1 // SPDX-License-Identifier: GPL-2.0+
3 * Read-Copy Update mechanism for mutual exclusion
5 * Copyright IBM Corporation, 2001
7 * Authors: Dipankar Sarma <dipankar@in.ibm.com>
8 * Manfred Spraul <manfred@colorfullife.com>
10 * Based on the original work by Paul McKenney <paulmck@linux.ibm.com>
11 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
13 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
14 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
16 * For detailed explanation of Read-Copy Update mechanism see -
17 * http://lse.sourceforge.net/locking/rcupdate.html
20 #include <linux/types.h>
21 #include <linux/kernel.h>
22 #include <linux/init.h>
23 #include <linux/spinlock.h>
24 #include <linux/smp.h>
25 #include <linux/interrupt.h>
26 #include <linux/sched/signal.h>
27 #include <linux/sched/debug.h>
28 #include <linux/atomic.h>
29 #include <linux/bitops.h>
30 #include <linux/percpu.h>
31 #include <linux/notifier.h>
32 #include <linux/cpu.h>
33 #include <linux/mutex.h>
34 #include <linux/export.h>
35 #include <linux/hardirq.h>
36 #include <linux/delay.h>
37 #include <linux/moduleparam.h>
38 #include <linux/kthread.h>
39 #include <linux/tick.h>
40 #include <linux/rcupdate_wait.h>
41 #include <linux/sched/isolation.h>
42 #include <linux/kprobes.h>
44 #define CREATE_TRACE_POINTS
48 #ifdef MODULE_PARAM_PREFIX
49 #undef MODULE_PARAM_PREFIX
51 #define MODULE_PARAM_PREFIX "rcupdate."
53 #ifndef CONFIG_TINY_RCU
54 extern int rcu_expedited; /* from sysctl */
55 module_param(rcu_expedited, int, 0);
56 extern int rcu_normal; /* from sysctl */
57 module_param(rcu_normal, int, 0);
58 static int rcu_normal_after_boot;
59 module_param(rcu_normal_after_boot, int, 0);
60 #endif /* #ifndef CONFIG_TINY_RCU */
62 #ifdef CONFIG_DEBUG_LOCK_ALLOC
64 * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
66 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
67 * RCU-sched read-side critical section. In absence of
68 * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
69 * critical section unless it can prove otherwise. Note that disabling
70 * of preemption (including disabling irqs) counts as an RCU-sched
71 * read-side critical section. This is useful for debug checks in functions
72 * that required that they be called within an RCU-sched read-side
75 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
76 * and while lockdep is disabled.
78 * Note that if the CPU is in the idle loop from an RCU point of
79 * view (ie: that we are in the section between rcu_idle_enter() and
80 * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU
81 * did an rcu_read_lock(). The reason for this is that RCU ignores CPUs
82 * that are in such a section, considering these as in extended quiescent
83 * state, so such a CPU is effectively never in an RCU read-side critical
84 * section regardless of what RCU primitives it invokes. This state of
85 * affairs is required --- we need to keep an RCU-free window in idle
86 * where the CPU may possibly enter into low power mode. This way we can
87 * notice an extended quiescent state to other CPUs that started a grace
88 * period. Otherwise we would delay any grace period as long as we run in
91 * Similarly, we avoid claiming an SRCU read lock held if the current
94 int rcu_read_lock_sched_held(void)
96 int lockdep_opinion = 0;
98 if (!debug_lockdep_rcu_enabled())
100 if (!rcu_is_watching())
102 if (!rcu_lockdep_current_cpu_online())
105 lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
106 return lockdep_opinion || !preemptible();
108 EXPORT_SYMBOL(rcu_read_lock_sched_held);
111 #ifndef CONFIG_TINY_RCU
114 * Should expedited grace-period primitives always fall back to their
115 * non-expedited counterparts? Intended for use within RCU. Note
116 * that if the user specifies both rcu_expedited and rcu_normal, then
117 * rcu_normal wins. (Except during the time period during boot from
118 * when the first task is spawned until the rcu_set_runtime_mode()
119 * core_initcall() is invoked, at which point everything is expedited.)
