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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 : :
44 : : #define CREATE_TRACE_POINTS
45 : :
46 : : #include "rcu.h"
47 : :
48 : : #ifdef MODULE_PARAM_PREFIX
49 : : #undef MODULE_PARAM_PREFIX
50 : : #endif
51 : : #define MODULE_PARAM_PREFIX "rcupdate."
52 : :
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 */
61 : :
62 : : #ifdef CONFIG_DEBUG_LOCK_ALLOC
63 : : /**
64 : : * rcu_read_lock_held_common() - might we be in RCU-sched read-side critical section?
65 : : * @ret: Best guess answer if lockdep cannot be relied on
66 : : *
67 : : * Returns true if lockdep must be ignored, in which case *ret contains
68 : : * the best guess described below. Otherwise returns false, in which
69 : : * case *ret tells the caller nothing and the caller should instead
70 : : * consult lockdep.
71 : : *
72 : : * If CONFIG_DEBUG_LOCK_ALLOC is selected, set *ret to nonzero iff in an
73 : : * RCU-sched read-side critical section. In absence of
74 : : * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
75 : : * critical section unless it can prove otherwise. Note that disabling
76 : : * of preemption (including disabling irqs) counts as an RCU-sched
77 : : * read-side critical section. This is useful for debug checks in functions
78 : : * that required that they be called within an RCU-sched read-side
79 : : * critical section.
80 : : *
81 : : * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
82 : : * and while lockdep is disabled.
83 : : *
84 : : * Note that if the CPU is in the idle loop from an RCU point of view (ie:
85 : : * that we are in the section between rcu_idle_enter() and rcu_idle_exit())
86 : : * then rcu_read_lock_held() sets *ret to false even if the CPU did an
87 : : * rcu_read_lock(). The reason for this is that RCU ignores CPUs that are
88 : : * in such a section, considering these as in extended quiescent state,
89 : : * so such a CPU is effectively never in an RCU read-side critical section
90 : : * regardless of what RCU primitives it invokes. This state of affairs is
91 : : * required --- we need to keep an RCU-free window in idle where the CPU may
92 : : * possibly enter into low power mode. This way we can notice an extended
93 : : * quiescent state to other CPUs that started a grace period. Otherwise
94 : : * we would delay any grace period as long as we run in the idle task.
95 : : *
96 : : * Similarly, we avoid claiming an RCU read lock held if the current
97 : : * CPU is offline.
98 : : */
99 : : static bool rcu_read_lock_held_common(bool *ret)
100 : : {
101 : : if (!debug_lockdep_rcu_enabled()) {
102 : : *ret = 1;
103 : : return true;
104 : : }
105 : : if (!rcu_is_watching()) {
106 : : *ret = 0;
107 : : return true;
108 : : }
109 : : if (!rcu_lockdep_current_cpu_online()) {
110 : : *ret = 0;
111 : : return true;
112 : : }
113 : : return false;
114 : : }
115 : :
116 : : int rcu_read_lock_sched_held(void)
117 : : {
118 : : bool ret;
119 : :
120 : : if (rcu_read_lock_held_common(&ret))
121 : : return ret;
122 : : return lock_is_held(&rcu_sched_lock_map) || !preemptible();
123 : : }
124 : : EXPORT_SYMBOL(rcu_read_lock_sched_held);
125 : : #endif
126 : :
127 : : #ifndef CONFIG_TINY_RCU
128 : :
129 : : /*
130 : : * Should expedited grace-period primitives always fall back to their
131 : : * non-expedited counterparts? Intended for use within RCU. Note
132 : : * that if the user specifies both rcu_expedited and rcu_normal, then
133 : : * rcu_normal wins. (Except during the time period during boot from
134 : : * when the first task is spawned until the rcu_set_runtime_mode()
135 : : * core_initcall() is invoked, at which point everything is expedited.)
