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Linux/kernel/rcu/update.c

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  1 // SPDX-License-Identifier: GPL-2.0+
  2 /*
  3  * Read-Copy Update mechanism for mutual exclusion
  4  *
  5  * Copyright IBM Corporation, 2001
  6  *
  7  * Authors: Dipankar Sarma <dipankar@in.ibm.com>
  8  *          Manfred Spraul <manfred@colorfullife.com>
  9  *
 10  * Based on the original work by Paul McKenney <paulmck@linux.ibm.com>
 11  * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
 12  * Papers:
 13  * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
 14  * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
 15  *
 16  * For detailed explanation of Read-Copy Update mechanism see -
 17  *              http://lse.sourceforge.net/locking/rcupdate.html
 18  *
 19  */
 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>
 43 #include <linux/slab.h>
 44 
 45 #define CREATE_TRACE_POINTS
 46 
 47 #include "rcu.h"
 48 
 49 #ifdef MODULE_PARAM_PREFIX
 50 #undef MODULE_PARAM_PREFIX
 51 #endif
 52 #define MODULE_PARAM_PREFIX "rcupdate."
 53 
 54 #ifndef CONFIG_TINY_RCU
 55 module_param(rcu_expedited, int, 0);
 56 module_param(rcu_normal, int, 0);
 57 static int rcu_normal_after_boot;
 58 module_param(rcu_normal_after_boot, int, 0);
 59 #endif /* #ifndef CONFIG_TINY_RCU */
 60 
 61 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 62 /**
 63  * rcu_read_lock_held_common() - might we be in RCU-sched read-side critical section?
 64  * @ret:        Best guess answer if lockdep cannot be relied on
 65  *
 66  * Returns true if lockdep must be ignored, in which case *ret contains
 67  * the best guess described below.  Otherwise returns false, in which
 68  * case *ret tells the caller nothing and the caller should instead
 69  * consult lockdep.
 70  *
 71  * If CONFIG_DEBUG_LOCK_ALLOC is selected, set *ret to nonzero iff in an
 72  * RCU-sched read-side critical section.  In absence of
 73  * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
 74  * critical section unless it can prove otherwise.  Note that disabling
 75  * of preemption (including disabling irqs) counts as an RCU-sched
 76  * read-side critical section.  This is useful for debug checks in functions
 77  * that required that they be called within an RCU-sched read-side
 78  * critical section.
 79  *
 80  * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
 81  * and while lockdep is disabled.
 82  *
 83  * Note that if the CPU is in the idle loop from an RCU point of view (ie:
 84  * that we are in the section between rcu_idle_enter() and rcu_idle_exit())
 85  * then rcu_read_lock_held() sets *ret to false even if the CPU did an
 86  * rcu_read_lock().  The reason for this is that RCU ignores CPUs that are
 87  * in such a section, considering these as in extended quiescent state,
 88  * so such a CPU is effectively never in an RCU read-side critical section
 89  * regardless of what RCU primitives it invokes.  This state of affairs is
 90  * required --- we need to keep an RCU-free window in idle where the CPU may
 91  * possibly enter into low power mode. This way we can notice an extended
 92  * quiescent state to other CPUs that started a grace period. Otherwise
 93  * we would delay any grace period as long as we run in the idle task.
 94  *
 95  * Similarly, we avoid claiming an RCU read lock held if the current
 96  * CPU is offline.
 97  */
 98 static bool rcu_read_lock_held_common(bool *ret)
 99 {
100         if (!debug_lockdep_rcu_enabled()) {
101                 *ret = 1;
102                 return true;
103         }
104         if (!rcu_is_watching()) {
105                 *ret = 0;
106                 return true;
107         }
108         if (!rcu_lockdep_current_cpu_online()) {
109                 *ret = 0;
110                 return true;
111         }
112         return false;
113 }
114 
115 int rcu_read_lock_sched_held(void)
116 {
117         bool ret;
118 
119         if (rcu_read_lock_held_common(&ret))
120                 return ret;
121         return lock_is_held(&rcu_sched_lock_map) || !preemptible();
122 }
123 EXPORT_SYMBOL(rcu_read_lock_sched_held);
124 #endif
125 
126 #ifndef CONFIG_TINY_RCU
127 
128 /*
129  * Should expedited grace-period primitives always fall back to their
130  * non-expedited counterparts?  Intended for use within RCU.  Note
131  * that if the user specifies both rcu_expedited and rcu_normal, then
132  * rcu_normal wins.  (Except during the time period during boot from
133  * when the first task is spawned until the rcu_set_runtime_mode()
134  * core_initcall() is invoked, at which point everything is expedited.)
