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

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  1 /*
  2  * RCU expedited grace periods
  3  *
  4  * This program is free software; you can redistribute it and/or modify
  5  * it under the terms of the GNU General Public License as published by
  6  * the Free Software Foundation; either version 2 of the License, or
  7  * (at your option) any later version.
  8  *
  9  * This program is distributed in the hope that it will be useful,
 10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
 11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 12  * GNU General Public License for more details.
 13  *
 14  * You should have received a copy of the GNU General Public License
 15  * along with this program; if not, you can access it online at
 16  * http://www.gnu.org/licenses/gpl-2.0.html.
 17  *
 18  * Copyright IBM Corporation, 2016
 19  *
 20  * Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
 21  */
 22 
 23 #include <linux/lockdep.h>
 24 
 25 /*
 26  * Record the start of an expedited grace period.
 27  */
 28 static void rcu_exp_gp_seq_start(void)
 29 {
 30         rcu_seq_start(&rcu_state.expedited_sequence);
 31 }
 32 
 33 /*
 34  * Return then value that expedited-grace-period counter will have
 35  * at the end of the current grace period.
 36  */
 37 static __maybe_unused unsigned long rcu_exp_gp_seq_endval(void)
 38 {
 39         return rcu_seq_endval(&rcu_state.expedited_sequence);
 40 }
 41 
 42 /*
 43  * Record the end of an expedited grace period.
 44  */
 45 static void rcu_exp_gp_seq_end(void)
 46 {
 47         rcu_seq_end(&rcu_state.expedited_sequence);
 48         smp_mb(); /* Ensure that consecutive grace periods serialize. */
 49 }
 50 
 51 /*
 52  * Take a snapshot of the expedited-grace-period counter.
 53  */
 54 static unsigned long rcu_exp_gp_seq_snap(void)
 55 {
 56         unsigned long s;
 57 
 58         smp_mb(); /* Caller's modifications seen first by other CPUs. */
 59         s = rcu_seq_snap(&rcu_state.expedited_sequence);
 60         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("snap"));
 61         return s;
 62 }
 63 
 64 /*
 65  * Given a counter snapshot from rcu_exp_gp_seq_snap(), return true
 66  * if a full expedited grace period has elapsed since that snapshot
 67  * was taken.
 68  */
 69 static bool rcu_exp_gp_seq_done(unsigned long s)
 70 {
 71         return rcu_seq_done(&rcu_state.expedited_sequence, s);
 72 }
 73 
 74 /*
 75  * Reset the ->expmaskinit values in the rcu_node tree to reflect any
 76  * recent CPU-online activity.  Note that these masks are not cleared
 77  * when CPUs go offline, so they reflect the union of all CPUs that have
 78  * ever been online.  This means that this function normally takes its
 79  * no-work-to-do fastpath.
 80  */
 81 static void sync_exp_reset_tree_hotplug(void)
 82 {
 83         bool done;
 84         unsigned long flags;
 85         unsigned long mask;
 86         unsigned long oldmask;
 87         int ncpus = smp_load_acquire(&rcu_state.ncpus); /* Order vs. locking. */
 88         struct rcu_node *rnp;
 89         struct rcu_node *rnp_up;
 90 
 91         /* If no new CPUs onlined since last time, nothing to do. */
 92         if (likely(ncpus == rcu_state.ncpus_snap))
 93                 return;
 94         rcu_state.ncpus_snap = ncpus;
 95 
 96         /*
 97          * Each pass through the following loop propagates newly onlined
 98          * CPUs for the current rcu_node structure up the rcu_node tree.
 99          */
100         rcu_for_each_leaf_node(rnp) {
101                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
102                 if (rnp->expmaskinit == rnp->expmaskinitnext) {
103                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
104                         continue;  /* No new CPUs, nothing to do. */
105                 }
106 
107                 /* Update this node's mask, track old value for propagation. */
108                 oldmask = rnp->expmaskinit;
109                 rnp->expmaskinit = rnp->expmaskinitnext;
110                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
111 
112                 /* If was already nonzero, nothing to propagate. */
113                 if (oldmask)
114                         continue;
115 
116                 /* Propagate the new CPU up the tree. */
117                 mask = rnp->grpmask;
118                 rnp_up = rnp->parent;
119                 done = false;
120                 while (rnp_up) {
121                         raw_spin_lock_irqsave_rcu_node(rnp_up, flags);
122                         if (rnp_up->expmaskinit)
123                                 done = true;
124                         rnp_up->expmaskinit |= mask;
125                         raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags);
126                         if (done)
127                                 break;
128                         mask = rnp_up->grpmask;
129                         rnp_up = rnp_up->parent;
130                 }
131         }
132 }
133 
134 /*
135  * Reset the ->expmask values in the rcu_node tree in preparation for
136  * a new expedited grace period.
