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Linux/kernel/rcu/update.c

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  1 /*
  2  * Read-Copy Update mechanism for mutual exclusion
  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, 2001
 19  *
 20  * Authors: Dipankar Sarma <dipankar@in.ibm.com>
 21  *          Manfred Spraul <manfred@colorfullife.com>
 22  *
 23  * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
 24  * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
 25  * Papers:
 26  * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
 27  * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
 28  *
 29  * For detailed explanation of Read-Copy Update mechanism see -
 30  *              http://lse.sourceforge.net/locking/rcupdate.html
 31  *
 32  */
 33 #include <linux/types.h>
 34 #include <linux/kernel.h>
 35 #include <linux/init.h>
 36 #include <linux/spinlock.h>
 37 #include <linux/smp.h>
 38 #include <linux/interrupt.h>
 39 #include <linux/sched.h>
 40 #include <linux/atomic.h>
 41 #include <linux/bitops.h>
 42 #include <linux/percpu.h>
 43 #include <linux/notifier.h>
 44 #include <linux/cpu.h>
 45 #include <linux/mutex.h>
 46 #include <linux/export.h>
 47 #include <linux/hardirq.h>
 48 #include <linux/delay.h>
 49 #include <linux/module.h>
 50 #include <linux/kthread.h>
 51 #include <linux/tick.h>
 52 
 53 #define CREATE_TRACE_POINTS
 54 
 55 #include "rcu.h"
 56 
 57 MODULE_ALIAS("rcupdate");
 58 #ifdef MODULE_PARAM_PREFIX
 59 #undef MODULE_PARAM_PREFIX
 60 #endif
 61 #define MODULE_PARAM_PREFIX "rcupdate."
 62 
 63 module_param(rcu_expedited, int, 0);
 64 
 65 #if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
 66 /**
 67  * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
 68  *
 69  * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
 70  * RCU-sched read-side critical section.  In absence of
 71  * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
 72  * critical section unless it can prove otherwise.  Note that disabling
 73  * of preemption (including disabling irqs) counts as an RCU-sched
 74  * read-side critical section.  This is useful for debug checks in functions
 75  * that required that they be called within an RCU-sched read-side
 76  * critical section.
 77  *
 78  * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
 79  * and while lockdep is disabled.
 80  *
 81  * Note that if the CPU is in the idle loop from an RCU point of
 82  * view (ie: that we are in the section between rcu_idle_enter() and
 83  * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU
 84  * did an rcu_read_lock().  The reason for this is that RCU ignores CPUs
 85  * that are in such a section, considering these as in extended quiescent
 86  * state, so such a CPU is effectively never in an RCU read-side critical
 87  * section regardless of what RCU primitives it invokes.  This state of
 88  * affairs is required --- we need to keep an RCU-free window in idle
 89  * where the CPU may possibly enter into low power mode. This way we can
 90  * notice an extended quiescent state to other CPUs that started a grace
 91  * period. Otherwise we would delay any grace period as long as we run in
 92  * the idle task.
 93  *
 94  * Similarly, we avoid claiming an SRCU read lock held if the current
 95  * CPU is offline.
 96  */
 97 int rcu_read_lock_sched_held(void)
 98 {
 99         int lockdep_opinion = 0;
100 
101         if (!debug_lockdep_rcu_enabled())
102                 return 1;
103         if (!rcu_is_watching())
104                 return 0;
105         if (!rcu_lockdep_current_cpu_online())
106                 return 0;
107         if (debug_locks)
108                 lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
109         return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
110 }
111 EXPORT_SYMBOL(rcu_read_lock_sched_held);
112 #endif
113 
114 #ifndef CONFIG_TINY_RCU
115 
116 static atomic_t rcu_expedited_nesting =
117         ATOMIC_INIT(IS_ENABLED(CONFIG_RCU_EXPEDITE_BOOT) ? 1 : 0);
118 
119 /*
120  * Should normal grace-period primitives be expedited?  Intended for
121  * use within RCU.  Note that this function takes the rcu_expedited
122  * sysfs/boot variable into account as well as the rcu_expedite_gp()
123  * nesting.  So looping on rcu_unexpedite_gp() until rcu_gp_is_expedited()
124  * returns false is a -really- bad idea.