121 bool rcu_gp_is_normal(void)
123 return READ_ONCE(rcu_normal) &&
124 rcu_scheduler_active != RCU_SCHEDULER_INIT;
126 EXPORT_SYMBOL_GPL(rcu_gp_is_normal);
128 static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);
131 * Should normal grace-period primitives be expedited? Intended for
132 * use within RCU. Note that this function takes the rcu_expedited
133 * sysfs/boot variable and rcu_scheduler_active into account as well
134 * as the rcu_expedite_gp() nesting. So looping on rcu_unexpedite_gp()
135 * until rcu_gp_is_expedited() returns false is a -really- bad idea.
137 bool rcu_gp_is_expedited(void)
139 return rcu_expedited || atomic_read(&rcu_expedited_nesting) ||
140 rcu_scheduler_active == RCU_SCHEDULER_INIT;
142 EXPORT_SYMBOL_GPL(rcu_gp_is_expedited);
145 * rcu_expedite_gp - Expedite future RCU grace periods
147 * After a call to this function, future calls to synchronize_rcu() and
148 * friends act as the corresponding synchronize_rcu_expedited() function
149 * had instead been called.
151 void rcu_expedite_gp(void)
153 atomic_inc(&rcu_expedited_nesting);
155 EXPORT_SYMBOL_GPL(rcu_expedite_gp);
158 * rcu_unexpedite_gp - Cancel prior rcu_expedite_gp() invocation
160 * Undo a prior call to rcu_expedite_gp(). If all prior calls to
161 * rcu_expedite_gp() are undone by a subsequent call to rcu_unexpedite_gp(),
162 * and if the rcu_expedited sysfs/boot parameter is not set, then all
163 * subsequent calls to synchronize_rcu() and friends will return to
164 * their normal non-expedited behavior.
166 void rcu_unexpedite_gp(void)
168 atomic_dec(&rcu_expedited_nesting);
170 EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
173 * Inform RCU of the end of the in-kernel boot sequence.
175 void rcu_end_inkernel_boot(void)
178 if (rcu_normal_after_boot)
179 WRITE_ONCE(rcu_normal, 1);
182 #endif /* #ifndef CONFIG_TINY_RCU */
185 * Test each non-SRCU synchronous grace-period wait API. This is
186 * useful just after a change in mode for these primitives, and
189 void rcu_test_sync_prims(void)
191 if (!IS_ENABLED(CONFIG_PROVE_RCU))
194 synchronize_rcu_expedited();
197 #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU)
200 * Switch to run-time mode once RCU has fully initialized.
202 static int __init rcu_set_runtime_mode(void)
204 rcu_test_sync_prims();
205 rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
206 rcu_test_sync_prims();
209 core_initcall(rcu_set_runtime_mode);
211 #endif /* #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU) */
213 #ifdef CONFIG_DEBUG_LOCK_ALLOC
214 static struct lock_class_key rcu_lock_key;
215 struct lockdep_map rcu_lock_map =
216 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
217 EXPORT_SYMBOL_GPL(rcu_lock_map);
219 static struct lock_class_key rcu_bh_lock_key;
220 struct lockdep_map rcu_bh_lock_map =
221 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key);
222 EXPORT_SYMBOL_GPL(rcu_bh_lock_map);
224 static struct lock_class_key rcu_sched_lock_key;
225 struct lockdep_map rcu_sched_lock_map =
226 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key);
227 EXPORT_SYMBOL_GPL(rcu_sched_lock_map);
229 static struct lock_class_key rcu_callback_key;
230 struct lockdep_map rcu_callback_map =
231 STATIC_LOCKDEP_MAP_INIT("rcu_callback", &rcu_callback_key);
232 EXPORT_SYMBOL_GPL(rcu_callback_map);
234 int notrace debug_lockdep_rcu_enabled(void)
236 return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&
237 current->lockdep_recursion == 0;
239 EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);
240 NOKPROBE_SYMBOL(debug_lockdep_rcu_enabled);
243 * rcu_read_lock_held() - might we be in RCU read-side critical section?