135  */
136 bool rcu_gp_is_normal(void)
137 {
138         return READ_ONCE(rcu_normal) &&
139                rcu_scheduler_active != RCU_SCHEDULER_INIT;
140 }
141 EXPORT_SYMBOL_GPL(rcu_gp_is_normal);
142 
143 static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);
144 
145 /*
146  * Should normal grace-period primitives be expedited?  Intended for
147  * use within RCU.  Note that this function takes the rcu_expedited
148  * sysfs/boot variable and rcu_scheduler_active into account as well
149  * as the rcu_expedite_gp() nesting.  So looping on rcu_unexpedite_gp()
150  * until rcu_gp_is_expedited() returns false is a -really- bad idea.
151  */
152 bool rcu_gp_is_expedited(void)
153 {
154         return rcu_expedited || atomic_read(&rcu_expedited_nesting);
155 }
156 EXPORT_SYMBOL_GPL(rcu_gp_is_expedited);
157 
158 /**
159  * rcu_expedite_gp - Expedite future RCU grace periods
160  *
161  * After a call to this function, future calls to synchronize_rcu() and
162  * friends act as the corresponding synchronize_rcu_expedited() function
163  * had instead been called.
164  */
165 void rcu_expedite_gp(void)
166 {
167         atomic_inc(&rcu_expedited_nesting);
168 }
169 EXPORT_SYMBOL_GPL(rcu_expedite_gp);
170 
171 /**
172  * rcu_unexpedite_gp - Cancel prior rcu_expedite_gp() invocation
173  *
174  * Undo a prior call to rcu_expedite_gp().  If all prior calls to
175  * rcu_expedite_gp() are undone by a subsequent call to rcu_unexpedite_gp(),
176  * and if the rcu_expedited sysfs/boot parameter is not set, then all
177  * subsequent calls to synchronize_rcu() and friends will return to
178  * their normal non-expedited behavior.
179  */
180 void rcu_unexpedite_gp(void)
181 {
182         atomic_dec(&rcu_expedited_nesting);
183 }
184 EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
185 
186 /*
187  * Inform RCU of the end of the in-kernel boot sequence.
188  */
189 void rcu_end_inkernel_boot(void)
190 {
191         rcu_unexpedite_gp();
192         if (rcu_normal_after_boot)
193                 WRITE_ONCE(rcu_normal, 1);
194 }
195 
196 #endif /* #ifndef CONFIG_TINY_RCU */
197 
198 /*
199  * Test each non-SRCU synchronous grace-period wait API.  This is
200  * useful just after a change in mode for these primitives, and
201  * during early boot.
202  */
203 void rcu_test_sync_prims(void)
204 {
205         if (!IS_ENABLED(CONFIG_PROVE_RCU))
206                 return;
207         synchronize_rcu();
208         synchronize_rcu_expedited();
209 }
210 
211 #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU)
212 
213 /*
214  * Switch to run-time mode once RCU has fully initialized.
215  */
216 static int __init rcu_set_runtime_mode(void)
217 {
218         rcu_test_sync_prims();
219         rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
220         kfree_rcu_scheduler_running();
221         rcu_test_sync_prims();
222         return 0;
223 }
224 core_initcall(rcu_set_runtime_mode);
225 
226 #endif /* #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU) */
227 
228 #ifdef CONFIG_DEBUG_LOCK_ALLOC
229 static struct lock_class_key rcu_lock_key;
230 struct lockdep_map rcu_lock_map =
231         STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
232 EXPORT_SYMBOL_GPL(rcu_lock_map);
233 
234 static struct lock_class_key rcu_bh_lock_key;
235 struct lockdep_map rcu_bh_lock_map =
236         STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key);
237 EXPORT_SYMBOL_GPL(rcu_bh_lock_map);
238 
239 static struct lock_class_key rcu_sched_lock_key;
240 struct lockdep_map rcu_sched_lock_map =
241         STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key);
242 EXPORT_SYMBOL_GPL(rcu_sched_lock_map);
243 
244 static struct lock_class_key rcu_callback_key;
245 struct lockdep_map rcu_callback_map =
246         STATIC_LOCKDEP_MAP_INIT("rcu_callback", &rcu_callback_key);
247 EXPORT_SYMBOL_GPL(rcu_callback_map);
248 
249 int notrace debug_lockdep_rcu_enabled(void)
250 {
251         return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&
252                current->lockdep_recursion == 0;
253 }
254 EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);
255 NOKPROBE_SYMBOL(debug_lockdep_rcu_enabled);
256 
257 /**
258  * rcu_read_lock_held() - might we be in RCU read-side critical section?
