
2381 
2382 /* Do a deferred wakeup of rcu_nocb_kthread(). */
2383 static void do_nocb_deferred_wakeup_common(struct rcu_data *rdp)
2384 {
2385         unsigned long flags;
2386         int ndw;
2387 
2388         raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
2389         if (!rcu_nocb_need_deferred_wakeup(rdp)) {
2390                 raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
2391                 return;
2392         }
2393         ndw = READ_ONCE(rdp->nocb_defer_wakeup);
2394         WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
2395         __wake_nocb_leader(rdp, ndw == RCU_NOCB_WAKE_FORCE, flags);
2396         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DeferredWake"));
2397 }
2398 
2399 /* Do a deferred wakeup of rcu_nocb_kthread() from a timer handler. */
2400 static void do_nocb_deferred_wakeup_timer(struct timer_list *t)
2401 {
2402         struct rcu_data *rdp = from_timer(rdp, t, nocb_timer);
2403 
2404         do_nocb_deferred_wakeup_common(rdp);
2405 }
2406 
2407 /*
2408  * Do a deferred wakeup of rcu_nocb_kthread() from fastpath.
2409  * This means we do an inexact common-case check.  Note that if
2410  * we miss, ->nocb_timer will eventually clean things up.
2411  */
2412 static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
2413 {
2414         if (rcu_nocb_need_deferred_wakeup(rdp))
2415                 do_nocb_deferred_wakeup_common(rdp);
2416 }
2417 
2418 void __init rcu_init_nohz(void)
2419 {
2420         int cpu;
2421         bool need_rcu_nocb_mask = false;
2422 
2423 #if defined(CONFIG_NO_HZ_FULL)
2424         if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
2425                 need_rcu_nocb_mask = true;
2426 #endif /* #if defined(CONFIG_NO_HZ_FULL) */
2427 
2428         if (!cpumask_available(rcu_nocb_mask) && need_rcu_nocb_mask) {
2429                 if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
2430                         pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
2431                         return;
2432                 }
2433         }
2434         if (!cpumask_available(rcu_nocb_mask))
2435                 return;
2436 
2437 #if defined(CONFIG_NO_HZ_FULL)
2438         if (tick_nohz_full_running)
2439                 cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
2440 #endif /* #if defined(CONFIG_NO_HZ_FULL) */
2441 
2442         if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
2443                 pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
2444                 cpumask_and(rcu_nocb_mask, cpu_possible_mask,
2445                             rcu_nocb_mask);
2446         }
2447         if (cpumask_empty(rcu_nocb_mask))
2448                 pr_info("\tOffload RCU callbacks from CPUs: (none).\n");
2449         else
2450                 pr_info("\tOffload RCU callbacks from CPUs: %*pbl.\n",
2451                         cpumask_pr_args(rcu_nocb_mask));
2452         if (rcu_nocb_poll)
2453                 pr_info("\tPoll for callbacks from no-CBs CPUs.\n");
2454 
2455         for_each_cpu(cpu, rcu_nocb_mask)
2456                 init_nocb_callback_list(per_cpu_ptr(&rcu_data, cpu));
2457         rcu_organize_nocb_kthreads();
2458 }
2459 
2460 /* Initialize per-rcu_data variables for no-CBs CPUs. */
2461 static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
2462 {
2463         rdp->nocb_tail = &rdp->nocb_head;
2464         init_swait_queue_head(&rdp->nocb_wq);
2465         rdp->nocb_follower_tail = &rdp->nocb_follower_head;
2466         raw_spin_lock_init(&rdp->nocb_lock);
2467         timer_setup(&rdp->nocb_timer, do_nocb_deferred_wakeup_timer, 0);
2468 }
2469 
2470 /*
2471  * If the specified CPU is a no-CBs CPU that does not already have its
2472  * rcuo kthread, spawn it.  If the CPUs are brought online out of order,
2473  * this can require re-organizing the leader-follower relationships.
2474  */
2475 static void rcu_spawn_one_nocb_kthread(int cpu)
2476 {
2477         struct rcu_data *rdp;
2478         struct rcu_data *rdp_last;
2479         struct rcu_data *rdp_old_leader;
2480         struct rcu_data *rdp_spawn = per_cpu_ptr(&rcu_data, cpu);
2481         struct task_struct *t;
2482 
2483         /*
2484          * If this isn't a no-CBs CPU or if it already has an rcuo kthread,
2485          * then nothing to do.