245 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU
246 * read-side critical section. In absence of CONFIG_DEBUG_LOCK_ALLOC,
247 * this assumes we are in an RCU read-side critical section unless it can
248 * prove otherwise. This is useful for debug checks in functions that
249 * require that they be called within an RCU read-side critical section.
251 * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
252 * and while lockdep is disabled.
254 * Note that rcu_read_lock() and the matching rcu_read_unlock() must
255 * occur in the same context, for example, it is illegal to invoke
256 * rcu_read_unlock() in process context if the matching rcu_read_lock()
257 * was invoked from within an irq handler.
259 * Note that rcu_read_lock() is disallowed if the CPU is either idle or
260 * offline from an RCU perspective, so check for those as well.
262 int rcu_read_lock_held(void)
264 if (!debug_lockdep_rcu_enabled())
266 if (!rcu_is_watching())
268 if (!rcu_lockdep_current_cpu_online())
270 return lock_is_held(&rcu_lock_map);
272 EXPORT_SYMBOL_GPL(rcu_read_lock_held);
275 * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section?
277 * Check for bottom half being disabled, which covers both the
278 * CONFIG_PROVE_RCU and not cases. Note that if someone uses
279 * rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled)
280 * will show the situation. This is useful for debug checks in functions
281 * that require that they be called within an RCU read-side critical
284 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot.
286 * Note that rcu_read_lock_bh() is disallowed if the CPU is either idle or
287 * offline from an RCU perspective, so check for those as well.
289 int rcu_read_lock_bh_held(void)
291 if (!debug_lockdep_rcu_enabled())
293 if (!rcu_is_watching())
295 if (!rcu_lockdep_current_cpu_online())
297 return in_softirq() || irqs_disabled();
299 EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
301 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
304 * wakeme_after_rcu() - Callback function to awaken a task after grace period
305 * @head: Pointer to rcu_head member within rcu_synchronize structure
307 * Awaken the corresponding task now that a grace period has elapsed.
309 void wakeme_after_rcu(struct rcu_head *head)
311 struct rcu_synchronize *rcu;
313 rcu = container_of(head, struct rcu_synchronize, head);
314 complete(&rcu->completion);
316 EXPORT_SYMBOL_GPL(wakeme_after_rcu);
318 void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
319 struct rcu_synchronize *rs_array)
324 /* Initialize and register callbacks for each crcu_array element. */
325 for (i = 0; i < n; i++) {
327 (crcu_array[i] == call_rcu)) {
331 init_rcu_head_on_stack(&rs_array[i].head);
332 init_completion(&rs_array[i].completion);
333 for (j = 0; j < i; j++)
334 if (crcu_array[j] == crcu_array[i])
337 (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
340 /* Wait for all callbacks to be invoked. */
341 for (i = 0; i < n; i++) {
343 (crcu_array[i] == call_rcu))
345 for (j = 0; j < i; j++)
346 if (crcu_array[j] == crcu_array[i])
349 wait_for_completion(&rs_array[i].completion);
350 destroy_rcu_head_on_stack(&rs_array[i].head);
353 EXPORT_SYMBOL_GPL(__wait_rcu_gp);
355 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
356 void init_rcu_head(struct rcu_head *head)
358 debug_object_init(head, &rcuhead_debug_descr);
360 EXPORT_SYMBOL_GPL(init_rcu_head);
362 void destroy_rcu_head(struct rcu_head *head)
364 debug_object_free(head, &rcuhead_debug_descr);
366 EXPORT_SYMBOL_GPL(destroy_rcu_head);
368 static bool rcuhead_is_static_object(void *addr)
374 * init_rcu_head_on_stack() - initialize on-stack rcu_head for debugobjects
375 * @head: pointer to rcu_head structure to be initialized
377 * This function informs debugobjects of a new rcu_head structure that
378 * has been allocated as an auto variable on the stack. This function
379 * is not required for rcu_head structures that are statically defined or
380 * that are dynamically allocated on the heap. This function has no
381 * effect for !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
383 void init_rcu_head_on_stack(struct rcu_head *head)
385 debug_object_init_on_stack(head, &rcuhead_debug_descr);
387 EXPORT_SYMBOL_GPL(init_rcu_head_on_stack);