259  *
260  * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU
261  * read-side critical section.  In absence of CONFIG_DEBUG_LOCK_ALLOC,
262  * this assumes we are in an RCU read-side critical section unless it can
263  * prove otherwise.  This is useful for debug checks in functions that
264  * require that they be called within an RCU read-side critical section.
265  *
266  * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
267  * and while lockdep is disabled.
268  *
269  * Note that rcu_read_lock() and the matching rcu_read_unlock() must
270  * occur in the same context, for example, it is illegal to invoke
271  * rcu_read_unlock() in process context if the matching rcu_read_lock()
272  * was invoked from within an irq handler.
273  *
274  * Note that rcu_read_lock() is disallowed if the CPU is either idle or
275  * offline from an RCU perspective, so check for those as well.
276  */
277 int rcu_read_lock_held(void)
278 {
279         bool ret;
280 
281         if (rcu_read_lock_held_common(&ret))
282                 return ret;
283         return lock_is_held(&rcu_lock_map);
284 }
285 EXPORT_SYMBOL_GPL(rcu_read_lock_held);
286 
287 /**
288  * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section?
289  *
290  * Check for bottom half being disabled, which covers both the
291  * CONFIG_PROVE_RCU and not cases.  Note that if someone uses
292  * rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled)
293  * will show the situation.  This is useful for debug checks in functions
294  * that require that they be called within an RCU read-side critical
295  * section.
296  *
297  * Check debug_lockdep_rcu_enabled() to prevent false positives during boot.
298  *
299  * Note that rcu_read_lock_bh() is disallowed if the CPU is either idle or
300  * offline from an RCU perspective, so check for those as well.
301  */
302 int rcu_read_lock_bh_held(void)
303 {
304         bool ret;
305 
306         if (rcu_read_lock_held_common(&ret))
307                 return ret;
308         return in_softirq() || irqs_disabled();
309 }
310 EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
311 
312 int rcu_read_lock_any_held(void)
313 {
314         bool ret;
315 
316         if (rcu_read_lock_held_common(&ret))
317                 return ret;
318         if (lock_is_held(&rcu_lock_map) ||
319             lock_is_held(&rcu_bh_lock_map) ||
320             lock_is_held(&rcu_sched_lock_map))
321                 return 1;
322         return !preemptible();
323 }
324 EXPORT_SYMBOL_GPL(rcu_read_lock_any_held);
325 
326 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
327 
328 /**
329  * wakeme_after_rcu() - Callback function to awaken a task after grace period
330  * @head: Pointer to rcu_head member within rcu_synchronize structure
331  *
332  * Awaken the corresponding task now that a grace period has elapsed.
