2 * Read-Copy Update mechanism for mutual exclusion
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.
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.
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.
18 * Copyright IBM Corporation, 2001
20 * Authors: Dipankar Sarma <dipankar@in.ibm.com>
21 * Manfred Spraul <manfred@colorfullife.com>
23 * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
24 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
26 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
27 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
29 * For detailed explanation of Read-Copy Update mechanism see -
30 * http://lse.sourceforge.net/locking/rcupdate.html
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/signal.h>
40 #include <linux/sched/debug.h>
41 #include <linux/atomic.h>
42 #include <linux/bitops.h>
43 #include <linux/percpu.h>
44 #include <linux/notifier.h>
45 #include <linux/cpu.h>
46 #include <linux/mutex.h>
47 #include <linux/export.h>
48 #include <linux/hardirq.h>
49 #include <linux/delay.h>
50 #include <linux/moduleparam.h>
51 #include <linux/kthread.h>
52 #include <linux/tick.h>
53 #include <linux/rcupdate_wait.h>
55 #define CREATE_TRACE_POINTS
59 #ifdef MODULE_PARAM_PREFIX
60 #undef MODULE_PARAM_PREFIX
62 #define MODULE_PARAM_PREFIX "rcupdate."
64 #ifndef CONFIG_TINY_RCU
65 module_param(rcu_expedited, int, 0);
66 module_param(rcu_normal, int, 0);
67 static int rcu_normal_after_boot;
68 module_param(rcu_normal_after_boot, int, 0);
69 #endif /* #ifndef CONFIG_TINY_RCU */
71 #ifdef CONFIG_DEBUG_LOCK_ALLOC
73 * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
75 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
76 * RCU-sched read-side critical section. In absence of
77 * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
78 * critical section unless it can prove otherwise. Note that disabling
79 * of preemption (including disabling irqs) counts as an RCU-sched
80 * read-side critical section. This is useful for debug checks in functions
81 * that required that they be called within an RCU-sched read-side
84 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
85 * and while lockdep is disabled.
87 * Note that if the CPU is in the idle loop from an RCU point of
88 * view (ie: that we are in the section between rcu_idle_enter() and
89 * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU
90 * did an rcu_read_lock(). The reason for this is that RCU ignores CPUs
91 * that are in such a section, considering these as in extended quiescent
92 * state, so such a CPU is effectively never in an RCU read-side critical
93 * section regardless of what RCU primitives it invokes. This state of
94 * affairs is required --- we need to keep an RCU-free window in idle
95 * where the CPU may possibly enter into low power mode. This way we can
96 * notice an extended quiescent state to other CPUs that started a grace
97 * period. Otherwise we would delay any grace period as long as we run in
100 * Similarly, we avoid claiming an SRCU read lock held if the current
103 int rcu_read_lock_sched_held(void)
105 int lockdep_opinion = 0;
107 if (!debug_lockdep_rcu_enabled())
109 if (!rcu_is_watching())
111 if (!rcu_lockdep_current_cpu_online())
114 lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
115 return lockdep_opinion || !preemptible();
117 EXPORT_SYMBOL(rcu_read_lock_sched_held);
120 #ifndef CONFIG_TINY_RCU
123 * Should expedited grace-period primitives always fall back to their
124 * non-expedited counterparts? Intended for use within RCU. Note
125 * that if the user specifies both rcu_expedited and rcu_normal, then
126 * rcu_normal wins. (Except during the time period during boot from
127 * when the first task is spawned until the rcu_set_runtime_mode()
128 * core_initcall() is invoked, at which point everything is expedited.)
130 bool rcu_gp_is_normal(void)
132 return READ_ONCE(rcu_normal) &&
133 rcu_scheduler_active != RCU_SCHEDULER_INIT;
135 EXPORT_SYMBOL_GPL(rcu_gp_is_normal);
137 static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);
140 * Should normal grace-period primitives be expedited? Intended for
141 * use within RCU. Note that this function takes the rcu_expedited
142 * sysfs/boot variable and rcu_scheduler_active into account as well
143 * as the rcu_expedite_gp() nesting. So looping on rcu_unexpedite_gp()
144 * until rcu_gp_is_expedited() returns false is a -really- bad idea.