390 * destroy_rcu_head_on_stack() - destroy on-stack rcu_head for debugobjects
391 * @head: pointer to rcu_head structure to be initialized
393 * This function informs debugobjects that an on-stack rcu_head structure
394 * is about to go out of scope. As with init_rcu_head_on_stack(), this
395 * function is not required for rcu_head structures that are statically
396 * defined or that are dynamically allocated on the heap. Also as with
397 * init_rcu_head_on_stack(), this function has no effect for
398 * !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
400 void destroy_rcu_head_on_stack(struct rcu_head *head)
402 debug_object_free(head, &rcuhead_debug_descr);
404 EXPORT_SYMBOL_GPL(destroy_rcu_head_on_stack);
406 struct debug_obj_descr rcuhead_debug_descr = {
408 .is_static_object = rcuhead_is_static_object,
410 EXPORT_SYMBOL_GPL(rcuhead_debug_descr);
411 #endif /* #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */
413 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU) || defined(CONFIG_RCU_TRACE)
414 void do_trace_rcu_torture_read(const char *rcutorturename, struct rcu_head *rhp,
416 unsigned long c_old, unsigned long c)
418 trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c);
420 EXPORT_SYMBOL_GPL(do_trace_rcu_torture_read);
422 #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \
426 #if IS_ENABLED(CONFIG_RCU_TORTURE_TEST) || IS_MODULE(CONFIG_RCU_TORTURE_TEST)
427 /* Get rcutorture access to sched_setaffinity(). */
428 long rcutorture_sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
432 ret = sched_setaffinity(pid, in_mask);
433 WARN_ONCE(ret, "%s: sched_setaffinity() returned %d\n", __func__, ret);
436 EXPORT_SYMBOL_GPL(rcutorture_sched_setaffinity);
439 #ifdef CONFIG_RCU_STALL_COMMON
440 int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */
441 EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress);
442 module_param(rcu_cpu_stall_suppress, int, 0644);
443 int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
444 module_param(rcu_cpu_stall_timeout, int, 0644);
445 #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
447 #ifdef CONFIG_TASKS_RCU
450 * Simple variant of RCU whose quiescent states are voluntary context
451 * switch, cond_resched_rcu_qs(), user-space execution, and idle.
452 * As such, grace periods can take one good long time. There are no
453 * read-side primitives similar to rcu_read_lock() and rcu_read_unlock()
454 * because this implementation is intended to get the system into a safe
455 * state for some of the manipulations involved in tracing and the like.
456 * Finally, this implementation does not support high call_rcu_tasks()
457 * rates from multiple CPUs. If this is required, per-CPU callback lists
461 /* Global list of callbacks and associated lock. */
462 static struct rcu_head *rcu_tasks_cbs_head;
463 static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
464 static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq);
465 static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);
467 /* Track exiting tasks in order to allow them to be waited for. */
468 DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
470 /* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */
471 #define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
472 static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
473 module_param(rcu_task_stall_timeout, int, 0644);
475 static struct task_struct *rcu_tasks_kthread_ptr;
478 * call_rcu_tasks() - Queue an RCU for invocation task-based grace period
479 * @rhp: structure to be used for queueing the RCU updates.
480 * @func: actual callback function to be invoked after the grace period
482 * The callback function will be invoked some time after a full grace
483 * period elapses, in other words after all currently executing RCU
484 * read-side critical sections have completed. call_rcu_tasks() assumes
485 * that the read-side critical sections end at a voluntary context
486 * switch (not a preemption!), cond_resched_rcu_qs(), entry into idle,
487 * or transition to usermode execution. As such, there are no read-side
488 * primitives analogous to rcu_read_lock() and rcu_read_unlock() because
489 * this primitive is intended to determine that all tasks have passed
490 * through a safe state, not so much for data-strcuture synchronization.