137  */
138 static void __maybe_unused sync_exp_reset_tree(void)
139 {
140         unsigned long flags;
141         struct rcu_node *rnp;
142 
143         sync_exp_reset_tree_hotplug();
144         rcu_for_each_node_breadth_first(rnp) {
145                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
146                 WARN_ON_ONCE(rnp->expmask);
147                 rnp->expmask = rnp->expmaskinit;
148                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
149         }
150 }
151 
152 /*
153  * Return non-zero if there is no RCU expedited grace period in progress
154  * for the specified rcu_node structure, in other words, if all CPUs and
155  * tasks covered by the specified rcu_node structure have done their bit
156  * for the current expedited grace period.  Works only for preemptible
157  * RCU -- other RCU implementation use other means.
158  *
159  * Caller must hold the specificed rcu_node structure's ->lock
160  */
161 static bool sync_rcu_preempt_exp_done(struct rcu_node *rnp)
162 {
163         raw_lockdep_assert_held_rcu_node(rnp);
164 
165         return rnp->exp_tasks == NULL &&
166                READ_ONCE(rnp->expmask) == 0;
167 }
168 
169 /*
170  * Like sync_rcu_preempt_exp_done(), but this function assumes the caller
171  * doesn't hold the rcu_node's ->lock, and will acquire and release the lock
172  * itself
173  */
174 static bool sync_rcu_preempt_exp_done_unlocked(struct rcu_node *rnp)
175 {
176         unsigned long flags;
177         bool ret;
178 
179         raw_spin_lock_irqsave_rcu_node(rnp, flags);
180         ret = sync_rcu_preempt_exp_done(rnp);
181         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
182 
183         return ret;
184 }
185 
186 
187 /*
188  * Report the exit from RCU read-side critical section for the last task
189  * that queued itself during or before the current expedited preemptible-RCU
190  * grace period.  This event is reported either to the rcu_node structure on
191  * which the task was queued or to one of that rcu_node structure's ancestors,
192  * recursively up the tree.  (Calm down, calm down, we do the recursion
193  * iteratively!)
194  *
195  * Caller must hold the specified rcu_node structure's ->lock.
196  */
197 static void __rcu_report_exp_rnp(struct rcu_node *rnp,
198                                  bool wake, unsigned long flags)
199         __releases(rnp->lock)
200 {
201         unsigned long mask;
202 
203         for (;;) {
204                 if (!sync_rcu_preempt_exp_done(rnp)) {
205                         if (!rnp->expmask)
206                                 rcu_initiate_boost(rnp, flags);
207                         else
208                                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
209                         break;
210                 }
211                 if (rnp->parent == NULL) {
212                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
213                         if (wake) {
214                                 smp_mb(); /* EGP done before wake_up(). */
215                                 swake_up_one(&rcu_state.expedited_wq);
216                         }
217                         break;
218                 }
219                 mask = rnp->grpmask;
220                 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */
221                 rnp = rnp->parent;
222                 raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
223                 WARN_ON_ONCE(!(rnp->expmask & mask));
224                 rnp->expmask &= ~mask;
225         }
226 }
227 
228 /*
229  * Report expedited quiescent state for specified node.  This is a
230  * lock-acquisition wrapper function for __rcu_report_exp_rnp().
231  */
232 static void __maybe_unused rcu_report_exp_rnp(struct rcu_node *rnp, bool wake)
233 {
234         unsigned long flags;
235 
236         raw_spin_lock_irqsave_rcu_node(rnp, flags);
237         __rcu_report_exp_rnp(rnp, wake, flags);
238 }
239 
240 /*
241  * Report expedited quiescent state for multiple CPUs, all covered by the
242  * specified leaf rcu_node structure.