125  */
126 bool rcu_gp_is_expedited(void)
127 {
128         return rcu_expedited || atomic_read(&rcu_expedited_nesting);
129 }
130 EXPORT_SYMBOL_GPL(rcu_gp_is_expedited);
131 
132 /**
133  * rcu_expedite_gp - Expedite future RCU grace periods
134  *
135  * After a call to this function, future calls to synchronize_rcu() and
136  * friends act as the corresponding synchronize_rcu_expedited() function
137  * had instead been called.
138  */
139 void rcu_expedite_gp(void)
140 {
141         atomic_inc(&rcu_expedited_nesting);
142 }
143 EXPORT_SYMBOL_GPL(rcu_expedite_gp);
144 
145 /**
146  * rcu_unexpedite_gp - Cancel prior rcu_expedite_gp() invocation
147  *
148  * Undo a prior call to rcu_expedite_gp().  If all prior calls to
149  * rcu_expedite_gp() are undone by a subsequent call to rcu_unexpedite_gp(),
150  * and if the rcu_expedited sysfs/boot parameter is not set, then all
151  * subsequent calls to synchronize_rcu() and friends will return to
152  * their normal non-expedited behavior.
153  */
154 void rcu_unexpedite_gp(void)
155 {
156         atomic_dec(&rcu_expedited_nesting);
157 }
158 EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
159 
160 #endif /* #ifndef CONFIG_TINY_RCU */
161 
162 /*
163  * Inform RCU of the end of the in-kernel boot sequence.
164  */
165 void rcu_end_inkernel_boot(void)
166 {
167         if (IS_ENABLED(CONFIG_RCU_EXPEDITE_BOOT))
168                 rcu_unexpedite_gp();
169 }
170 
171 #ifdef CONFIG_PREEMPT_RCU
172 
173 /*
174  * Preemptible RCU implementation for rcu_read_lock().
175  * Just increment ->rcu_read_lock_nesting, shared state will be updated
176  * if we block.
177  */
178 void __rcu_read_lock(void)
179 {
180         current->rcu_read_lock_nesting++;
181         barrier();  /* critical section after entry code. */
182 }
183 EXPORT_SYMBOL_GPL(__rcu_read_lock);
184 
185 /*
186  * Preemptible RCU implementation for rcu_read_unlock().
187  * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
188  * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
189  * invoke rcu_read_unlock_special() to clean up after a context switch
190  * in an RCU read-side critical section and other special cases.
191  */
192 void __rcu_read_unlock(void)
193 {
194         struct task_struct *t = current;
195 
196         if (t->rcu_read_lock_nesting != 1) {
197                 --t->rcu_read_lock_nesting;
198         } else {
199                 barrier();  /* critical section before exit code. */
200                 t->rcu_read_lock_nesting = INT_MIN;
201                 barrier();  /* assign before ->rcu_read_unlock_special load */
202                 if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
203                         rcu_read_unlock_special(t);
204                 barrier();  /* ->rcu_read_unlock_special load before assign */
205                 t->rcu_read_lock_nesting = 0;
206         }
207 #ifdef CONFIG_PROVE_LOCKING
208         {
209                 int rrln = READ_ONCE(t->rcu_read_lock_nesting);
210 
211                 WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2);
212         }
213 #endif /* #ifdef CONFIG_PROVE_LOCKING */
214 }
215 EXPORT_SYMBOL_GPL(__rcu_read_unlock);
216 
217 #endif /* #ifdef CONFIG_PREEMPT_RCU */
218 
219 #ifdef CONFIG_DEBUG_LOCK_ALLOC
220 static struct lock_class_key rcu_lock_key;
221 struct lockdep_map rcu_lock_map =
222         STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
223 EXPORT_SYMBOL_GPL(rcu_lock_map);
224 
225 static struct lock_class_key rcu_bh_lock_key;
226 struct lockdep_map rcu_bh_lock_map =
227         STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key);
228 EXPORT_SYMBOL_GPL(rcu_bh_lock_map);
229 
230 static struct lock_class_key rcu_sched_lock_key;
231 struct lockdep_map rcu_sched_lock_map =
232         STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key);
233 EXPORT_SYMBOL_GPL(rcu_sched_lock_map);
234 
235 static struct lock_class_key rcu_callback_key;
236 struct lockdep_map rcu_callback_map =
237         STATIC_LOCKDEP_MAP_INIT("rcu_callback", &rcu_callback_key);
238 EXPORT_SYMBOL_GPL(rcu_callback_map);
239 
240 int notrace debug_lockdep_rcu_enabled(void)
241 {
242         return rcu_scheduler_active && debug_locks &&
243                current->lockdep_recursion == 0;
244 }
245 EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);
246 
247 /**
248  * rcu_read_lock_held() - might we be in RCU read-side critical section?