2486          */
2487         if (!rcu_is_nocb_cpu(cpu) || rdp_spawn->nocb_kthread)
2488                 return;
2489 
2490         /* If we didn't spawn the leader first, reorganize! */
2491         rdp_old_leader = rdp_spawn->nocb_leader;
2492         if (rdp_old_leader != rdp_spawn && !rdp_old_leader->nocb_kthread) {
2493                 rdp_last = NULL;
2494                 rdp = rdp_old_leader;
2495                 do {
2496                         rdp->nocb_leader = rdp_spawn;
2497                         if (rdp_last && rdp != rdp_spawn)
2498                                 rdp_last->nocb_next_follower = rdp;
2499                         if (rdp == rdp_spawn) {
2500                                 rdp = rdp->nocb_next_follower;
2501                         } else {
2502                                 rdp_last = rdp;
2503                                 rdp = rdp->nocb_next_follower;
2504                                 rdp_last->nocb_next_follower = NULL;
2505                         }
2506                 } while (rdp);
2507                 rdp_spawn->nocb_next_follower = rdp_old_leader;
2508         }
2509 
2510         /* Spawn the kthread for this CPU. */
2511         t = kthread_run(rcu_nocb_kthread, rdp_spawn,
2512                         "rcuo%c/%d", rcu_state.abbr, cpu);
2513         if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo kthread, OOM is now expected behavior\n", __func__))
2514                 return;
2515         WRITE_ONCE(rdp_spawn->nocb_kthread, t);
2516 }
2517 
2518 /*
2519  * If the specified CPU is a no-CBs CPU that does not already have its
2520  * rcuo kthreads, spawn them.
2521  */
2522 static void rcu_spawn_all_nocb_kthreads(int cpu)
2523 {
2524         if (rcu_scheduler_fully_active)
2525                 rcu_spawn_one_nocb_kthread(cpu);
2526 }
2527 
2528 /*
2529  * Once the scheduler is running, spawn rcuo kthreads for all online
2530  * no-CBs CPUs.  This assumes that the early_initcall()s happen before
2531  * non-boot CPUs come online -- if this changes, we will need to add
2532  * some mutual exclusion.
2533  */
2534 static void __init rcu_spawn_nocb_kthreads(void)
2535 {
2536         int cpu;
2537 
2538         for_each_online_cpu(cpu)
2539                 rcu_spawn_all_nocb_kthreads(cpu);
2540 }
2541 
2542 /* How many follower CPU IDs per leader?  Default of -1 for sqrt(nr_cpu_ids). */
2543 static int rcu_nocb_leader_stride = -1;
2544 module_param(rcu_nocb_leader_stride, int, 0444);
2545 
2546 /*
2547  * Initialize leader-follower relationships for all no-CBs CPU.
2548  */
2549 static void __init rcu_organize_nocb_kthreads(void)
2550 {
2551         int cpu;
2552         int ls = rcu_nocb_leader_stride;
2553         int nl = 0;  /* Next leader. */
2554         struct rcu_data *rdp;
2555         struct rcu_data *rdp_leader = NULL;  /* Suppress misguided gcc warn. */
2556         struct rcu_data *rdp_prev = NULL;
2557 
2558         if (!cpumask_available(rcu_nocb_mask))
2559                 return;
2560         if (ls == -1) {
2561                 ls = int_sqrt(nr_cpu_ids);
2562                 rcu_nocb_leader_stride = ls;
2563         }
2564 
2565         /*
2566          * Each pass through this loop sets up one rcu_data structure.
2567          * Should the corresponding CPU come online in the future, then
2568          * we will spawn the needed set of rcu_nocb_kthread() kthreads.
2569          */
2570         for_each_cpu(cpu, rcu_nocb_mask) {
2571                 rdp = per_cpu_ptr(&rcu_data, cpu);
2572                 if (rdp->cpu >= nl) {
2573                         /* New leader, set up for followers & next leader. */
2574                         nl = DIV_ROUND_UP(rdp->cpu + 1, ls) * ls;
2575                         rdp->nocb_leader = rdp;
2576                         rdp_leader = rdp;
2577                 } else {
2578                         /* Another follower, link to previous leader. */
2579                         rdp->nocb_leader = rdp_leader;
2580                         rdp_prev->nocb_next_follower = rdp;
2581                 }
2582                 rdp_prev = rdp;
2583         }
2584 }
2585 
2586 /* Prevent __call_rcu() from enqueuing callbacks on no-CBs CPUs */
2587 static bool init_nocb_callback_list(struct rcu_data *rdp)
2588 {
2589         if (!rcu_is_nocb_cpu(rdp->cpu))
2590                 return false;
2591 
2592         /* If there are early-boot callbacks, move them to nocb lists. */
2593         if (!rcu_segcblist_empty(&rdp->cblist)) {
2594                 rdp->nocb_head = rcu_segcblist_head(&rdp->cblist);
2595                 rdp->nocb_tail = rcu_segcblist_tail(&rdp->cblist);
2596                 atomic_long_set(&rdp->nocb_q_count,
2597                                 rcu_segcblist_n_cbs(&rdp->cblist));
2598                 atomic_long_set(&rdp->nocb_q_count_lazy,
2599                                 rcu_segcblist_n_lazy_cbs(&rdp->cblist));
2600                 rcu_segcblist_init(&rdp->cblist);
2601         }
2602         rcu_segcblist_disable(&rdp->cblist);
2603         return true;
2604 }
2605 
2606 /*
2607  * Bind the current task to the offloaded CPUs.  If there are no offloaded
2608  * CPUs, leave the task unbound.  Splat if the bind attempt fails.