146 bool rcu_gp_is_expedited(void)
148 return rcu_expedited || atomic_read(&rcu_expedited_nesting) ||
149 rcu_scheduler_active == RCU_SCHEDULER_INIT;
151 EXPORT_SYMBOL_GPL(rcu_gp_is_expedited);
154 * rcu_expedite_gp - Expedite future RCU grace periods
156 * After a call to this function, future calls to synchronize_rcu() and
157 * friends act as the corresponding synchronize_rcu_expedited() function
158 * had instead been called.
160 void rcu_expedite_gp(void)
162 atomic_inc(&rcu_expedited_nesting);
164 EXPORT_SYMBOL_GPL(rcu_expedite_gp);
167 * rcu_unexpedite_gp - Cancel prior rcu_expedite_gp() invocation
169 * Undo a prior call to rcu_expedite_gp(). If all prior calls to
170 * rcu_expedite_gp() are undone by a subsequent call to rcu_unexpedite_gp(),
171 * and if the rcu_expedited sysfs/boot parameter is not set, then all
172 * subsequent calls to synchronize_rcu() and friends will return to
173 * their normal non-expedited behavior.
175 void rcu_unexpedite_gp(void)
177 atomic_dec(&rcu_expedited_nesting);
179 EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
182 * Inform RCU of the end of the in-kernel boot sequence.
184 void rcu_end_inkernel_boot(void)
187 if (rcu_normal_after_boot)
188 WRITE_ONCE(rcu_normal, 1);
191 #endif /* #ifndef CONFIG_TINY_RCU */
194 * Test each non-SRCU synchronous grace-period wait API. This is
195 * useful just after a change in mode for these primitives, and
198 void rcu_test_sync_prims(void)
200 if (!IS_ENABLED(CONFIG_PROVE_RCU))
203 synchronize_rcu_bh();
205 synchronize_rcu_expedited();
206 synchronize_rcu_bh_expedited();
207 synchronize_sched_expedited();
210 #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU)
213 * Switch to run-time mode once RCU has fully initialized.
215 static int __init rcu_set_runtime_mode(void)
217 rcu_test_sync_prims();
218 rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
219 rcu_test_sync_prims();
222 core_initcall(rcu_set_runtime_mode);
224 #endif /* #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU) */
226 #ifdef CONFIG_PREEMPT_RCU
229 * Preemptible RCU implementation for rcu_read_lock().
230 * Just increment ->rcu_read_lock_nesting, shared state will be updated
233 void __rcu_read_lock(void)
235 current->rcu_read_lock_nesting++;
236 barrier(); /* critical section after entry code. */
238 EXPORT_SYMBOL_GPL(__rcu_read_lock);
241 * Preemptible RCU implementation for rcu_read_unlock().
242 * Decrement ->rcu_read_lock_nesting. If the result is zero (outermost
243 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
244 * invoke rcu_read_unlock_special() to clean up after a context switch
245 * in an RCU read-side critical section and other special cases.