333  */
334 void wakeme_after_rcu(struct rcu_head *head)
335 {
336         struct rcu_synchronize *rcu;
337 
338         rcu = container_of(head, struct rcu_synchronize, head);
339         complete(&rcu->completion);
340 }
341 EXPORT_SYMBOL_GPL(wakeme_after_rcu);
342 
343 void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
344                    struct rcu_synchronize *rs_array)
345 {
346         int i;
347         int j;
348 
349         /* Initialize and register callbacks for each crcu_array element. */
350         for (i = 0; i < n; i++) {
351                 if (checktiny &&
352                     (crcu_array[i] == call_rcu)) {
353                         might_sleep();
354                         continue;
355                 }
356                 init_rcu_head_on_stack(&rs_array[i].head);
357                 init_completion(&rs_array[i].completion);
358                 for (j = 0; j < i; j++)
359                         if (crcu_array[j] == crcu_array[i])
360                                 break;
361                 if (j == i)
362                         (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
363         }
364 
365         /* Wait for all callbacks to be invoked. */
366         for (i = 0; i < n; i++) {
367                 if (checktiny &&
368                     (crcu_array[i] == call_rcu))
369                         continue;
370                 for (j = 0; j < i; j++)
371                         if (crcu_array[j] == crcu_array[i])
372                                 break;
373                 if (j == i)
374                         wait_for_completion(&rs_array[i].completion);
375                 destroy_rcu_head_on_stack(&rs_array[i].head);
376         }
377 }
378 EXPORT_SYMBOL_GPL(__wait_rcu_gp);
379 
380 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
381 void init_rcu_head(struct rcu_head *head)
382 {
383         debug_object_init(head, &rcuhead_debug_descr);
384 }
385 EXPORT_SYMBOL_GPL(init_rcu_head);
386 
387 void destroy_rcu_head(struct rcu_head *head)
388 {
389         debug_object_free(head, &rcuhead_debug_descr);
390 }
391 EXPORT_SYMBOL_GPL(destroy_rcu_head);
392 
393 static bool rcuhead_is_static_object(void *addr)
394 {
395         return true;
396 }
397 
398 /**
399  * init_rcu_head_on_stack() - initialize on-stack rcu_head for debugobjects
400  * @head: pointer to rcu_head structure to be initialized
401  *
402  * This function informs debugobjects of a new rcu_head structure that
403  * has been allocated as an auto variable on the stack.  This function
404  * is not required for rcu_head structures that are statically defined or
405  * that are dynamically allocated on the heap.  This function has no
406  * effect for !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
407  */
408 void init_rcu_head_on_stack(struct rcu_head *head)
409 {
410         debug_object_init_on_stack(head, &rcuhead_debug_descr);
411 }
412 EXPORT_SYMBOL_GPL(init_rcu_head_on_stack);
413 
414 /**
415  * destroy_rcu_head_on_stack() - destroy on-stack rcu_head for debugobjects
416  * @head: pointer to rcu_head structure to be initialized
417  *
418  * This function informs debugobjects that an on-stack rcu_head structure
419  * is about to go out of scope.  As with init_rcu_head_on_stack(), this
420  * function is not required for rcu_head structures that are statically
421  * defined or that are dynamically allocated on the heap.  Also as with
422  * init_rcu_head_on_stack(), this function has no effect for
423  * !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
424  */
425 void destroy_rcu_head_on_stack(struct rcu_head *head)
426 {
427         debug_object_free(head, &rcuhead_debug_descr);
428 }
429 EXPORT_SYMBOL_GPL(destroy_rcu_head_on_stack);
430 
431 struct debug_obj_descr rcuhead_debug_descr = {
432         .name = "rcu_head",
433         .is_static_object = rcuhead_is_static_object,
434 };
435 EXPORT_SYMBOL_GPL(rcuhead_debug_descr);
436 #endif /* #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */
437 
438 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_RCU_TRACE)
439 void do_trace_rcu_torture_read(const char *rcutorturename, struct rcu_head *rhp,
440                                unsigned long secs,
441                                unsigned long c_old, unsigned long c)
442 {
443         trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c);
444 }
445 EXPORT_SYMBOL_GPL(do_trace_rcu_torture_read);
446 #else
447 #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \
448         do { } while (0)
449 #endif
450 
451 #if IS_ENABLED(CONFIG_RCU_TORTURE_TEST) || IS_MODULE(CONFIG_RCU_TORTURE_TEST)
452 /* Get rcutorture access to sched_setaffinity(). */
453 long rcutorture_sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
454 {
455         int ret;
456 
457         ret = sched_setaffinity(pid, in_mask);
458         WARN_ONCE(ret, "%s: sched_setaffinity() returned %d\n", __func__, ret);
459         return ret;
460 }
461 EXPORT_SYMBOL_GPL(rcutorture_sched_setaffinity);
462 #endif
463 
464 #ifdef CONFIG_RCU_STALL_COMMON
465 int rcu_cpu_stall_ftrace_dump __read_mostly;
466 module_param(rcu_cpu_stall_ftrace_dump, int, 0644);
467 int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */
468 EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress);
469 module_param(rcu_cpu_stall_suppress, int, 0644);
470 int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
471 module_param(rcu_cpu_stall_timeout, int, 0644);
472 #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
473 
474 #ifdef CONFIG_TASKS_RCU
475 
476 /*
477  * Simple variant of RCU whose quiescent states are voluntary context
478  * switch, cond_resched_rcu_qs(), user-space execution, and idle.