136 : : */
137 : 3 : bool rcu_gp_is_normal(void)
138 : : {
139 : 3 : return READ_ONCE(rcu_normal) &&
140 : 0 : rcu_scheduler_active != RCU_SCHEDULER_INIT;
141 : : }
142 : : EXPORT_SYMBOL_GPL(rcu_gp_is_normal);
143 : :
144 : : static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);
145 : :
146 : : /*
147 : : * Should normal grace-period primitives be expedited? Intended for
148 : : * use within RCU. Note that this function takes the rcu_expedited
149 : : * sysfs/boot variable and rcu_scheduler_active into account as well
150 : : * as the rcu_expedite_gp() nesting. So looping on rcu_unexpedite_gp()
151 : : * until rcu_gp_is_expedited() returns false is a -really- bad idea.
152 : : */
153 : 3 : bool rcu_gp_is_expedited(void)
154 : : {
155 : 3 : return rcu_expedited || atomic_read(&rcu_expedited_nesting);
156 : : }
157 : : EXPORT_SYMBOL_GPL(rcu_gp_is_expedited);
158 : :
159 : : /**
160 : : * rcu_expedite_gp - Expedite future RCU grace periods
161 : : *
162 : : * After a call to this function, future calls to synchronize_rcu() and
163 : : * friends act as the corresponding synchronize_rcu_expedited() function
164 : : * had instead been called.
165 : : */
166 : 0 : void rcu_expedite_gp(void)
167 : : {
168 : : atomic_inc(&rcu_expedited_nesting);
169 : 0 : }
170 : : EXPORT_SYMBOL_GPL(rcu_expedite_gp);
171 : :
172 : : /**
173 : : * rcu_unexpedite_gp - Cancel prior rcu_expedite_gp() invocation
174 : : *
175 : : * Undo a prior call to rcu_expedite_gp(). If all prior calls to
176 : : * rcu_expedite_gp() are undone by a subsequent call to rcu_unexpedite_gp(),
177 : : * and if the rcu_expedited sysfs/boot parameter is not set, then all
178 : : * subsequent calls to synchronize_rcu() and friends will return to
179 : : * their normal non-expedited behavior.
180 : : */
181 : 3 : void rcu_unexpedite_gp(void)
182 : : {
183 : : atomic_dec(&rcu_expedited_nesting);
184 : 3 : }
185 : : EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
186 : :
187 : : /*
188 : : * Inform RCU of the end of the in-kernel boot sequence.
189 : : */
190 : 3 : void rcu_end_inkernel_boot(void)
191 : : {
192 : 3 : rcu_unexpedite_gp();
193 : 3 : if (rcu_normal_after_boot)
194 : : WRITE_ONCE(rcu_normal, 1);
195 : 3 : }
196 : :
197 : : #endif /* #ifndef CONFIG_TINY_RCU */
198 : :
199 : : /*
200 : : * Test each non-SRCU synchronous grace-period wait API. This is
201 : : * useful just after a change in mode for these primitives, and
202 : : * during early boot.
203 : : */
204 : 3 : void rcu_test_sync_prims(void)
205 : : {
206 : : if (!IS_ENABLED(CONFIG_PROVE_RCU))
207 : 3 : return;
208 : : synchronize_rcu();
209 : : synchronize_rcu_expedited();
210 : : }
211 : :
212 : : #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU)
213 : :
214 : : /*
215 : : * Switch to run-time mode once RCU has fully initialized.