249  *
250  * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU
251  * read-side critical section.  In absence of CONFIG_DEBUG_LOCK_ALLOC,
252  * this assumes we are in an RCU read-side critical section unless it can
253  * prove otherwise.  This is useful for debug checks in functions that
254  * require that they be called within an RCU read-side critical section.
255  *
256  * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
257  * and while lockdep is disabled.
258  *
259  * Note that rcu_read_lock() and the matching rcu_read_unlock() must
260  * occur in the same context, for example, it is illegal to invoke
261  * rcu_read_unlock() in process context if the matching rcu_read_lock()
262  * was invoked from within an irq handler.
263  *
264  * Note that rcu_read_lock() is disallowed if the CPU is either idle or
265  * offline from an RCU perspective, so check for those as well.
266  */
267 int rcu_read_lock_held(void)
268 {
269         if (!debug_lockdep_rcu_enabled())
270                 return 1;
271         if (!rcu_is_watching())
272                 return 0;
273         if (!rcu_lockdep_current_cpu_online())
274                 return 0;
275         return lock_is_held(&rcu_lock_map);
276 }
277 EXPORT_SYMBOL_GPL(rcu_read_lock_held);
278 
279 /**
280  * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section?
281  *
282  * Check for bottom half being disabled, which covers both the
283  * CONFIG_PROVE_RCU and not cases.  Note that if someone uses
284  * rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled)
285  * will show the situation.  This is useful for debug checks in functions
286  * that require that they be called within an RCU read-side critical
287  * section.
288  *
289  * Check debug_lockdep_rcu_enabled() to prevent false positives during boot.
290  *
291  * Note that rcu_read_lock() is disallowed if the CPU is either idle or
292  * offline from an RCU perspective, so check for those as well.
293  */
294 int rcu_read_lock_bh_held(void)
295 {
296         if (!debug_lockdep_rcu_enabled())
297                 return 1;
298         if (!rcu_is_watching())
299                 return 0;
300         if (!rcu_lockdep_current_cpu_online())
301                 return 0;
302         return in_softirq() || irqs_disabled();
303 }
304 EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
305 
306 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
307 
308 /**
309  * wakeme_after_rcu() - Callback function to awaken a task after grace period
310  * @head: Pointer to rcu_head member within rcu_synchronize structure
311  *
312  * Awaken the corresponding task now that a grace period has elapsed.