479  * As such, grace periods can take one good long time.  There are no
480  * read-side primitives similar to rcu_read_lock() and rcu_read_unlock()
481  * because this implementation is intended to get the system into a safe
482  * state for some of the manipulations involved in tracing and the like.
483  * Finally, this implementation does not support high call_rcu_tasks()
484  * rates from multiple CPUs.  If this is required, per-CPU callback lists
485  * will be needed.
486  */
487 
488 /* Global list of callbacks and associated lock. */
489 static struct rcu_head *rcu_tasks_cbs_head;
490 static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
491 static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq);
492 static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);
493 
494 /* Track exiting tasks in order to allow them to be waited for. */
495 DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
496 
497 /* Control stall timeouts.  Disable with <= 0, otherwise jiffies till stall. */
498 #define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
499 static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
500 module_param(rcu_task_stall_timeout, int, 0644);
501 
502 static struct task_struct *rcu_tasks_kthread_ptr;
503 
504 /**
505  * call_rcu_tasks() - Queue an RCU for invocation task-based grace period
506  * @rhp: structure to be used for queueing the RCU updates.
507  * @func: actual callback function to be invoked after the grace period
508  *
509  * The callback function will be invoked some time after a full grace
510  * period elapses, in other words after all currently executing RCU
511  * read-side critical sections have completed. call_rcu_tasks() assumes
512  * that the read-side critical sections end at a voluntary context
513  * switch (not a preemption!), cond_resched_rcu_qs(), entry into idle,
514  * or transition to usermode execution.  As such, there are no read-side
515  * primitives analogous to rcu_read_lock() and rcu_read_unlock() because
516  * this primitive is intended to determine that all tasks have passed
517  * through a safe state, not so much for data-strcuture synchronization.
518  *
519  * See the description of call_rcu() for more detailed information on
520  * memory ordering guarantees.
521  */
522 void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
523 {
524         unsigned long flags;
525         bool needwake;
526 
527         rhp->next = NULL;
528         rhp->func = func;
529         raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
530         needwake = !rcu_tasks_cbs_head;
531         *rcu_tasks_cbs_tail = rhp;
532         rcu_tasks_cbs_tail = &rhp->next;
533         raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
534         /* We can't create the thread unless interrupts are enabled. */
535         if (needwake && READ_ONCE(rcu_tasks_kthread_ptr))
536                 wake_up(&rcu_tasks_cbs_wq);
537 }
538 EXPORT_SYMBOL_GPL(call_rcu_tasks);
539 
540 /**
541  * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
542  *
543  * Control will return to the caller some time after a full rcu-tasks
544  * grace period has elapsed, in other words after all currently
545  * executing rcu-tasks read-side critical sections have elapsed.  These
546  * read-side critical sections are delimited by calls to schedule(),
547  * cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls
548  * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
549  *
550  * This is a very specialized primitive, intended only for a few uses in
551  * tracing and other situations requiring manipulation of function
552  * preambles and profiling hooks.  The synchronize_rcu_tasks() function
553  * is not (yet) intended for heavy use from multiple CPUs.
554  *
555  * Note that this guarantee implies further memory-ordering guarantees.
556  * On systems with more than one CPU, when synchronize_rcu_tasks() returns,
557  * each CPU is guaranteed to have executed a full memory barrier since the
558  * end of its last RCU-tasks read-side critical section whose beginning
559  * preceded the call to synchronize_rcu_tasks().  In addition, each CPU
560  * having an RCU-tasks read-side critical section that extends beyond
561  * the return from synchronize_rcu_tasks() is guaranteed to have executed
562  * a full memory barrier after the beginning of synchronize_rcu_tasks()
563  * and before the beginning of that RCU-tasks read-side critical section.
564  * Note that these guarantees include CPUs that are offline, idle, or
565  * executing in user mode, as well as CPUs that are executing in the kernel.
566  *
567  * Furthermore, if CPU A invoked synchronize_rcu_tasks(), which returned
568  * to its caller on CPU B, then both CPU A and CPU B are guaranteed
569  * to have executed a full memory barrier during the execution of
570  * synchronize_rcu_tasks() -- even if CPU A and CPU B are the same CPU
571  * (but again only if the system has more than one CPU).
