2 * Read-Copy Update mechanism for mutual exclusion
4 * This program is free software; you can redistribute it and/or modify
5 * it under the terms of the GNU General Public License as published by
6 * the Free Software Foundation; either version 2 of the License, or
7 * (at your option) any later version.
9 * This program is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 * GNU General Public License for more details.
14 * You should have received a copy of the GNU General Public License
15 * along with this program; if not, write to the Free Software
16 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
18 * Copyright IBM Corporation, 2001
20 * Author: Dipankar Sarma <dipankar@in.ibm.com>
22 * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
23 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
25 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
26 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
28 * For detailed explanation of Read-Copy Update mechanism see -
29 * http://lse.sourceforge.net/locking/rcupdate.html
33 #ifndef __LINUX_RCUPDATE_H
34 #define __LINUX_RCUPDATE_H
36 #include <linux/types.h>
37 #include <linux/cache.h>
38 #include <linux/spinlock.h>
39 #include <linux/threads.h>
40 #include <linux/cpumask.h>
41 #include <linux/seqlock.h>
42 #include <linux/lockdep.h>
43 #include <linux/completion.h>
44 #include <linux/debugobjects.h>
45 #include <linux/compiler.h>
47 #ifdef CONFIG_RCU_TORTURE_TEST
48 extern int rcutorture_runnable; /* for sysctl */
49 #endif /* #ifdef CONFIG_RCU_TORTURE_TEST */
51 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU)
52 extern void rcutorture_record_test_transition(void);
53 extern void rcutorture_record_progress(unsigned long vernum);
54 extern void do_trace_rcu_torture_read(char *rcutorturename,
55 struct rcu_head *rhp);
57 static inline void rcutorture_record_test_transition(void)
60 static inline void rcutorture_record_progress(unsigned long vernum)
63 #ifdef CONFIG_RCU_TRACE
64 extern void do_trace_rcu_torture_read(char *rcutorturename,
65 struct rcu_head *rhp);
67 #define do_trace_rcu_torture_read(rcutorturename, rhp) do { } while (0)
71 #define UINT_CMP_GE(a, b) (UINT_MAX / 2 >= (a) - (b))
72 #define UINT_CMP_LT(a, b) (UINT_MAX / 2 < (a) - (b))
73 #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
74 #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
76 /* Exported common interfaces */
78 #ifdef CONFIG_PREEMPT_RCU
81 * call_rcu() - Queue an RCU callback for invocation after a grace period.
82 * @head: structure to be used for queueing the RCU updates.
83 * @func: actual callback function to be invoked after the grace period
85 * The callback function will be invoked some time after a full grace
86 * period elapses, in other words after all pre-existing RCU read-side
87 * critical sections have completed. However, the callback function
88 * might well execute concurrently with RCU read-side critical sections
89 * that started after call_rcu() was invoked. RCU read-side critical
90 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
93 extern void call_rcu(struct rcu_head *head,
94 void (*func)(struct rcu_head *head));
96 #else /* #ifdef CONFIG_PREEMPT_RCU */
98 /* In classic RCU, call_rcu() is just call_rcu_sched(). */
99 #define call_rcu call_rcu_sched
101 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
104 * call_rcu_bh() - Queue an RCU for invocation after a quicker grace period.
105 * @head: structure to be used for queueing the RCU updates.
106 * @func: actual callback function to be invoked after the grace period
108 * The callback function will be invoked some time after a full grace
109 * period elapses, in other words after all currently executing RCU
110 * read-side critical sections have completed. call_rcu_bh() assumes
111 * that the read-side critical sections end on completion of a softirq
112 * handler. This means that read-side critical sections in process
113 * context must not be interrupted by softirqs. This interface is to be
114 * used when most of the read-side critical sections are in softirq context.
115 * RCU read-side critical sections are delimited by :
116 * - rcu_read_lock() and rcu_read_unlock(), if in interrupt context.
118 * - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context.
119 * These may be nested.
121 extern void call_rcu_bh(struct rcu_head *head,
122 void (*func)(struct rcu_head *head));
125 * call_rcu_sched() - Queue an RCU for invocation after sched grace period.
126 * @head: structure to be used for queueing the RCU updates.