247 void __rcu_read_unlock(void)
249 struct task_struct *t = current;
251 if (t->rcu_read_lock_nesting != 1) {
252 --t->rcu_read_lock_nesting;
254 barrier(); /* critical section before exit code. */
255 t->rcu_read_lock_nesting = INT_MIN;
256 barrier(); /* assign before ->rcu_read_unlock_special load */
257 if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
258 rcu_read_unlock_special(t);
259 barrier(); /* ->rcu_read_unlock_special load before assign */
260 t->rcu_read_lock_nesting = 0;
262 #ifdef CONFIG_PROVE_LOCKING
264 int rrln = READ_ONCE(t->rcu_read_lock_nesting);
266 WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2);
268 #endif /* #ifdef CONFIG_PROVE_LOCKING */
270 EXPORT_SYMBOL_GPL(__rcu_read_unlock);
272 #endif /* #ifdef CONFIG_PREEMPT_RCU */
274 #ifdef CONFIG_DEBUG_LOCK_ALLOC
275 static struct lock_class_key rcu_lock_key;
276 struct lockdep_map rcu_lock_map =
277 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
278 EXPORT_SYMBOL_GPL(rcu_lock_map);
280 static struct lock_class_key rcu_bh_lock_key;
281 struct lockdep_map rcu_bh_lock_map =
282 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key);
283 EXPORT_SYMBOL_GPL(rcu_bh_lock_map);
285 static struct lock_class_key rcu_sched_lock_key;
286 struct lockdep_map rcu_sched_lock_map =
287 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key);
288 EXPORT_SYMBOL_GPL(rcu_sched_lock_map);
290 static struct lock_class_key rcu_callback_key;
291 struct lockdep_map rcu_callback_map =
292 STATIC_LOCKDEP_MAP_INIT("rcu_callback", &rcu_callback_key);
293 EXPORT_SYMBOL_GPL(rcu_callback_map);
295 int notrace debug_lockdep_rcu_enabled(void)
297 return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&
298 current->lockdep_recursion == 0;
300 EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);
303 * rcu_read_lock_held() - might we be in RCU read-side critical section?
305 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU
306 * read-side critical section. In absence of CONFIG_DEBUG_LOCK_ALLOC,
307 * this assumes we are in an RCU read-side critical section unless it can
308 * prove otherwise. This is useful for debug checks in functions that
309 * require that they be called within an RCU read-side critical section.
311 * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
312 * and while lockdep is disabled.
314 * Note that rcu_read_lock() and the matching rcu_read_unlock() must
315 * occur in the same context, for example, it is illegal to invoke
316 * rcu_read_unlock() in process context if the matching rcu_read_lock()
317 * was invoked from within an irq handler.
319 * Note that rcu_read_lock() is disallowed if the CPU is either idle or
320 * offline from an RCU perspective, so check for those as well.
322 int rcu_read_lock_held(void)
324 if (!debug_lockdep_rcu_enabled())
326 if (!rcu_is_watching())
328 if (!rcu_lockdep_current_cpu_online())
330 return lock_is_held(&rcu_lock_map);
332 EXPORT_SYMBOL_GPL(rcu_read_lock_held);
335 * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section?
337 * Check for bottom half being disabled, which covers both the
338 * CONFIG_PROVE_RCU and not cases. Note that if someone uses
339 * rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled)
340 * will show the situation. This is useful for debug checks in functions
341 * that require that they be called within an RCU read-side critical
344 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot.
346 * Note that rcu_read_lock() is disallowed if the CPU is either idle or
347 * offline from an RCU perspective, so check for those as well.
349 int rcu_read_lock_bh_held(void)
351 if (!debug_lockdep_rcu_enabled())
353 if (!rcu_is_watching())
355 if (!rcu_lockdep_current_cpu_online())
357 return in_softirq() || irqs_disabled();
359 EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
361 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
364 * wakeme_after_rcu() - Callback function to awaken a task after grace period
365 * @head: Pointer to rcu_head member within rcu_synchronize structure
367 * Awaken the corresponding task now that a grace period has elapsed.