313  */
314 void wakeme_after_rcu(struct rcu_head *head)
315 {
316         struct rcu_synchronize *rcu;
317 
318         rcu = container_of(head, struct rcu_synchronize, head);
319         complete(&rcu->completion);
320 }
321 EXPORT_SYMBOL_GPL(wakeme_after_rcu);
322 
323 void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
324                    struct rcu_synchronize *rs_array)
325 {
326         int i;
327 
328         /* Initialize and register callbacks for each flavor specified. */
329         for (i = 0; i < n; i++) {
330                 if (checktiny &&
331                     (crcu_array[i] == call_rcu ||
332                      crcu_array[i] == call_rcu_bh)) {
333                         might_sleep();
334                         continue;
335                 }
336                 init_rcu_head_on_stack(&rs_array[i].head);
337                 init_completion(&rs_array[i].completion);
338                 (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
339         }
340 
341         /* Wait for all callbacks to be invoked. */
342         for (i = 0; i < n; i++) {
343                 if (checktiny &&
344                     (crcu_array[i] == call_rcu ||
345                      crcu_array[i] == call_rcu_bh))
346                         continue;
347                 wait_for_completion(&rs_array[i].completion);
348                 destroy_rcu_head_on_stack(&rs_array[i].head);
349         }
350 }
351 EXPORT_SYMBOL_GPL(__wait_rcu_gp);
352 
353 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
354 void init_rcu_head(struct rcu_head *head)
355 {
356         debug_object_init(head, &rcuhead_debug_descr);
357 }
358 
359 void destroy_rcu_head(struct rcu_head *head)
360 {
361         debug_object_free(head, &rcuhead_debug_descr);
362 }
363 
364 /*
365  * fixup_activate is called when:
366  * - an active object is activated
367  * - an unknown object is activated (might be a statically initialized object)
368  * Activation is performed internally by call_rcu().
369  */
370 static int rcuhead_fixup_activate(void *addr, enum debug_obj_state state)
371 {
372         struct rcu_head *head = addr;
373 
374         switch (state) {
375 
376         case ODEBUG_STATE_NOTAVAILABLE:
377                 /*
378                  * This is not really a fixup. We just make sure that it is
379                  * tracked in the object tracker.
380                  */
381                 debug_object_init(head, &rcuhead_debug_descr);
382                 debug_object_activate(head, &rcuhead_debug_descr);
383                 return 0;
384         default:
385                 return 1;
386         }
387 }
388 
389 /**
390  * init_rcu_head_on_stack() - initialize on-stack rcu_head for debugobjects
391  * @head: pointer to rcu_head structure to be initialized
392  *
393  * This function informs debugobjects of a new rcu_head structure that
394  * has been allocated as an auto variable on the stack.  This function
395  * is not required for rcu_head structures that are statically defined or
396  * that are dynamically allocated on the heap.  This function has no
397  * effect for !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
398  */
399 void init_rcu_head_on_stack(struct rcu_head *head)
400 {
401         debug_object_init_on_stack(head, &rcuhead_debug_descr);
402 }
403 EXPORT_SYMBOL_GPL(init_rcu_head_on_stack);
404 
405 /**
406  * destroy_rcu_head_on_stack() - destroy on-stack rcu_head for debugobjects
407  * @head: pointer to rcu_head structure to be initialized
408  *
409  * This function informs debugobjects that an on-stack rcu_head structure
410  * is about to go out of scope.  As with init_rcu_head_on_stack(), this
411  * function is not required for rcu_head structures that are statically
412  * defined or that are dynamically allocated on the heap.  Also as with
413  * init_rcu_head_on_stack(), this function has no effect for
414  * !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
415  */
416 void destroy_rcu_head_on_stack(struct rcu_head *head)
417 {
418         debug_object_free(head, &rcuhead_debug_descr);
419 }
420 EXPORT_SYMBOL_GPL(destroy_rcu_head_on_stack);
421 
422 struct debug_obj_descr rcuhead_debug_descr = {
423         .name = "rcu_head",
424         .fixup_activate = rcuhead_fixup_activate,
425 };
426 EXPORT_SYMBOL_GPL(rcuhead_debug_descr);
427 #endif /* #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */
428 
429 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU) || defined(CONFIG_RCU_TRACE)
430 void do_trace_rcu_torture_read(const char *rcutorturename, struct rcu_head *rhp,
431                                unsigned long secs,
432                                unsigned long c_old, unsigned long c)
433 {
434         trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c);
435 }
436 EXPORT_SYMBOL_GPL(do_trace_rcu_torture_read);
437 #else
438 #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \
439         do { } while (0)
440 #endif
441 
442 #ifdef CONFIG_RCU_STALL_COMMON
443 
444 #ifdef CONFIG_PROVE_RCU
445 #define RCU_STALL_DELAY_DELTA          (5 * HZ)
446 #else
447 #define RCU_STALL_DELAY_DELTA          0
448 #endif
449 
450 int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */
451 static int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
452 
453 module_param(rcu_cpu_stall_suppress, int, 0644);
454 module_param(rcu_cpu_stall_timeout, int, 0644);
455 
456 int rcu_jiffies_till_stall_check(void)
457 {
458         int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
459 
460         /*
461          * Limit check must be consistent with the Kconfig limits
462          * for CONFIG_RCU_CPU_STALL_TIMEOUT.