243  */
244 static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
245                                     unsigned long mask, bool wake)
246 {
247         unsigned long flags;
248 
249         raw_spin_lock_irqsave_rcu_node(rnp, flags);
250         if (!(rnp->expmask & mask)) {
251                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
252                 return;
253         }
254         rnp->expmask &= ~mask;
255         __rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */
256 }
257 
258 /*
259  * Report expedited quiescent state for specified rcu_data (CPU).
260  */
261 static void rcu_report_exp_rdp(struct rcu_data *rdp)
262 {
263         WRITE_ONCE(rdp->deferred_qs, false);
264         rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true);
265 }
266 
267 /* Common code for work-done checking. */
268 static bool sync_exp_work_done(unsigned long s)
269 {
270         if (rcu_exp_gp_seq_done(s)) {
271                 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
272                 /* Ensure test happens before caller kfree(). */
273                 smp_mb__before_atomic(); /* ^^^ */
274                 return true;
275         }
276         return false;
277 }
278 
279 /*
280  * Funnel-lock acquisition for expedited grace periods.  Returns true
281  * if some other task completed an expedited grace period that this task
282  * can piggy-back on, and with no mutex held.  Otherwise, returns false
283  * with the mutex held, indicating that the caller must actually do the
284  * expedited grace period.
285  */
286 static bool exp_funnel_lock(unsigned long s)
287 {
288         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
289         struct rcu_node *rnp = rdp->mynode;
290         struct rcu_node *rnp_root = rcu_get_root();
291 
292         /* Low-contention fastpath. */
293         if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
294             (rnp == rnp_root ||
295              ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
296             mutex_trylock(&rcu_state.exp_mutex))
297                 goto fastpath;
298 
299         /*
300          * Each pass through the following loop works its way up
301          * the rcu_node tree, returning if others have done the work or
302          * otherwise falls through to acquire ->exp_mutex.  The mapping
303          * from CPU to rcu_node structure can be inexact, as it is just
304          * promoting locality and is not strictly needed for correctness.
305          */
306         for (; rnp != NULL; rnp = rnp->parent) {
307                 if (sync_exp_work_done(s))
308                         return true;
309 
310                 /* Work not done, either wait here or go up. */
311                 spin_lock(&rnp->exp_lock);
312                 if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {
313 
314                         /* Someone else doing GP, so wait for them. */
315                         spin_unlock(&rnp->exp_lock);
316                         trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
317                                                   rnp->grplo, rnp->grphi,
318                                                   TPS("wait"));
319                         wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
320                                    sync_exp_work_done(s));
321                         return true;
322                 }
323                 rnp->exp_seq_rq = s; /* Followers can wait on us. */
324                 spin_unlock(&rnp->exp_lock);
325                 trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
326                                           rnp->grplo, rnp->grphi, TPS("nxtlvl"));
327         }
328         mutex_lock(&rcu_state.exp_mutex);
329 fastpath:
330         if (sync_exp_work_done(s)) {
331                 mutex_unlock(&rcu_state.exp_mutex);
332                 return true;
333         }
334         rcu_exp_gp_seq_start();
335         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("start"));
336         return false;
337 }
338 
339 /*
340  * Select the CPUs within the specified rcu_node that the upcoming
341  * expedited grace period needs to wait for.
342  */
343 static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
344 {
345         int cpu;
346         unsigned long flags;
347         smp_call_func_t func;
348         unsigned long mask_ofl_test;
349         unsigned long mask_ofl_ipi;
350         int ret;
351         struct rcu_exp_work *rewp =
352                 container_of(wp, struct rcu_exp_work, rew_work);
353         struct rcu_node *rnp = container_of(rewp, struct rcu_node, rew);
354 
355         func = rewp->rew_func;
356         raw_spin_lock_irqsave_rcu_node(rnp, flags);
357 
358         /* Each pass checks a CPU for identity, offline, and idle. */
359         mask_ofl_test = 0;
360         for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
361                 unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
362                 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
363                 int snap;
364 
365                 if (raw_smp_processor_id() == cpu ||
366                     !(rnp->qsmaskinitnext & mask)) {
367                         mask_ofl_test |= mask;
368                 } else {
369                         snap = rcu_dynticks_snap(rdp);
370                         if (rcu_dynticks_in_eqs(snap))
371                                 mask_ofl_test |= mask;
372                         else
373                                 rdp->exp_dynticks_snap = snap;
374                 }
375         }
376         mask_ofl_ipi = rnp->expmask & ~mask_ofl_test;
377 
378         /*
379          * Need to wait for any blocked tasks as well.  Note that
380          * additional blocking tasks will also block the expedited GP
381          * until such time as the ->expmask bits are cleared.