572  */
573 void synchronize_rcu_tasks(void)
574 {
575         /* Complain if the scheduler has not started.  */
576         RCU_LOCKDEP_WARN(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
577                          "synchronize_rcu_tasks called too soon");
578 
579         /* Wait for the grace period. */
580         wait_rcu_gp(call_rcu_tasks);
581 }
582 EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);
583 
584 /**
585  * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
586  *
587  * Although the current implementation is guaranteed to wait, it is not
588  * obligated to, for example, if there are no pending callbacks.
589  */
590 void rcu_barrier_tasks(void)
591 {
592         /* There is only one callback queue, so this is easy.  ;-) */
593         synchronize_rcu_tasks();
594 }
595 EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
596 
597 /* See if tasks are still holding out, complain if so. */
598 static void check_holdout_task(struct task_struct *t,
599                                bool needreport, bool *firstreport)
600 {
601         int cpu;
602 
603         if (!READ_ONCE(t->rcu_tasks_holdout) ||
604             t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
605             !READ_ONCE(t->on_rq) ||
606             (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
607              !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
608                 WRITE_ONCE(t->rcu_tasks_holdout, false);
609                 list_del_init(&t->rcu_tasks_holdout_list);
610                 put_task_struct(t);
611                 return;
612         }
613         rcu_request_urgent_qs_task(t);
614         if (!needreport)
615                 return;
616         if (*firstreport) {
617                 pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
618                 *firstreport = false;
619         }
620         cpu = task_cpu(t);
621         pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n",
622                  t, ".I"[is_idle_task(t)],
623                  "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
624                  t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
625                  t->rcu_tasks_idle_cpu, cpu);
626         sched_show_task(t);
627 }
628 
629 /* RCU-tasks kthread that detects grace periods and invokes callbacks. */
630 static int __noreturn rcu_tasks_kthread(void *arg)
631 {
632         unsigned long flags;
633         struct task_struct *g, *t;
634         unsigned long lastreport;
635         struct rcu_head *list;
636         struct rcu_head *next;
637         LIST_HEAD(rcu_tasks_holdouts);
638         int fract;
639 
640         /* Run on housekeeping CPUs by default.  Sysadm can move if desired. */
641         housekeeping_affine(current, HK_FLAG_RCU);
642 
643         /*
644          * Each pass through the following loop makes one check for
645          * newly arrived callbacks, and, if there are some, waits for
646          * one RCU-tasks grace period and then invokes the callbacks.
647          * This loop is terminated by the system going down.  ;-)
648          */
649         for (;;) {
650 
651                 /* Pick up any new callbacks. */
652                 raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
653                 list = rcu_tasks_cbs_head;
654                 rcu_tasks_cbs_head = NULL;
655                 rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
656                 raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
657 
658                 /* If there were none, wait a bit and start over. */
659                 if (!list) {
660                         wait_event_interruptible(rcu_tasks_cbs_wq,
661                                                  rcu_tasks_cbs_head);
662                         if (!rcu_tasks_cbs_head) {
663                                 WARN_ON(signal_pending(current));
664                                 schedule_timeout_interruptible(HZ/10);
665                         }
666                         continue;
667                 }
668 
669                 /*
670                  * Wait for all pre-existing t->on_rq and t->nvcsw
671                  * transitions to complete.  Invoking synchronize_rcu()
672                  * suffices because all these transitions occur with
673                  * interrupts disabled.  Without this synchronize_rcu(),
674                  * a read-side critical section that started before the
675                  * grace period might be incorrectly seen as having started
676                  * after the grace period.
677                  *
678                  * This synchronize_rcu() also dispenses with the
679                  * need for a memory barrier on the first store to
680                  * ->rcu_tasks_holdout, as it forces the store to happen
681                  * after the beginning of the grace period.
682                  */
683                 synchronize_rcu();
684 
685                 /*
686                  * There were callbacks, so we need to wait for an
687                  * RCU-tasks grace period.  Start off by scanning
688                  * the task list for tasks that are not already
689                  * voluntarily blocked.  Mark these tasks and make
690                  * a list of them in rcu_tasks_holdouts.