463          */
464         if (till_stall_check < 3) {
465                 WRITE_ONCE(rcu_cpu_stall_timeout, 3);
466                 till_stall_check = 3;
467         } else if (till_stall_check > 300) {
468                 WRITE_ONCE(rcu_cpu_stall_timeout, 300);
469                 till_stall_check = 300;
470         }
471         return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
472 }
473 
474 void rcu_sysrq_start(void)
475 {
476         if (!rcu_cpu_stall_suppress)
477                 rcu_cpu_stall_suppress = 2;
478 }
479 
480 void rcu_sysrq_end(void)
481 {
482         if (rcu_cpu_stall_suppress == 2)
483                 rcu_cpu_stall_suppress = 0;
484 }
485 
486 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
487 {
488         rcu_cpu_stall_suppress = 1;
489         return NOTIFY_DONE;
490 }
491 
492 static struct notifier_block rcu_panic_block = {
493         .notifier_call = rcu_panic,
494 };
495 
496 static int __init check_cpu_stall_init(void)
497 {
498         atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
499         return 0;
500 }
501 early_initcall(check_cpu_stall_init);
502 
503 #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
504 
505 #ifdef CONFIG_TASKS_RCU
506 
507 /*
508  * Simple variant of RCU whose quiescent states are voluntary context switch,
509  * user-space execution, and idle.  As such, grace periods can take one good
510  * long time.  There are no read-side primitives similar to rcu_read_lock()
511  * and rcu_read_unlock() because this implementation is intended to get
512  * the system into a safe state for some of the manipulations involved in
513  * tracing and the like.  Finally, this implementation does not support
514  * high call_rcu_tasks() rates from multiple CPUs.  If this is required,
515  * per-CPU callback lists will be needed.
516  */
517 
518 /* Global list of callbacks and associated lock. */
519 static struct rcu_head *rcu_tasks_cbs_head;
520 static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
521 static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq);
522 static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);
523 
524 /* Track exiting tasks in order to allow them to be waited for. */
525 DEFINE_SRCU(tasks_rcu_exit_srcu);
526 
527 /* Control stall timeouts.  Disable with <= 0, otherwise jiffies till stall. */
528 static int rcu_task_stall_timeout __read_mostly = HZ * 60 * 10;
529 module_param(rcu_task_stall_timeout, int, 0644);
530 
531 static void rcu_spawn_tasks_kthread(void);
532 
533 /*
534  * Post an RCU-tasks callback.  First call must be from process context
535  * after the scheduler if fully operational.
536  */
537 void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
538 {
539         unsigned long flags;
540         bool needwake;
541 
542         rhp->next = NULL;
543         rhp->func = func;
544         raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
545         needwake = !rcu_tasks_cbs_head;
546         *rcu_tasks_cbs_tail = rhp;
547         rcu_tasks_cbs_tail = &rhp->next;
548         raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
549         if (needwake) {
550                 rcu_spawn_tasks_kthread();
551                 wake_up(&rcu_tasks_cbs_wq);
552         }
553 }
554 EXPORT_SYMBOL_GPL(call_rcu_tasks);
555 
556 /**
557  * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
558  *
559  * Control will return to the caller some time after a full rcu-tasks
560  * grace period has elapsed, in other words after all currently
561  * executing rcu-tasks read-side critical sections have elapsed.  These
562  * read-side critical sections are delimited by calls to schedule(),
563  * cond_resched_rcu_qs(), idle execution, userspace execution, calls
564  * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
565  *
566  * This is a very specialized primitive, intended only for a few uses in
567  * tracing and other situations requiring manipulation of function
568  * preambles and profiling hooks.  The synchronize_rcu_tasks() function
569  * is not (yet) intended for heavy use from multiple CPUs.