492 * See the description of call_rcu() for more detailed information on
493 * memory ordering guarantees.
495 void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
502 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
503 needwake = !rcu_tasks_cbs_head;
504 *rcu_tasks_cbs_tail = rhp;
505 rcu_tasks_cbs_tail = &rhp->next;
506 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
507 /* We can't create the thread unless interrupts are enabled. */
508 if (needwake && READ_ONCE(rcu_tasks_kthread_ptr))
509 wake_up(&rcu_tasks_cbs_wq);
511 EXPORT_SYMBOL_GPL(call_rcu_tasks);
514 * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
516 * Control will return to the caller some time after a full rcu-tasks
517 * grace period has elapsed, in other words after all currently
518 * executing rcu-tasks read-side critical sections have elapsed. These
519 * read-side critical sections are delimited by calls to schedule(),
520 * cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls
521 * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
523 * This is a very specialized primitive, intended only for a few uses in
524 * tracing and other situations requiring manipulation of function
525 * preambles and profiling hooks. The synchronize_rcu_tasks() function
526 * is not (yet) intended for heavy use from multiple CPUs.
528 * Note that this guarantee implies further memory-ordering guarantees.
529 * On systems with more than one CPU, when synchronize_rcu_tasks() returns,
530 * each CPU is guaranteed to have executed a full memory barrier since the
531 * end of its last RCU-tasks read-side critical section whose beginning
532 * preceded the call to synchronize_rcu_tasks(). In addition, each CPU
533 * having an RCU-tasks read-side critical section that extends beyond
534 * the return from synchronize_rcu_tasks() is guaranteed to have executed
535 * a full memory barrier after the beginning of synchronize_rcu_tasks()
536 * and before the beginning of that RCU-tasks read-side critical section.
537 * Note that these guarantees include CPUs that are offline, idle, or
538 * executing in user mode, as well as CPUs that are executing in the kernel.
540 * Furthermore, if CPU A invoked synchronize_rcu_tasks(), which returned
541 * to its caller on CPU B, then both CPU A and CPU B are guaranteed
542 * to have executed a full memory barrier during the execution of
543 * synchronize_rcu_tasks() -- even if CPU A and CPU B are the same CPU
544 * (but again only if the system has more than one CPU).
546 void synchronize_rcu_tasks(void)
548 /* Complain if the scheduler has not started. */
549 RCU_LOCKDEP_WARN(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
550 "synchronize_rcu_tasks called too soon");
552 /* Wait for the grace period. */
553 wait_rcu_gp(call_rcu_tasks);
555 EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);
558 * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
560 * Although the current implementation is guaranteed to wait, it is not
561 * obligated to, for example, if there are no pending callbacks.
563 void rcu_barrier_tasks(void)
565 /* There is only one callback queue, so this is easy. ;-) */
566 synchronize_rcu_tasks();
568 EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
570 /* See if tasks are still holding out, complain if so. */
571 static void check_holdout_task(struct task_struct *t,
572 bool needreport, bool *firstreport)
576 if (!READ_ONCE(t->rcu_tasks_holdout) ||
577 t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
578 !READ_ONCE(t->on_rq) ||
579 (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
580 !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
581 WRITE_ONCE(t->rcu_tasks_holdout, false);
582 list_del_init(&t->rcu_tasks_holdout_list);
586 rcu_request_urgent_qs_task(t);
590 pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
591 *firstreport = false;
594 pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n",
595 t, ".I"[is_idle_task(t)],
596 "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
597 t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
598 t->rcu_tasks_idle_cpu, cpu);
602 /* RCU-tasks kthread that detects grace periods and invokes callbacks. */
603 static int __noreturn rcu_tasks_kthread(void *arg)
606 struct task_struct *g, *t;
607 unsigned long lastreport;
608 struct rcu_head *list;
609 struct rcu_head *next;
610 LIST_HEAD(rcu_tasks_holdouts);
613 /* Run on housekeeping CPUs by default. Sysadm can move if desired. */
614 housekeeping_affine(current, HK_FLAG_RCU);
617 * Each pass through the following loop makes one check for
618 * newly arrived callbacks, and, if there are some, waits for
619 * one RCU-tasks grace period and then invokes the callbacks.