382          */
383         if (rcu_preempt_has_tasks(rnp))
384                 rnp->exp_tasks = rnp->blkd_tasks.next;
385         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
386 
387         /* IPI the remaining CPUs for expedited quiescent state. */
388         for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
389                 unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
390                 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
391 
392                 if (!(mask_ofl_ipi & mask))
393                         continue;
394 retry_ipi:
395                 if (rcu_dynticks_in_eqs_since(rdp, rdp->exp_dynticks_snap)) {
396                         mask_ofl_test |= mask;
397                         continue;
398                 }
399                 ret = smp_call_function_single(cpu, func, NULL, 0);
400                 if (!ret) {
401                         mask_ofl_ipi &= ~mask;
402                         continue;
403                 }
404                 /* Failed, raced with CPU hotplug operation. */
405                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
406                 if ((rnp->qsmaskinitnext & mask) &&
407                     (rnp->expmask & mask)) {
408                         /* Online, so delay for a bit and try again. */
409                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
410                         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("selectofl"));
411                         schedule_timeout_uninterruptible(1);
412                         goto retry_ipi;
413                 }
414                 /* CPU really is offline, so we can ignore it. */
415                 if (!(rnp->expmask & mask))
416                         mask_ofl_ipi &= ~mask;
417                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
418         }
419         /* Report quiescent states for those that went offline. */
420         mask_ofl_test |= mask_ofl_ipi;
421         if (mask_ofl_test)
422                 rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
423 }
424 
425 /*
426  * Select the nodes that the upcoming expedited grace period needs
427  * to wait for.
428  */
429 static void sync_rcu_exp_select_cpus(smp_call_func_t func)
430 {
431         int cpu;
432         struct rcu_node *rnp;
433 
434         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("reset"));
435         sync_exp_reset_tree();
436         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("select"));
437 
438         /* Schedule work for each leaf rcu_node structure. */
439         rcu_for_each_leaf_node(rnp) {
440                 rnp->exp_need_flush = false;
441                 if (!READ_ONCE(rnp->expmask))
442                         continue; /* Avoid early boot non-existent wq. */
443                 rnp->rew.rew_func = func;
444                 if (!READ_ONCE(rcu_par_gp_wq) ||
445                     rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
446                     rcu_is_last_leaf_node(rnp)) {
447                         /* No workqueues yet or last leaf, do direct call. */
448                         sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
449                         continue;
450                 }
451                 INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
452                 preempt_disable();
453                 cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask);
454                 /* If all offline, queue the work on an unbound CPU. */
455                 if (unlikely(cpu > rnp->grphi))
456                         cpu = WORK_CPU_UNBOUND;
457                 queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
458                 preempt_enable();
459                 rnp->exp_need_flush = true;
460         }
461 
462         /* Wait for workqueue jobs (if any) to complete. */
463         rcu_for_each_leaf_node(rnp)
464                 if (rnp->exp_need_flush)
465                         flush_work(&rnp->rew.rew_work);
466 }
467 
468 static void synchronize_sched_expedited_wait(void)
469 {
470         int cpu;
471         unsigned long jiffies_stall;
472         unsigned long jiffies_start;
473         unsigned long mask;
474         int ndetected;
475         struct rcu_node *rnp;
476         struct rcu_node *rnp_root = rcu_get_root();
477         int ret;
478 
479         trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
480         jiffies_stall = rcu_jiffies_till_stall_check();
481         jiffies_start = jiffies;
482 
483         for (;;) {
484                 ret = swait_event_timeout_exclusive(
485                                 rcu_state.expedited_wq,
486                                 sync_rcu_preempt_exp_done_unlocked(rnp_root),
487                                 jiffies_stall);
488                 if (ret > 0 || sync_rcu_preempt_exp_done_unlocked(rnp_root))
489                         return;
490                 WARN_ON(ret < 0);  /* workqueues should not be signaled. */
491                 if (rcu_cpu_stall_suppress)
492                         continue;
493                 panic_on_rcu_stall();