570  *
571  * Note that this guarantee implies further memory-ordering guarantees.
572  * On systems with more than one CPU, when synchronize_rcu_tasks() returns,
573  * each CPU is guaranteed to have executed a full memory barrier since the
574  * end of its last RCU-tasks read-side critical section whose beginning
575  * preceded the call to synchronize_rcu_tasks().  In addition, each CPU
576  * having an RCU-tasks read-side critical section that extends beyond
577  * the return from synchronize_rcu_tasks() is guaranteed to have executed
578  * a full memory barrier after the beginning of synchronize_rcu_tasks()
579  * and before the beginning of that RCU-tasks read-side critical section.
580  * Note that these guarantees include CPUs that are offline, idle, or
581  * executing in user mode, as well as CPUs that are executing in the kernel.
582  *
583  * Furthermore, if CPU A invoked synchronize_rcu_tasks(), which returned
584  * to its caller on CPU B, then both CPU A and CPU B are guaranteed
585  * to have executed a full memory barrier during the execution of
586  * synchronize_rcu_tasks() -- even if CPU A and CPU B are the same CPU
587  * (but again only if the system has more than one CPU).
588  */
589 void synchronize_rcu_tasks(void)
590 {
591         /* Complain if the scheduler has not started.  */
592         RCU_LOCKDEP_WARN(!rcu_scheduler_active,
593                          "synchronize_rcu_tasks called too soon");
594 
595         /* Wait for the grace period. */
596         wait_rcu_gp(call_rcu_tasks);
597 }
598 EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);
599 
600 /**
601  * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
602  *
603  * Although the current implementation is guaranteed to wait, it is not
604  * obligated to, for example, if there are no pending callbacks.
605  */
606 void rcu_barrier_tasks(void)
607 {
608         /* There is only one callback queue, so this is easy.  ;-) */
609         synchronize_rcu_tasks();
610 }
611 EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
612 
613 /* See if tasks are still holding out, complain if so. */
614 static void check_holdout_task(struct task_struct *t,
615                                bool needreport, bool *firstreport)
616 {
617         int cpu;
618 
619         if (!READ_ONCE(t->rcu_tasks_holdout) ||
620             t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
621             !READ_ONCE(t->on_rq) ||
622             (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
623              !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
624                 WRITE_ONCE(t->rcu_tasks_holdout, false);
625                 list_del_init(&t->rcu_tasks_holdout_list);
626                 put_task_struct(t);
627                 return;
628         }
629         if (!needreport)
630                 return;
631         if (*firstreport) {
632                 pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
633                 *firstreport = false;
634         }
635         cpu = task_cpu(t);
636         pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n",
637                  t, ".I"[is_idle_task(t)],
638                  "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
639                  t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
640                  t->rcu_tasks_idle_cpu, cpu);
641         sched_show_task(t);
642 }
643 
644 /* RCU-tasks kthread that detects grace periods and invokes callbacks. */
645 static int __noreturn rcu_tasks_kthread(void *arg)
646 {
647         unsigned long flags;
648         struct task_struct *g, *t;
649         unsigned long lastreport;
650         struct rcu_head *list;
651         struct rcu_head *next;
652         LIST_HEAD(rcu_tasks_holdouts);
653 
654         /* Run on housekeeping CPUs by default.  Sysadm can move if desired. */
655         housekeeping_affine(current);
656 
657         /*
658          * Each pass through the following loop makes one check for
659          * newly arrived callbacks, and, if there are some, waits for
660          * one RCU-tasks grace period and then invokes the callbacks.