216 : : */
217 : 3 : static int __init rcu_set_runtime_mode(void)
218 : : {
219 : : rcu_test_sync_prims();
220 : 3 : rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
221 : : rcu_test_sync_prims();
222 : 3 : 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 : 3 : 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 : 3 : complete(&rcu->completion);
340 : 3 : }
341 : : EXPORT_SYMBOL_GPL(wakeme_after_rcu);
342 : :
343 : 3 : 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 : 3 : for (i = 0; i < n; i++) {
351 : 3 : if (checktiny &&
352 : 0 : (crcu_array[i] == call_rcu)) {
353 : 0 : might_sleep();
354 : 0 : continue;
355 : : }
356 : : init_rcu_head_on_stack(&rs_array[i].head);
357 : 3 : init_completion(&rs_array[i].completion);
358 : 3 : for (j = 0; j < i; j++)
359 : 0 : if (crcu_array[j] == crcu_array[i])
360 : : break;
361 : 3 : if (j == i)
362 : 3 : (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
363 : : }
364 : :
365 : : /* Wait for all callbacks to be invoked. */
366 : 3 : for (i = 0; i < n; i++) {
367 : 3 : if (checktiny &&
368 : 0 : (crcu_array[i] == call_rcu))
369 : 0 : continue;
370 : 0 : for (j = 0; j < i; j++)
371 : 0 : if (crcu_array[j] == crcu_array[i])
372 : : break;
373 : 3 : if (j == i)
374 : 3 : wait_for_completion(&rs_array[i].completion);
375 : : destroy_rcu_head_on_stack(&rs_array[i].head);
376 : : }
377 : 3 : }
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_PREEMPT_RCU) || defined(CONFIG_RCU_TRACE)
439 : 0 : 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 : 0 : }
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 : : static void early_boot_test_call_rcu(void)
857 : : {
858 : : static struct rcu_head head;
859 : : static struct rcu_head shead;
860 : :
861 : : call_rcu(&head, test_callback);
862 : : if (IS_ENABLED(CONFIG_SRCU))
863 : : call_srcu(&early_srcu, &shead, test_callback);
864 : : }
865 : :
866 : : void rcu_early_boot_tests(void)
867 : : {
868 : : pr_info("Running RCU self tests\n");
869 : :
870 : : if (rcu_self_test)
871 : : early_boot_test_call_rcu();
872 : : rcu_test_sync_prims();
873 : : }
874 : :
875 : : static int rcu_verify_early_boot_tests(void)
876 : : {
877 : : int ret = 0;
878 : : int early_boot_test_counter = 0;
879 : :
880 : : if (rcu_self_test) {
881 : : early_boot_test_counter++;
882 : : rcu_barrier();
883 : : if (IS_ENABLED(CONFIG_SRCU)) {
884 : : early_boot_test_counter++;
885 : : srcu_barrier(&early_srcu);
886 : : }
887 : : }
888 : : if (rcu_self_test_counter != early_boot_test_counter) {
889 : : WARN_ON(1);
890 : : ret = -1;
891 : : }
892 : :
893 : : return ret;
894 : : }
895 : : late_initcall(rcu_verify_early_boot_tests);
896 : : #else
897 : 3 : void rcu_early_boot_tests(void) {}
898 : : #endif /* CONFIG_PROVE_RCU */
899 : :
900 : : #ifndef CONFIG_TINY_RCU
901 : :
902 : : /*
903 : : * Print any significant non-default boot-time settings.
904 : : */
905 : 3 : void __init rcupdate_announce_bootup_oddness(void)
906 : : {
907 : 3 : if (rcu_normal)
908 : 0 : pr_info("\tNo expedited grace period (rcu_normal).\n");
909 : 3 : else if (rcu_normal_after_boot)
910 : 0 : pr_info("\tNo expedited grace period (rcu_normal_after_boot).\n");
911 : 3 : else if (rcu_expedited)
912 : 0 : pr_info("\tAll grace periods are expedited (rcu_expedited).\n");
913 : 3 : if (rcu_cpu_stall_suppress)
914 : 0 : pr_info("\tRCU CPU stall warnings suppressed (rcu_cpu_stall_suppress).\n");
915 : 3 : if (rcu_cpu_stall_timeout != CONFIG_RCU_CPU_STALL_TIMEOUT)
916 : 0 : pr_info("\tRCU CPU stall warnings timeout set to %d (rcu_cpu_stall_timeout).\n", rcu_cpu_stall_timeout);
917 : : rcu_tasks_bootup_oddness();
918 : 3 : }
919 : :
920 : : #endif /* #ifndef CONFIG_TINY_RCU */
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