494                 pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
495                        rcu_state.name);
496                 ndetected = 0;
497                 rcu_for_each_leaf_node(rnp) {
498                         ndetected += rcu_print_task_exp_stall(rnp);
499                         for_each_leaf_node_possible_cpu(rnp, cpu) {
500                                 struct rcu_data *rdp;
501 
502                                 mask = leaf_node_cpu_bit(rnp, cpu);
503                                 if (!(rnp->expmask & mask))
504                                         continue;
505                                 ndetected++;
506                                 rdp = per_cpu_ptr(&rcu_data, cpu);
507                                 pr_cont(" %d-%c%c%c", cpu,
508                                         "O."[!!cpu_online(cpu)],
509                                         "o."[!!(rdp->grpmask & rnp->expmaskinit)],
510                                         "N."[!!(rdp->grpmask & rnp->expmaskinitnext)]);
511                         }
512                 }
513                 pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
514                         jiffies - jiffies_start, rcu_state.expedited_sequence,
515                         rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]);
516                 if (ndetected) {
517                         pr_err("blocking rcu_node structures:");
518                         rcu_for_each_node_breadth_first(rnp) {
519                                 if (rnp == rnp_root)
520                                         continue; /* printed unconditionally */
521                                 if (sync_rcu_preempt_exp_done_unlocked(rnp))
522                                         continue;
523                                 pr_cont(" l=%u:%d-%d:%#lx/%c",
524                                         rnp->level, rnp->grplo, rnp->grphi,
525                                         rnp->expmask,
526                                         ".T"[!!rnp->exp_tasks]);
527                         }
528                         pr_cont("\n");
529                 }
530                 rcu_for_each_leaf_node(rnp) {
531                         for_each_leaf_node_possible_cpu(rnp, cpu) {
532                                 mask = leaf_node_cpu_bit(rnp, cpu);
533                                 if (!(rnp->expmask & mask))
534                                         continue;
535                                 dump_cpu_task(cpu);
536                         }
537                 }
538                 jiffies_stall = 3 * rcu_jiffies_till_stall_check() + 3;
539         }
540 }
541 
542 /*
543  * Wait for the current expedited grace period to complete, and then
544  * wake up everyone who piggybacked on the just-completed expedited
545  * grace period.  Also update all the ->exp_seq_rq counters as needed
546  * in order to avoid counter-wrap problems.
547  */
548 static void rcu_exp_wait_wake(unsigned long s)
549 {
550         struct rcu_node *rnp;
551 
552         synchronize_sched_expedited_wait();
553         rcu_exp_gp_seq_end();
554         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));
555 
556         /*
557          * Switch over to wakeup mode, allowing the next GP, but -only- the
558          * next GP, to proceed.
559          */
560         mutex_lock(&rcu_state.exp_wake_mutex);
561 
562         rcu_for_each_node_breadth_first(rnp) {
563                 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
564                         spin_lock(&rnp->exp_lock);
565                         /* Recheck, avoid hang in case someone just arrived. */
566                         if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
567                                 rnp->exp_seq_rq = s;
568                         spin_unlock(&rnp->exp_lock);
569                 }
570                 smp_mb(); /* All above changes before wakeup. */
571                 wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rcu_state.expedited_sequence) & 0x3]);
572         }
573         trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake"));
574         mutex_unlock(&rcu_state.exp_wake_mutex);
575 }
576 
577 /*
578  * Common code to drive an expedited grace period forward, used by
579  * workqueues and mid-boot-time tasks.
580  */
581 static void rcu_exp_sel_wait_wake(smp_call_func_t func, unsigned long s)
582 {
583         /* Initialize the rcu_node tree in preparation for the wait. */
584         sync_rcu_exp_select_cpus(func);
585 
586         /* Wait and clean up, including waking everyone. */
587         rcu_exp_wait_wake(s);
588 }
589 
590 /*
591  * Work-queue handler to drive an expedited grace period forward.
592  */
593 static void wait_rcu_exp_gp(struct work_struct *wp)
594 {
595         struct rcu_exp_work *rewp;
596 
597         rewp = container_of(wp, struct rcu_exp_work, rew_work);
598         rcu_exp_sel_wait_wake(rewp->rew_func, rewp->rew_s);
599 }
600 
601 /*
602  * Given a smp_call_function() handler, kick off the specified
603  * implementation of expedited grace period.