661          * This loop is terminated by the system going down.  ;-)
662          */
663         for (;;) {
664 
665                 /* Pick up any new callbacks. */
666                 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
667                 list = rcu_tasks_cbs_head;
668                 rcu_tasks_cbs_head = NULL;
669                 rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
670                 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
671 
672                 /* If there were none, wait a bit and start over. */
673                 if (!list) {
674                         wait_event_interruptible(rcu_tasks_cbs_wq,
675                                                  rcu_tasks_cbs_head);
676                         if (!rcu_tasks_cbs_head) {
677                                 WARN_ON(signal_pending(current));
678                                 schedule_timeout_interruptible(HZ/10);
679                         }
680                         continue;
681                 }
682 
683                 /*
684                  * Wait for all pre-existing t->on_rq and t->nvcsw
685                  * transitions to complete.  Invoking synchronize_sched()
686                  * suffices because all these transitions occur with
687                  * interrupts disabled.  Without this synchronize_sched(),
688                  * a read-side critical section that started before the
689                  * grace period might be incorrectly seen as having started
690                  * after the grace period.
691                  *
692                  * This synchronize_sched() also dispenses with the
693                  * need for a memory barrier on the first store to
694                  * ->rcu_tasks_holdout, as it forces the store to happen
695                  * after the beginning of the grace period.
696                  */
697                 synchronize_sched();
698 
699                 /*
700                  * There were callbacks, so we need to wait for an
701                  * RCU-tasks grace period.  Start off by scanning
702                  * the task list for tasks that are not already
703                  * voluntarily blocked.  Mark these tasks and make
704                  * a list of them in rcu_tasks_holdouts.
705                  */
706                 rcu_read_lock();
707                 for_each_process_thread(g, t) {
708                         if (t != current && READ_ONCE(t->on_rq) &&
709                             !is_idle_task(t)) {
710                                 get_task_struct(t);
711                                 t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
712                                 WRITE_ONCE(t->rcu_tasks_holdout, true);
713                                 list_add(&t->rcu_tasks_holdout_list,
714                                          &rcu_tasks_holdouts);
715                         }
716                 }
717                 rcu_read_unlock();
718 
719                 /*
720                  * Wait for tasks that are in the process of exiting.
721                  * This does only part of the job, ensuring that all
722                  * tasks that were previously exiting reach the point
723                  * where they have disabled preemption, allowing the
724                  * later synchronize_sched() to finish the job.
725                  */
726                 synchronize_srcu(&tasks_rcu_exit_srcu);
727 
728                 /*
729                  * Each pass through the following loop scans the list
730                  * of holdout tasks, removing any that are no longer
731                  * holdouts.  When the list is empty, we are done.
732                  */
733                 lastreport = jiffies;
734                 while (!list_empty(&rcu_tasks_holdouts)) {
735                         bool firstreport;
736                         bool needreport;
737                         int rtst;
738                         struct task_struct *t1;
739 
740                         schedule_timeout_interruptible(HZ);
741                         rtst = READ_ONCE(rcu_task_stall_timeout);
742                         needreport = rtst > 0 &&
743                                      time_after(jiffies, lastreport + rtst);
744                         if (needreport)
745                                 lastreport = jiffies;
746                         firstreport = true;
747                         WARN_ON(signal_pending(current));
748                         list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts,
749                                                 rcu_tasks_holdout_list) {
750                                 check_holdout_task(t, needreport, &firstreport);
751                                 cond_resched();
752                         }
753                 }
754 
755                 /*
756                  * Because ->on_rq and ->nvcsw are not guaranteed
757                  * to have a full memory barriers prior to them in the
758                  * schedule() path, memory reordering on other CPUs could
759                  * cause their RCU-tasks read-side critical sections to
760                  * extend past the end of the grace period.  However,
761                  * because these ->nvcsw updates are carried out with
762                  * interrupts disabled, we can use synchronize_sched()
763                  * to force the needed ordering on all such CPUs.
764                  *
765                  * This synchronize_sched() also confines all
766                  * ->rcu_tasks_holdout accesses to be within the grace
767                  * period, avoiding the need for memory barriers for
768                  * ->rcu_tasks_holdout accesses.
769                  *
770                  * In addition, this synchronize_sched() waits for exiting
771                  * tasks to complete their final preempt_disable() region
772                  * of execution, cleaning up after the synchronize_srcu()
773                  * above.