691                  */
692                 rcu_read_lock();
693                 for_each_process_thread(g, t) {
694                         if (t != current && READ_ONCE(t->on_rq) &&
695                             !is_idle_task(t)) {
696                                 get_task_struct(t);
697                                 t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
698                                 WRITE_ONCE(t->rcu_tasks_holdout, true);
699                                 list_add(&t->rcu_tasks_holdout_list,
700                                          &rcu_tasks_holdouts);
701                         }
702                 }
703                 rcu_read_unlock();
704 
705                 /*
706                  * Wait for tasks that are in the process of exiting.
707                  * This does only part of the job, ensuring that all
708                  * tasks that were previously exiting reach the point
709                  * where they have disabled preemption, allowing the
710                  * later synchronize_rcu() to finish the job.
711                  */
712                 synchronize_srcu(&tasks_rcu_exit_srcu);
713 
714                 /*
715                  * Each pass through the following loop scans the list
716                  * of holdout tasks, removing any that are no longer
717                  * holdouts.  When the list is empty, we are done.
718                  */
719                 lastreport = jiffies;
720 
721                 /* Start off with HZ/10 wait and slowly back off to 1 HZ wait*/
722                 fract = 10;
723 
724                 for (;;) {
725                         bool firstreport;
726                         bool needreport;
727                         int rtst;
728                         struct task_struct *t1;
729 
730                         if (list_empty(&rcu_tasks_holdouts))
731                                 break;
732 
733                         /* Slowly back off waiting for holdouts */
734                         schedule_timeout_interruptible(HZ/fract);
735 
736                         if (fract > 1)
737                                 fract--;
738 
739                         rtst = READ_ONCE(rcu_task_stall_timeout);
740                         needreport = rtst > 0 &&
741                                      time_after(jiffies, lastreport + rtst);
742                         if (needreport)
743                                 lastreport = jiffies;
744                         firstreport = true;
745                         WARN_ON(signal_pending(current));
746                         list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts,
747                                                 rcu_tasks_holdout_list) {
748                                 check_holdout_task(t, needreport, &firstreport);
749                                 cond_resched();
750                         }
751                 }
752 
753                 /*
754                  * Because ->on_rq and ->nvcsw are not guaranteed
755                  * to have a full memory barriers prior to them in the
756                  * schedule() path, memory reordering on other CPUs could
757                  * cause their RCU-tasks read-side critical sections to
758                  * extend past the end of the grace period.  However,
759                  * because these ->nvcsw updates are carried out with
760                  * interrupts disabled, we can use synchronize_rcu()
761                  * to force the needed ordering on all such CPUs.
762                  *
763                  * This synchronize_rcu() also confines all
764                  * ->rcu_tasks_holdout accesses to be within the grace
765                  * period, avoiding the need for memory barriers for
766                  * ->rcu_tasks_holdout accesses.
767                  *
768                  * In addition, this synchronize_rcu() waits for exiting
769                  * tasks to complete their final preempt_disable() region
770                  * of execution, cleaning up after the synchronize_srcu()
771                  * above.
772                  */
773                 synchronize_rcu();
774 
775                 /* Invoke the callbacks. */
776                 while (list) {
777                         next = list->next;
778                         local_bh_disable();
779                         list->func(list);
780                         local_bh_enable();
781                         list = next;
782                         cond_resched();
783                 }
784                 /* Paranoid sleep to keep this from entering a tight loop */
785                 schedule_timeout_uninterruptible(HZ/10);
786         }
787 }
788 
789 /* Spawn rcu_tasks_kthread() at core_initcall() time. */
790 static int __init rcu_spawn_tasks_kthread(void)
791 {
792         struct task_struct *t;
793 
794         t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread");
795         if (WARN_ONCE(IS_ERR(t), "%s: Could not start Tasks-RCU grace-period kthread, OOM is now expected behavior\n", __func__))
796                 return 0;
797         smp_mb(); /* Ensure others see full kthread. */
798         WRITE_ONCE(rcu_tasks_kthread_ptr, t);
799         return 0;
800 }
801 core_initcall(rcu_spawn_tasks_kthread);
802 
803 /* Do the srcu_read_lock() for the above synchronize_srcu().  */
804 void exit_tasks_rcu_start(void)
805 {
806         preempt_disable();
807         current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
808         preempt_enable();
809 }
810 
811 /* Do the srcu_read_unlock() for the above synchronize_srcu().  */
812 void exit_tasks_rcu_finish(void)
813 {
814         preempt_disable();
815         __srcu_read_unlock(&tasks_rcu_exit_srcu, current->rcu_tasks_idx);
816         preempt_enable();
817 }
818 
819 #endif /* #ifdef CONFIG_TASKS_RCU */
820 
821 #ifndef CONFIG_TINY_RCU
822 
823 /*
824  * Print any non-default Tasks RCU settings.