127 * @func: actual callback function to be invoked after the grace period
129 * The callback function will be invoked some time after a full grace
130 * period elapses, in other words after all currently executing RCU
131 * read-side critical sections have completed. call_rcu_sched() assumes
132 * that the read-side critical sections end on enabling of preemption
133 * or on voluntary preemption.
134 * RCU read-side critical sections are delimited by :
135 * - rcu_read_lock_sched() and rcu_read_unlock_sched(),
137 * anything that disables preemption.
138 * These may be nested.
140 extern void call_rcu_sched(struct rcu_head *head,
141 void (*func)(struct rcu_head *rcu));
143 extern void synchronize_sched(void);
145 #ifdef CONFIG_PREEMPT_RCU
147 extern void __rcu_read_lock(void);
148 extern void __rcu_read_unlock(void);
149 void synchronize_rcu(void);
152 * Defined as a macro as it is a very low level header included from
153 * areas that don't even know about current. This gives the rcu_read_lock()
154 * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
155 * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
157 #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
159 #else /* #ifdef CONFIG_PREEMPT_RCU */
161 static inline void __rcu_read_lock(void)
166 static inline void __rcu_read_unlock(void)
171 static inline void synchronize_rcu(void)
176 static inline int rcu_preempt_depth(void)
181 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
183 /* Internal to kernel */
184 extern void rcu_sched_qs(int cpu);
185 extern void rcu_bh_qs(int cpu);
186 extern void rcu_check_callbacks(int cpu, int user);
187 struct notifier_block;
188 extern void rcu_idle_enter(void);
189 extern void rcu_idle_exit(void);
190 extern void rcu_irq_enter(void);
191 extern void rcu_irq_exit(void);
194 * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
195 * @a: Code that RCU needs to pay attention to.
197 * RCU, RCU-bh, and RCU-sched read-side critical sections are forbidden
198 * in the inner idle loop, that is, between the rcu_idle_enter() and
199 * the rcu_idle_exit() -- RCU will happily ignore any such read-side
200 * critical sections. However, things like powertop need tracepoints
201 * in the inner idle loop.
203 * This macro provides the way out: RCU_NONIDLE(do_something_with_RCU())
204 * will tell RCU that it needs to pay attending, invoke its argument
205 * (in this example, a call to the do_something_with_RCU() function),
206 * and then tell RCU to go back to ignoring this CPU. It is permissible
207 * to nest RCU_NONIDLE() wrappers, but the nesting level is currently
208 * quite limited. If deeper nesting is required, it will be necessary
209 * to adjust DYNTICK_TASK_NESTING_VALUE accordingly.
211 * This macro may be used from process-level code only.
213 #define RCU_NONIDLE(a) \
216 do { a; } while (0); \
221 * Infrastructure to implement the synchronize_() primitives in
222 * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
225 typedef void call_rcu_func_t(struct rcu_head *head,
226 void (*func)(struct rcu_head *head));
227 void wait_rcu_gp(call_rcu_func_t crf);
229 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU)
230 #include <linux/rcutree.h>
231 #elif defined(CONFIG_TINY_RCU) || defined(CONFIG_TINY_PREEMPT_RCU)
232 #include <linux/rcutiny.h>
234 #error "Unknown RCU implementation specified to kernel configuration"
238 * init_rcu_head_on_stack()/destroy_rcu_head_on_stack() are needed for dynamic
239 * initialization and destruction of rcu_head on the stack. rcu_head structures
240 * allocated dynamically in the heap or defined statically don't need any
243 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
244 extern void init_rcu_head_on_stack(struct rcu_head *head);
245 extern void destroy_rcu_head_on_stack(struct rcu_head *head);
246 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
247 static inline void init_rcu_head_on_stack(struct rcu_head *head)
251 static inline void destroy_rcu_head_on_stack(struct rcu_head *head)
254 #endif /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
256 #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
257 bool rcu_lockdep_current_cpu_online(void);
258 #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
259 static inline bool rcu_lockdep_current_cpu_online(void)
263 #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
265 #ifdef CONFIG_DEBUG_LOCK_ALLOC
267 #ifdef CONFIG_PROVE_RCU
268 extern int rcu_is_cpu_idle(void);
269 #else /* !CONFIG_PROVE_RCU */
270 static inline int rcu_is_cpu_idle(void)
274 #endif /* else !CONFIG_PROVE_RCU */
276 static inline void rcu_lock_acquire(struct lockdep_map *map)
278 lock_acquire(map, 0, 0, 2, 1, NULL, _THIS_IP_);
281 static inline void rcu_lock_release(struct lockdep_map *map)
283 lock_release(map, 1, _THIS_IP_);
286 extern struct lockdep_map rcu_lock_map;
287 extern struct lockdep_map rcu_bh_lock_map;
288 extern struct lockdep_map rcu_sched_lock_map;
289 extern int debug_lockdep_rcu_enabled(void);
292 * rcu_read_lock_held() - might we be in RCU read-side critical section?