2609  */
2610 void rcu_bind_current_to_nocb(void)
2611 {
2612         if (cpumask_available(rcu_nocb_mask) && cpumask_weight(rcu_nocb_mask))
2613                 WARN_ON(sched_setaffinity(current->pid, rcu_nocb_mask));
2614 }
2615 EXPORT_SYMBOL_GPL(rcu_bind_current_to_nocb);
2616 
2617 /*
2618  * Return the number of RCU callbacks still queued from the specified
2619  * CPU, which must be a nocbs CPU.
2620  */
2621 static unsigned long rcu_get_n_cbs_nocb_cpu(struct rcu_data *rdp)
2622 {
2623         return atomic_long_read(&rdp->nocb_q_count);
2624 }
2625 
2626 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
2627 
2628 static bool rcu_nocb_cpu_needs_barrier(int cpu)
2629 {
2630         WARN_ON_ONCE(1); /* Should be dead code. */
2631         return false;
2632 }
2633 
2634 static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
2635 {
2636 }
2637 
2638 static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
2639 {
2640         return NULL;
2641 }
2642 
2643 static void rcu_init_one_nocb(struct rcu_node *rnp)
2644 {
2645 }
2646 
2647 static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
2648                             bool lazy, unsigned long flags)
2649 {
2650         return false;
2651 }
2652 
2653 static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_data *my_rdp,
2654                                                      struct rcu_data *rdp,
2655                                                      unsigned long flags)
2656 {
2657         return false;
2658 }
2659 
2660 static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
2661 {
2662 }
2663 
2664 static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp)
2665 {
2666         return false;
2667 }
2668 
2669 static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
2670 {
2671 }
2672 
2673 static void rcu_spawn_all_nocb_kthreads(int cpu)
2674 {
2675 }
2676 
2677 static void __init rcu_spawn_nocb_kthreads(void)
2678 {
2679 }
2680 
2681 static bool init_nocb_callback_list(struct rcu_data *rdp)
2682 {
2683         return false;
2684 }
2685 
2686 static unsigned long rcu_get_n_cbs_nocb_cpu(struct rcu_data *rdp)
2687 {
2688         return 0;
2689 }
2690 
2691 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
2692 
2693 /*
2694  * Is this CPU a NO_HZ_FULL CPU that should ignore RCU so that the
2695  * grace-period kthread will do force_quiescent_state() processing?
2696  * The idea is to avoid waking up RCU core processing on such a
2697  * CPU unless the grace period has extended for too long.
2698  *
2699  * This code relies on the fact that all NO_HZ_FULL CPUs are also
2700  * CONFIG_RCU_NOCB_CPU CPUs.
2701  */
2702 static bool rcu_nohz_full_cpu(void)
2703 {
2704 #ifdef CONFIG_NO_HZ_FULL
2705         if (tick_nohz_full_cpu(smp_processor_id()) &&
2706             (!rcu_gp_in_progress() ||
2707              ULONG_CMP_LT(jiffies, READ_ONCE(rcu_state.gp_start) + HZ)))
2708                 return true;
2709 #endif /* #ifdef CONFIG_NO_HZ_FULL */
2710         return false;
2711 }
2712 
2713 /*
2714  * Bind the RCU grace-period kthreads to the housekeeping CPU.
2715  */
2716 static void rcu_bind_gp_kthread(void)
2717 {
2718         if (!tick_nohz_full_enabled())
2719                 return;
2720         housekeeping_affine(current, HK_FLAG_RCU);
2721 }
2722 
2723 /* Record the current task on dyntick-idle entry. */
2724 static void rcu_dynticks_task_enter(void)
2725 {
2726 #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
2727         WRITE_ONCE(current->rcu_tasks_idle_cpu, smp_processor_id());
2728 #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
2729 }
2730 
2731 /* Record no current task on dyntick-idle exit. */
2732 static void rcu_dynticks_task_exit(void)
2733 {
2734 #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
2735         WRITE_ONCE(current->rcu_tasks_idle_cpu, -1);
2736 #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
2737 }
2738 

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