825  */
826 static void __init rcu_tasks_bootup_oddness(void)
827 {
828 #ifdef CONFIG_TASKS_RCU
829         if (rcu_task_stall_timeout != RCU_TASK_STALL_TIMEOUT)
830                 pr_info("\tTasks-RCU CPU stall warnings timeout set to %d (rcu_task_stall_timeout).\n", rcu_task_stall_timeout);
831         else
832                 pr_info("\tTasks RCU enabled.\n");
833 #endif /* #ifdef CONFIG_TASKS_RCU */
834 }
835 
836 #endif /* #ifndef CONFIG_TINY_RCU */
837 
838 #ifdef CONFIG_PROVE_RCU
839 
840 /*
841  * Early boot self test parameters.
842  */
843 static bool rcu_self_test;
844 module_param(rcu_self_test, bool, 0444);
845 
846 static int rcu_self_test_counter;
847 
848 static void test_callback(struct rcu_head *r)
849 {
850         rcu_self_test_counter++;
851         pr_info("RCU test callback executed %d\n", rcu_self_test_counter);
852 }
853 
854 DEFINE_STATIC_SRCU(early_srcu);
855 
856 struct early_boot_kfree_rcu {
857         struct rcu_head rh;
858 };
859 
860 static void early_boot_test_call_rcu(void)
861 {
862         static struct rcu_head head;
863         static struct rcu_head shead;
864         struct early_boot_kfree_rcu *rhp;
865 
866         call_rcu(&head, test_callback);
867         if (IS_ENABLED(CONFIG_SRCU))
868                 call_srcu(&early_srcu, &shead, test_callback);
869         rhp = kmalloc(sizeof(*rhp), GFP_KERNEL);
870         if (!WARN_ON_ONCE(!rhp))
871                 kfree_rcu(rhp, rh);
872 }
873 
874 void rcu_early_boot_tests(void)
875 {
876         pr_info("Running RCU self tests\n");
877 
878         if (rcu_self_test)
879                 early_boot_test_call_rcu();
880         rcu_test_sync_prims();
881 }
882 
883 static int rcu_verify_early_boot_tests(void)
884 {
885         int ret = 0;
886         int early_boot_test_counter = 0;
887 
888         if (rcu_self_test) {
889                 early_boot_test_counter++;
890                 rcu_barrier();
891                 if (IS_ENABLED(CONFIG_SRCU)) {
892                         early_boot_test_counter++;
893                         srcu_barrier(&early_srcu);
894                 }
895         }
896         if (rcu_self_test_counter != early_boot_test_counter) {
897                 WARN_ON(1);
898                 ret = -1;
899         }
900 
901         return ret;
902 }
903 late_initcall(rcu_verify_early_boot_tests);
904 #else
905 void rcu_early_boot_tests(void) {}
906 #endif /* CONFIG_PROVE_RCU */
907 
908 #ifndef CONFIG_TINY_RCU
909 
910 /*
911  * Print any significant non-default boot-time settings.
912  */
913 void __init rcupdate_announce_bootup_oddness(void)
914 {
915         if (rcu_normal)
916                 pr_info("\tNo expedited grace period (rcu_normal).\n");
917         else if (rcu_normal_after_boot)
918                 pr_info("\tNo expedited grace period (rcu_normal_after_boot).\n");
919         else if (rcu_expedited)
920                 pr_info("\tAll grace periods are expedited (rcu_expedited).\n");
921         if (rcu_cpu_stall_suppress)
922                 pr_info("\tRCU CPU stall warnings suppressed (rcu_cpu_stall_suppress).\n");
923         if (rcu_cpu_stall_timeout != CONFIG_RCU_CPU_STALL_TIMEOUT)
924                 pr_info("\tRCU CPU stall warnings timeout set to %d (rcu_cpu_stall_timeout).\n", rcu_cpu_stall_timeout);
925         rcu_tasks_bootup_oddness();
926 }
927 
928 #endif /* #ifndef CONFIG_TINY_RCU */
929 

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