604  */
605 static void _synchronize_rcu_expedited(smp_call_func_t func)
606 {
607         struct rcu_data *rdp;
608         struct rcu_exp_work rew;
609         struct rcu_node *rnp;
610         unsigned long s;
611 
612         /* If expedited grace periods are prohibited, fall back to normal. */
613         if (rcu_gp_is_normal()) {
614                 wait_rcu_gp(call_rcu);
615                 return;
616         }
617 
618         /* Take a snapshot of the sequence number.  */
619         s = rcu_exp_gp_seq_snap();
620         if (exp_funnel_lock(s))
621                 return;  /* Someone else did our work for us. */
622 
623         /* Ensure that load happens before action based on it. */
624         if (unlikely(rcu_scheduler_active == RCU_SCHEDULER_INIT)) {
625                 /* Direct call during scheduler init and early_initcalls(). */
626                 rcu_exp_sel_wait_wake(func, s);
627         } else {
628                 /* Marshall arguments & schedule the expedited grace period. */
629                 rew.rew_func = func;
630                 rew.rew_s = s;
631                 INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp);
632                 queue_work(rcu_gp_wq, &rew.rew_work);
633         }
634 
635         /* Wait for expedited grace period to complete. */
636         rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
637         rnp = rcu_get_root();
638         wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
639                    sync_exp_work_done(s));
640         smp_mb(); /* Workqueue actions happen before return. */
641 
642         /* Let the next expedited grace period start. */
643         mutex_unlock(&rcu_state.exp_mutex);
644 }
645 
646 #ifdef CONFIG_PREEMPT_RCU
647 
648 /*
649  * Remote handler for smp_call_function_single().  If there is an
650  * RCU read-side critical section in effect, request that the
651  * next rcu_read_unlock() record the quiescent state up the
652  * ->expmask fields in the rcu_node tree.  Otherwise, immediately
653  * report the quiescent state.
654  */
655 static void sync_rcu_exp_handler(void *unused)
656 {
657         unsigned long flags;
658         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
659         struct rcu_node *rnp = rdp->mynode;
660         struct task_struct *t = current;
661 
662         /*
663          * First, the common case of not being in an RCU read-side
664          * critical section.  If also enabled or idle, immediately
665          * report the quiescent state, otherwise defer.
666          */
667         if (!t->rcu_read_lock_nesting) {
668                 if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
669                     rcu_dynticks_curr_cpu_in_eqs()) {
670                         rcu_report_exp_rdp(rdp);
671                 } else {
672                         rdp->deferred_qs = true;
673                         set_tsk_need_resched(t);
674                         set_preempt_need_resched();
675                 }
676                 return;
677         }
678 
679         /*
680          * Second, the less-common case of being in an RCU read-side
681          * critical section.  In this case we can count on a future
682          * rcu_read_unlock().  However, this rcu_read_unlock() might
683          * execute on some other CPU, but in that case there will be
684          * a future context switch.  Either way, if the expedited
685          * grace period is still waiting on this CPU, set ->deferred_qs
686          * so that the eventual quiescent state will be reported.
687          * Note that there is a large group of race conditions that
688          * can have caused this quiescent state to already have been
689          * reported, so we really do need to check ->expmask.
690          */
691         if (t->rcu_read_lock_nesting > 0) {
692                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
693                 if (rnp->expmask & rdp->grpmask)
694                         rdp->deferred_qs = true;
695                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
696         }
697 
698         /*
699          * The final and least likely case is where the interrupted
700          * code was just about to or just finished exiting the RCU-preempt
701          * read-side critical section, and no, we can't tell which.
702          * So either way, set ->deferred_qs to flag later code that
703          * a quiescent state is required.
704          *
705          * If the CPU is fully enabled (or if some buggy RCU-preempt
706          * read-side critical section is being used from idle), just
707          * invoke rcu_preempt_defer_qs() to immediately report the
708          * quiescent state.  We cannot use rcu_read_unlock_special()
709          * because we are in an interrupt handler, which will cause that
710          * function to take an early exit without doing anything.
711          *
712          * Otherwise, force a context switch after the CPU enables everything.