369 void wakeme_after_rcu(struct rcu_head *head)
371 struct rcu_synchronize *rcu;
373 rcu = container_of(head, struct rcu_synchronize, head);
374 complete(&rcu->completion);
376 EXPORT_SYMBOL_GPL(wakeme_after_rcu);
378 void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
379 struct rcu_synchronize *rs_array)
383 /* Initialize and register callbacks for each flavor specified. */
384 for (i = 0; i < n; i++) {
386 (crcu_array[i] == call_rcu ||
387 crcu_array[i] == call_rcu_bh)) {
391 init_rcu_head_on_stack(&rs_array[i].head);
392 init_completion(&rs_array[i].completion);
393 (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
396 /* Wait for all callbacks to be invoked. */
397 for (i = 0; i < n; i++) {
399 (crcu_array[i] == call_rcu ||
400 crcu_array[i] == call_rcu_bh))
402 wait_for_completion(&rs_array[i].completion);
403 destroy_rcu_head_on_stack(&rs_array[i].head);
406 EXPORT_SYMBOL_GPL(__wait_rcu_gp);
408 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
409 void init_rcu_head(struct rcu_head *head)
411 debug_object_init(head, &rcuhead_debug_descr);
414 void destroy_rcu_head(struct rcu_head *head)
416 debug_object_free(head, &rcuhead_debug_descr);
419 static bool rcuhead_is_static_object(void *addr)
425 * init_rcu_head_on_stack() - initialize on-stack rcu_head for debugobjects
426 * @head: pointer to rcu_head structure to be initialized
428 * This function informs debugobjects of a new rcu_head structure that
429 * has been allocated as an auto variable on the stack. This function
430 * is not required for rcu_head structures that are statically defined or
431 * that are dynamically allocated on the heap. This function has no
432 * effect for !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
434 void init_rcu_head_on_stack(struct rcu_head *head)
436 debug_object_init_on_stack(head, &rcuhead_debug_descr);
438 EXPORT_SYMBOL_GPL(init_rcu_head_on_stack);
441 * destroy_rcu_head_on_stack() - destroy on-stack rcu_head for debugobjects
442 * @head: pointer to rcu_head structure to be initialized
444 * This function informs debugobjects that an on-stack rcu_head structure
445 * is about to go out of scope. As with init_rcu_head_on_stack(), this
446 * function is not required for rcu_head structures that are statically
447 * defined or that are dynamically allocated on the heap. Also as with
448 * init_rcu_head_on_stack(), this function has no effect for
449 * !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
451 void destroy_rcu_head_on_stack(struct rcu_head *head)
453 debug_object_free(head, &rcuhead_debug_descr);
455 EXPORT_SYMBOL_GPL(destroy_rcu_head_on_stack);
457 struct debug_obj_descr rcuhead_debug_descr = {
459 .is_static_object = rcuhead_is_static_object,
461 EXPORT_SYMBOL_GPL(rcuhead_debug_descr);
462 #endif /* #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */
464 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU) || defined(CONFIG_RCU_TRACE)
465 void do_trace_rcu_torture_read(const char *rcutorturename, struct rcu_head *rhp,
467 unsigned long c_old, unsigned long c)
469 trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c);
471 EXPORT_SYMBOL_GPL(do_trace_rcu_torture_read);
473 #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \
477 #ifdef CONFIG_RCU_STALL_COMMON
479 #ifdef CONFIG_PROVE_RCU
480 #define RCU_STALL_DELAY_DELTA (5 * HZ)
482 #define RCU_STALL_DELAY_DELTA 0
485 int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */
486 static int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
488 module_param(rcu_cpu_stall_suppress, int, 0644);
489 module_param(rcu_cpu_stall_timeout, int, 0644);
491 int rcu_jiffies_till_stall_check(void)
493 int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
496 * Limit check must be consistent with the Kconfig limits
497 * for CONFIG_RCU_CPU_STALL_TIMEOUT.
499 if (till_stall_check < 3) {
500 WRITE_ONCE(rcu_cpu_stall_timeout, 3);
501 till_stall_check = 3;
502 } else if (till_stall_check > 300) {
503 WRITE_ONCE(rcu_cpu_stall_timeout, 300);
504 till_stall_check = 300;
506 return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
509 void rcu_sysrq_start(void)
511 if (!rcu_cpu_stall_suppress)
512 rcu_cpu_stall_suppress = 2;
515 void rcu_sysrq_end(void)
517 if (rcu_cpu_stall_suppress == 2)
518 rcu_cpu_stall_suppress = 0;
521 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
523 rcu_cpu_stall_suppress = 1;
527 static struct notifier_block rcu_panic_block = {
528 .notifier_call = rcu_panic,
531 static int __init check_cpu_stall_init(void)
533 atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
536 early_initcall(check_cpu_stall_init);
538 #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
540 #ifdef CONFIG_TASKS_RCU
543 * Simple variant of RCU whose quiescent states are voluntary context switch,
544 * user-space execution, and idle. As such, grace periods can take one good
545 * long time. There are no read-side primitives similar to rcu_read_lock()
546 * and rcu_read_unlock() because this implementation is intended to get
547 * the system into a safe state for some of the manipulations involved in
548 * tracing and the like. Finally, this implementation does not support
549 * high call_rcu_tasks() rates from multiple CPUs. If this is required,
550 * per-CPU callback lists will be needed.