620 * This loop is terminated by the system going down. ;-)
624 /* Pick up any new callbacks. */
625 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
626 list = rcu_tasks_cbs_head;
627 rcu_tasks_cbs_head = NULL;
628 rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
629 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
631 /* If there were none, wait a bit and start over. */
633 wait_event_interruptible(rcu_tasks_cbs_wq,
635 if (!rcu_tasks_cbs_head) {
636 WARN_ON(signal_pending(current));
637 schedule_timeout_interruptible(HZ/10);
643 * Wait for all pre-existing t->on_rq and t->nvcsw
644 * transitions to complete. Invoking synchronize_rcu()
645 * suffices because all these transitions occur with
646 * interrupts disabled. Without this synchronize_rcu(),
647 * a read-side critical section that started before the
648 * grace period might be incorrectly seen as having started
649 * after the grace period.
651 * This synchronize_rcu() also dispenses with the
652 * need for a memory barrier on the first store to
653 * ->rcu_tasks_holdout, as it forces the store to happen
654 * after the beginning of the grace period.
659 * There were callbacks, so we need to wait for an
660 * RCU-tasks grace period. Start off by scanning
661 * the task list for tasks that are not already
662 * voluntarily blocked. Mark these tasks and make
663 * a list of them in rcu_tasks_holdouts.
666 for_each_process_thread(g, t) {
667 if (t != current && READ_ONCE(t->on_rq) &&
670 t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
671 WRITE_ONCE(t->rcu_tasks_holdout, true);
672 list_add(&t->rcu_tasks_holdout_list,
673 &rcu_tasks_holdouts);
679 * Wait for tasks that are in the process of exiting.
680 * This does only part of the job, ensuring that all
681 * tasks that were previously exiting reach the point
682 * where they have disabled preemption, allowing the
683 * later synchronize_rcu() to finish the job.
685 synchronize_srcu(&tasks_rcu_exit_srcu);
688 * Each pass through the following loop scans the list
689 * of holdout tasks, removing any that are no longer
690 * holdouts. When the list is empty, we are done.
692 lastreport = jiffies;
694 /* Start off with HZ/10 wait and slowly back off to 1 HZ wait*/
701 struct task_struct *t1;
703 if (list_empty(&rcu_tasks_holdouts))
706 /* Slowly back off waiting for holdouts */
707 schedule_timeout_interruptible(HZ/fract);
712 rtst = READ_ONCE(rcu_task_stall_timeout);
713 needreport = rtst > 0 &&
714 time_after(jiffies, lastreport + rtst);
716 lastreport = jiffies;
718 WARN_ON(signal_pending(current));
719 list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts,
720 rcu_tasks_holdout_list) {
721 check_holdout_task(t, needreport, &firstreport);
727 * Because ->on_rq and ->nvcsw are not guaranteed
728 * to have a full memory barriers prior to them in the
729 * schedule() path, memory reordering on other CPUs could
730 * cause their RCU-tasks read-side critical sections to
731 * extend past the end of the grace period. However,
732 * because these ->nvcsw updates are carried out with
733 * interrupts disabled, we can use synchronize_rcu()
734 * to force the needed ordering on all such CPUs.
736 * This synchronize_rcu() also confines all
737 * ->rcu_tasks_holdout accesses to be within the grace
738 * period, avoiding the need for memory barriers for
739 * ->rcu_tasks_holdout accesses.