713          */
714         rdp->deferred_qs = true;
715         if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
716             WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs())) {
717                 rcu_preempt_deferred_qs(t);
718         } else {
719                 set_tsk_need_resched(t);
720                 set_preempt_need_resched();
721         }
722 }
723 
724 /* PREEMPT=y, so no PREEMPT=n expedited grace period to clean up after. */
725 static void sync_sched_exp_online_cleanup(int cpu)
726 {
727 }
728 
729 /**
730  * synchronize_rcu_expedited - Brute-force RCU grace period
731  *
732  * Wait for an RCU-preempt grace period, but expedite it.  The basic
733  * idea is to IPI all non-idle non-nohz online CPUs.  The IPI handler
734  * checks whether the CPU is in an RCU-preempt critical section, and
735  * if so, it sets a flag that causes the outermost rcu_read_unlock()
736  * to report the quiescent state.  On the other hand, if the CPU is
737  * not in an RCU read-side critical section, the IPI handler reports
738  * the quiescent state immediately.
739  *
740  * Although this is a greate improvement over previous expedited
741  * implementations, it is still unfriendly to real-time workloads, so is
742  * thus not recommended for any sort of common-case code.  In fact, if
743  * you are using synchronize_rcu_expedited() in a loop, please restructure
744  * your code to batch your updates, and then Use a single synchronize_rcu()
745  * instead.
746  *
747  * This has the same semantics as (but is more brutal than) synchronize_rcu().
748  */
749 void synchronize_rcu_expedited(void)
750 {
751         RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
752                          lock_is_held(&rcu_lock_map) ||
753                          lock_is_held(&rcu_sched_lock_map),
754                          "Illegal synchronize_rcu_expedited() in RCU read-side critical section");
755 
756         if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
757                 return;
758         _synchronize_rcu_expedited(sync_rcu_exp_handler);
759 }
760 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
761 
762 #else /* #ifdef CONFIG_PREEMPT_RCU */
763 
764 /* Invoked on each online non-idle CPU for expedited quiescent state. */
765 static void sync_sched_exp_handler(void *unused)
766 {
767         struct rcu_data *rdp;
768         struct rcu_node *rnp;
769 
770         rdp = this_cpu_ptr(&rcu_data);
771         rnp = rdp->mynode;
772         if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
773             __this_cpu_read(rcu_data.cpu_no_qs.b.exp))
774                 return;
775         if (rcu_is_cpu_rrupt_from_idle()) {
776                 rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
777                 return;
778         }
779         __this_cpu_write(rcu_data.cpu_no_qs.b.exp, true);
780         /* Store .exp before .rcu_urgent_qs. */
781         smp_store_release(this_cpu_ptr(&rcu_data.rcu_urgent_qs), true);
782         set_tsk_need_resched(current);
783         set_preempt_need_resched();
784 }
785 
786 /* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */
787 static void sync_sched_exp_online_cleanup(int cpu)
788 {
789         struct rcu_data *rdp;
790         int ret;
791         struct rcu_node *rnp;
792 
793         rdp = per_cpu_ptr(&rcu_data, cpu);
794         rnp = rdp->mynode;
795         if (!(READ_ONCE(rnp->expmask) & rdp->grpmask))
796                 return;
797         ret = smp_call_function_single(cpu, sync_sched_exp_handler, NULL, 0);
798         WARN_ON_ONCE(ret);
799 }
800 
801 /*
802  * Because a context switch is a grace period for !PREEMPT, any
803  * blocking grace-period wait automatically implies a grace period if
804  * there is only one CPU online at any point time during execution of
805  * either synchronize_rcu() or synchronize_rcu_expedited().  It is OK to
806  * occasionally incorrectly indicate that there are multiple CPUs online
807  * when there was in fact only one the whole time, as this just adds some
808  * overhead: RCU still operates correctly.
809  */
810 static int rcu_blocking_is_gp(void)
811 {
812         int ret;
813 
814         might_sleep();  /* Check for RCU read-side critical section. */
815         preempt_disable();
816         ret = num_online_cpus() <= 1;
817         preempt_enable();
818         return ret;
819 }
820 
821 /* PREEMPT=n implementation of synchronize_rcu_expedited(). */
822 void synchronize_rcu_expedited(void)
823 {
824         RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
825                          lock_is_held(&rcu_lock_map) ||
826                          lock_is_held(&rcu_sched_lock_map),
827                          "Illegal synchronize_rcu_expedited() in RCU read-side critical section");
828 
829         /* If only one CPU, this is automatically a grace period. */
830         if (rcu_blocking_is_gp())
831                 return;
832 
833         _synchronize_rcu_expedited(sync_sched_exp_handler);
834 }
835 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
836 
837 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
838 

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