553 /* Global list of callbacks and associated lock. */
554 static struct rcu_head *rcu_tasks_cbs_head;
555 static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
556 static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq);
557 static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);
559 /* Track exiting tasks in order to allow them to be waited for. */
560 DEFINE_SRCU(tasks_rcu_exit_srcu);
562 /* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */
563 static int rcu_task_stall_timeout __read_mostly = HZ * 60 * 10;
564 module_param(rcu_task_stall_timeout, int, 0644);
566 static void rcu_spawn_tasks_kthread(void);
567 static struct task_struct *rcu_tasks_kthread_ptr;
570 * Post an RCU-tasks callback. First call must be from process context
571 * after the scheduler if fully operational.
573 void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
577 bool havetask = READ_ONCE(rcu_tasks_kthread_ptr);
581 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
582 needwake = !rcu_tasks_cbs_head;
583 *rcu_tasks_cbs_tail = rhp;
584 rcu_tasks_cbs_tail = &rhp->next;
585 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
586 /* We can't create the thread unless interrupts are enabled. */
587 if ((needwake && havetask) ||
588 (!havetask && !irqs_disabled_flags(flags))) {
589 rcu_spawn_tasks_kthread();
590 wake_up(&rcu_tasks_cbs_wq);
593 EXPORT_SYMBOL_GPL(call_rcu_tasks);
596 * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
598 * Control will return to the caller some time after a full rcu-tasks
599 * grace period has elapsed, in other words after all currently
600 * executing rcu-tasks read-side critical sections have elapsed. These
601 * read-side critical sections are delimited by calls to schedule(),
602 * cond_resched_rcu_qs(), idle execution, userspace execution, calls
603 * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
605 * This is a very specialized primitive, intended only for a few uses in
606 * tracing and other situations requiring manipulation of function
607 * preambles and profiling hooks. The synchronize_rcu_tasks() function
608 * is not (yet) intended for heavy use from multiple CPUs.
610 * Note that this guarantee implies further memory-ordering guarantees.
611 * On systems with more than one CPU, when synchronize_rcu_tasks() returns,
612 * each CPU is guaranteed to have executed a full memory barrier since the
613 * end of its last RCU-tasks read-side critical section whose beginning
614 * preceded the call to synchronize_rcu_tasks(). In addition, each CPU
615 * having an RCU-tasks read-side critical section that extends beyond
616 * the return from synchronize_rcu_tasks() is guaranteed to have executed
617 * a full memory barrier after the beginning of synchronize_rcu_tasks()
618 * and before the beginning of that RCU-tasks read-side critical section.
619 * Note that these guarantees include CPUs that are offline, idle, or
620 * executing in user mode, as well as CPUs that are executing in the kernel.
622 * Furthermore, if CPU A invoked synchronize_rcu_tasks(), which returned
623 * to its caller on CPU B, then both CPU A and CPU B are guaranteed
624 * to have executed a full memory barrier during the execution of
625 * synchronize_rcu_tasks() -- even if CPU A and CPU B are the same CPU
626 * (but again only if the system has more than one CPU).
628 void synchronize_rcu_tasks(void)
630 /* Complain if the scheduler has not started. */
631 RCU_LOCKDEP_WARN(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
632 "synchronize_rcu_tasks called too soon");
634 /* Wait for the grace period. */
635 wait_rcu_gp(call_rcu_tasks);
637 EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);
640 * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
642 * Although the current implementation is guaranteed to wait, it is not
643 * obligated to, for example, if there are no pending callbacks.