774                  */
775                 synchronize_sched();
776 
777                 /* Invoke the callbacks. */
778                 while (list) {
779                         next = list->next;
780                         local_bh_disable();
781                         list->func(list);
782                         local_bh_enable();
783                         list = next;
784                         cond_resched();
785                 }
786                 schedule_timeout_uninterruptible(HZ/10);
787         }
788 }
789 
790 /* Spawn rcu_tasks_kthread() at first call to call_rcu_tasks(). */
791 static void rcu_spawn_tasks_kthread(void)
792 {
793         static DEFINE_MUTEX(rcu_tasks_kthread_mutex);
794         static struct task_struct *rcu_tasks_kthread_ptr;
795         struct task_struct *t;
796 
797         if (READ_ONCE(rcu_tasks_kthread_ptr)) {
798                 smp_mb(); /* Ensure caller sees full kthread. */
799                 return;
800         }
801         mutex_lock(&rcu_tasks_kthread_mutex);
802         if (rcu_tasks_kthread_ptr) {
803                 mutex_unlock(&rcu_tasks_kthread_mutex);
804                 return;
805         }
806         t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread");
807         BUG_ON(IS_ERR(t));
808         smp_mb(); /* Ensure others see full kthread. */
809         WRITE_ONCE(rcu_tasks_kthread_ptr, t);
810         mutex_unlock(&rcu_tasks_kthread_mutex);
811 }
812 
813 #endif /* #ifdef CONFIG_TASKS_RCU */
814 
815 #ifdef CONFIG_PROVE_RCU
816 
817 /*
818  * Early boot self test parameters, one for each flavor
819  */
820 static bool rcu_self_test;
821 static bool rcu_self_test_bh;
822 static bool rcu_self_test_sched;
823 
824 module_param(rcu_self_test, bool, 0444);
825 module_param(rcu_self_test_bh, bool, 0444);
826 module_param(rcu_self_test_sched, bool, 0444);
827 
828 static int rcu_self_test_counter;
829 
830 static void test_callback(struct rcu_head *r)
831 {
832         rcu_self_test_counter++;
833         pr_info("RCU test callback executed %d\n", rcu_self_test_counter);
834 }
835 
836 static void early_boot_test_call_rcu(void)
837 {
838         static struct rcu_head head;
839 
840         call_rcu(&head, test_callback);
841 }
842 
843 static void early_boot_test_call_rcu_bh(void)
844 {
845         static struct rcu_head head;
846 
847         call_rcu_bh(&head, test_callback);
848 }
849 
850 static void early_boot_test_call_rcu_sched(void)
851 {
852         static struct rcu_head head;
853 
854         call_rcu_sched(&head, test_callback);
855 }
856 
857 void rcu_early_boot_tests(void)
858 {
859         pr_info("Running RCU self tests\n");
860 
861         if (rcu_self_test)
862                 early_boot_test_call_rcu();
863         if (rcu_self_test_bh)
864                 early_boot_test_call_rcu_bh();
865         if (rcu_self_test_sched)
866                 early_boot_test_call_rcu_sched();
867 }
868 
869 static int rcu_verify_early_boot_tests(void)
870 {
871         int ret = 0;
872         int early_boot_test_counter = 0;
873 
874         if (rcu_self_test) {
875                 early_boot_test_counter++;
876                 rcu_barrier();
877         }
878         if (rcu_self_test_bh) {
879                 early_boot_test_counter++;
880                 rcu_barrier_bh();
881         }
882         if (rcu_self_test_sched) {
883                 early_boot_test_counter++;
884                 rcu_barrier_sched();
885         }
886 
887         if (rcu_self_test_counter != early_boot_test_counter) {
888                 WARN_ON(1);
889                 ret = -1;
890         }
891 
892         return ret;
893 }
894 late_initcall(rcu_verify_early_boot_tests);
895 #else
896 void rcu_early_boot_tests(void) {}
897 #endif /* CONFIG_PROVE_RCU */
898 

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