741 * In addition, this synchronize_rcu() waits for exiting
742 * tasks to complete their final preempt_disable() region
743 * of execution, cleaning up after the synchronize_srcu()
748 /* Invoke the callbacks. */
757 /* Paranoid sleep to keep this from entering a tight loop */
758 schedule_timeout_uninterruptible(HZ/10);
762 /* Spawn rcu_tasks_kthread() at core_initcall() time. */
763 static int __init rcu_spawn_tasks_kthread(void)
765 struct task_struct *t;
767 t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread");
768 if (WARN_ONCE(IS_ERR(t), "%s: Could not start Tasks-RCU grace-period kthread, OOM is now expected behavior\n", __func__))
770 smp_mb(); /* Ensure others see full kthread. */
771 WRITE_ONCE(rcu_tasks_kthread_ptr, t);
774 core_initcall(rcu_spawn_tasks_kthread);
776 /* Do the srcu_read_lock() for the above synchronize_srcu(). */
777 void exit_tasks_rcu_start(void)
780 current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
784 /* Do the srcu_read_unlock() for the above synchronize_srcu(). */
785 void exit_tasks_rcu_finish(void)
788 __srcu_read_unlock(&tasks_rcu_exit_srcu, current->rcu_tasks_idx);
792 #endif /* #ifdef CONFIG_TASKS_RCU */
794 #ifndef CONFIG_TINY_RCU
797 * Print any non-default Tasks RCU settings.
799 static void __init rcu_tasks_bootup_oddness(void)
801 #ifdef CONFIG_TASKS_RCU
802 if (rcu_task_stall_timeout != RCU_TASK_STALL_TIMEOUT)
803 pr_info("\tTasks-RCU CPU stall warnings timeout set to %d (rcu_task_stall_timeout).\n", rcu_task_stall_timeout);
805 pr_info("\tTasks RCU enabled.\n");
806 #endif /* #ifdef CONFIG_TASKS_RCU */
809 #endif /* #ifndef CONFIG_TINY_RCU */
811 #ifdef CONFIG_PROVE_RCU
814 * Early boot self test parameters.
816 static bool rcu_self_test;
817 module_param(rcu_self_test, bool, 0444);
819 static int rcu_self_test_counter;
821 static void test_callback(struct rcu_head *r)
823 rcu_self_test_counter++;
824 pr_info("RCU test callback executed %d\n", rcu_self_test_counter);
827 DEFINE_STATIC_SRCU(early_srcu);
829 static void early_boot_test_call_rcu(void)
831 static struct rcu_head head;
832 static struct rcu_head shead;
834 call_rcu(&head, test_callback);
835 if (IS_ENABLED(CONFIG_SRCU))
836 call_srcu(&early_srcu, &shead, test_callback);
839 void rcu_early_boot_tests(void)
841 pr_info("Running RCU self tests\n");
844 early_boot_test_call_rcu();
845 rcu_test_sync_prims();
848 static int rcu_verify_early_boot_tests(void)
851 int early_boot_test_counter = 0;
854 early_boot_test_counter++;
856 if (IS_ENABLED(CONFIG_SRCU)) {
857 early_boot_test_counter++;
858 srcu_barrier(&early_srcu);
861 if (rcu_self_test_counter != early_boot_test_counter) {
868 late_initcall(rcu_verify_early_boot_tests);
870 void rcu_early_boot_tests(void) {}
871 #endif /* CONFIG_PROVE_RCU */
873 #ifndef CONFIG_TINY_RCU
876 * Print any significant non-default boot-time settings.
878 void __init rcupdate_announce_bootup_oddness(void)
881 pr_info("\tNo expedited grace period (rcu_normal).\n");
882 else if (rcu_normal_after_boot)
883 pr_info("\tNo expedited grace period (rcu_normal_after_boot).\n");
884 else if (rcu_expedited)
885 pr_info("\tAll grace periods are expedited (rcu_expedited).\n");
886 if (rcu_cpu_stall_suppress)
887 pr_info("\tRCU CPU stall warnings suppressed (rcu_cpu_stall_suppress).\n");
888 if (rcu_cpu_stall_timeout != CONFIG_RCU_CPU_STALL_TIMEOUT)
889 pr_info("\tRCU CPU stall warnings timeout set to %d (rcu_cpu_stall_timeout).\n", rcu_cpu_stall_timeout);
890 rcu_tasks_bootup_oddness();
893 #endif /* #ifndef CONFIG_TINY_RCU */