645 void rcu_barrier_tasks(void)
647 /* There is only one callback queue, so this is easy. ;-) */
648 synchronize_rcu_tasks();
650 EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
652 /* See if tasks are still holding out, complain if so. */
653 static void check_holdout_task(struct task_struct *t,
654 bool needreport, bool *firstreport)
658 if (!READ_ONCE(t->rcu_tasks_holdout) ||
659 t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
660 !READ_ONCE(t->on_rq) ||
661 (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
662 !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
663 WRITE_ONCE(t->rcu_tasks_holdout, false);
664 list_del_init(&t->rcu_tasks_holdout_list);
668 rcu_request_urgent_qs_task(t);
672 pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
673 *firstreport = false;
676 pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n",
677 t, ".I"[is_idle_task(t)],
678 "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
679 t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
680 t->rcu_tasks_idle_cpu, cpu);
684 /* RCU-tasks kthread that detects grace periods and invokes callbacks. */
685 static int __noreturn rcu_tasks_kthread(void *arg)
688 struct task_struct *g, *t;
689 unsigned long lastreport;
690 struct rcu_head *list;
691 struct rcu_head *next;
692 LIST_HEAD(rcu_tasks_holdouts);
694 /* Run on housekeeping CPUs by default. Sysadm can move if desired. */
695 housekeeping_affine(current);
698 * Each pass through the following loop makes one check for
699 * newly arrived callbacks, and, if there are some, waits for
700 * one RCU-tasks grace period and then invokes the callbacks.
701 * This loop is terminated by the system going down. ;-)
705 /* Pick up any new callbacks. */
706 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
707 list = rcu_tasks_cbs_head;
708 rcu_tasks_cbs_head = NULL;
709 rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
710 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
712 /* If there were none, wait a bit and start over. */
714 wait_event_interruptible(rcu_tasks_cbs_wq,
716 if (!rcu_tasks_cbs_head) {
717 WARN_ON(signal_pending(current));
718 schedule_timeout_interruptible(HZ/10);
724 * Wait for all pre-existing t->on_rq and t->nvcsw
725 * transitions to complete. Invoking synchronize_sched()
726 * suffices because all these transitions occur with
727 * interrupts disabled. Without this synchronize_sched(),
728 * a read-side critical section that started before the
729 * grace period might be incorrectly seen as having started
730 * after the grace period.
732 * This synchronize_sched() also dispenses with the
733 * need for a memory barrier on the first store to
734 * ->rcu_tasks_holdout, as it forces the store to happen
735 * after the beginning of the grace period.
740 * There were callbacks, so we need to wait for an
741 * RCU-tasks grace period. Start off by scanning
742 * the task list for tasks that are not already
743 * voluntarily blocked. Mark these tasks and make
744 * a list of them in rcu_tasks_holdouts.
747 for_each_process_thread(g, t) {
748 if (t != current && READ_ONCE(t->on_rq) &&
751 t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
752 WRITE_ONCE(t->rcu_tasks_holdout, true);
753 list_add(&t->rcu_tasks_holdout_list,
754 &rcu_tasks_holdouts);
760 * Wait for tasks that are in the process of exiting.
761 * This does only part of the job, ensuring that all
762 * tasks that were previously exiting reach the point
763 * where they have disabled preemption, allowing the
764 * later synchronize_sched() to finish the job.
766 synchronize_srcu(&tasks_rcu_exit_srcu);
769 * Each pass through the following loop scans the list
770 * of holdout tasks, removing any that are no longer
771 * holdouts. When the list is empty, we are done.
773 lastreport = jiffies;
774 while (!list_empty(&rcu_tasks_holdouts)) {
778 struct task_struct *t1;
780 schedule_timeout_interruptible(HZ);
781 rtst = READ_ONCE(rcu_task_stall_timeout);
782 needreport = rtst > 0 &&
783 time_after(jiffies, lastreport + rtst);
785 lastreport = jiffies;
787 WARN_ON(signal_pending(current));
788 list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts,
789 rcu_tasks_holdout_list) {
790 check_holdout_task(t, needreport, &firstreport);
796 * Because ->on_rq and ->nvcsw are not guaranteed
797 * to have a full memory barriers prior to them in the
798 * schedule() path, memory reordering on other CPUs could
799 * cause their RCU-tasks read-side critical sections to
800 * extend past the end of the grace period. However,
801 * because these ->nvcsw updates are carried out with
802 * interrupts disabled, we can use synchronize_sched()
803 * to force the needed ordering on all such CPUs.
805 * This synchronize_sched() also confines all
806 * ->rcu_tasks_holdout accesses to be within the grace
807 * period, avoiding the need for memory barriers for
808 * ->rcu_tasks_holdout accesses.
810 * In addition, this synchronize_sched() waits for exiting
811 * tasks to complete their final preempt_disable() region
812 * of execution, cleaning up after the synchronize_srcu()
817 /* Invoke the callbacks. */
826 schedule_timeout_uninterruptible(HZ/10);
830 /* Spawn rcu_tasks_kthread() at first call to call_rcu_tasks(). */
831 static void rcu_spawn_tasks_kthread(void)
833 static DEFINE_MUTEX(rcu_tasks_kthread_mutex);
834 struct task_struct *t;
836 if (READ_ONCE(rcu_tasks_kthread_ptr)) {
837 smp_mb(); /* Ensure caller sees full kthread. */
840 mutex_lock(&rcu_tasks_kthread_mutex);
841 if (rcu_tasks_kthread_ptr) {
842 mutex_unlock(&rcu_tasks_kthread_mutex);
845 t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread");
847 smp_mb(); /* Ensure others see full kthread. */
848 WRITE_ONCE(rcu_tasks_kthread_ptr, t);
849 mutex_unlock(&rcu_tasks_kthread_mutex);
852 #endif /* #ifdef CONFIG_TASKS_RCU */
854 #ifdef CONFIG_PROVE_RCU
857 * Early boot self test parameters, one for each flavor
859 static bool rcu_self_test;
860 static bool rcu_self_test_bh;
861 static bool rcu_self_test_sched;
863 module_param(rcu_self_test, bool, 0444);
864 module_param(rcu_self_test_bh, bool, 0444);
865 module_param(rcu_self_test_sched, bool, 0444);
867 static int rcu_self_test_counter;
869 static void test_callback(struct rcu_head *r)
871 rcu_self_test_counter++;
872 pr_info("RCU test callback executed %d\n", rcu_self_test_counter);
875 static void early_boot_test_call_rcu(void)
877 static struct rcu_head head;
879 call_rcu(&head, test_callback);
882 static void early_boot_test_call_rcu_bh(void)
884 static struct rcu_head head;
886 call_rcu_bh(&head, test_callback);
889 static void early_boot_test_call_rcu_sched(void)
891 static struct rcu_head head;
893 call_rcu_sched(&head, test_callback);
896 void rcu_early_boot_tests(void)
898 pr_info("Running RCU self tests\n");
901 early_boot_test_call_rcu();
902 if (rcu_self_test_bh)
903 early_boot_test_call_rcu_bh();
904 if (rcu_self_test_sched)
905 early_boot_test_call_rcu_sched();
906 rcu_test_sync_prims();
909 static int rcu_verify_early_boot_tests(void)
912 int early_boot_test_counter = 0;
915 early_boot_test_counter++;
918 if (rcu_self_test_bh) {
919 early_boot_test_counter++;
922 if (rcu_self_test_sched) {
923 early_boot_test_counter++;
927 if (rcu_self_test_counter != early_boot_test_counter) {
934 late_initcall(rcu_verify_early_boot_tests);
936 void rcu_early_boot_tests(void) {}
937 #endif /* CONFIG_PROVE_RCU */