294 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU
295 * read-side critical section. In absence of CONFIG_DEBUG_LOCK_ALLOC,
296 * this assumes we are in an RCU read-side critical section unless it can
297 * prove otherwise. This is useful for debug checks in functions that
298 * require that they be called within an RCU read-side critical section.
300 * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
301 * and while lockdep is disabled.
303 * Note that rcu_read_lock() and the matching rcu_read_unlock() must
304 * occur in the same context, for example, it is illegal to invoke
305 * rcu_read_unlock() in process context if the matching rcu_read_lock()
306 * was invoked from within an irq handler.
308 * Note that rcu_read_lock() is disallowed if the CPU is either idle or
309 * offline from an RCU perspective, so check for those as well.
311 static inline int rcu_read_lock_held(void)
313 if (!debug_lockdep_rcu_enabled())
315 if (rcu_is_cpu_idle())
317 if (!rcu_lockdep_current_cpu_online())
319 return lock_is_held(&rcu_lock_map);
323 * rcu_read_lock_bh_held() is defined out of line to avoid #include-file
326 extern int rcu_read_lock_bh_held(void);
329 * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
331 * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
332 * RCU-sched read-side critical section. In absence of
333 * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
334 * critical section unless it can prove otherwise. Note that disabling
335 * of preemption (including disabling irqs) counts as an RCU-sched
336 * read-side critical section. This is useful for debug checks in functions
337 * that required that they be called within an RCU-sched read-side
340 * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
341 * and while lockdep is disabled.
343 * Note that if the CPU is in the idle loop from an RCU point of
344 * view (ie: that we are in the section between rcu_idle_enter() and
345 * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU
346 * did an rcu_read_lock(). The reason for this is that RCU ignores CPUs
347 * that are in such a section, considering these as in extended quiescent
348 * state, so such a CPU is effectively never in an RCU read-side critical
349 * section regardless of what RCU primitives it invokes. This state of
350 * affairs is required --- we need to keep an RCU-free window in idle
351 * where the CPU may possibly enter into low power mode. This way we can
352 * notice an extended quiescent state to other CPUs that started a grace
353 * period. Otherwise we would delay any grace period as long as we run in
356 * Similarly, we avoid claiming an SRCU read lock held if the current
359 #ifdef CONFIG_PREEMPT_COUNT
360 static inline int rcu_read_lock_sched_held(void)
362 int lockdep_opinion = 0;
364 if (!debug_lockdep_rcu_enabled())
366 if (rcu_is_cpu_idle())
368 if (!rcu_lockdep_current_cpu_online())
371 lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
372 return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
374 #else /* #ifdef CONFIG_PREEMPT_COUNT */
375 static inline int rcu_read_lock_sched_held(void)
379 #endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
381 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
383 # define rcu_lock_acquire(a) do { } while (0)
384 # define rcu_lock_release(a) do { } while (0)
386 static inline int rcu_read_lock_held(void)
391 static inline int rcu_read_lock_bh_held(void)
396 #ifdef CONFIG_PREEMPT_COUNT
397 static inline int rcu_read_lock_sched_held(void)
399 return preempt_count() != 0 || irqs_disabled();
401 #else /* #ifdef CONFIG_PREEMPT_COUNT */
402 static inline int rcu_read_lock_sched_held(void)
406 #endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
408 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
410 #ifdef CONFIG_PROVE_RCU
412 extern int rcu_my_thread_group_empty(void);
415 * rcu_lockdep_assert - emit lockdep splat if specified condition not met
416 * @c: condition to check
417 * @s: informative message
419 #define rcu_lockdep_assert(c, s) \
421 static bool __section(.data.unlikely) __warned; \
422 if (debug_lockdep_rcu_enabled() && !__warned && !(c)) { \
424 lockdep_rcu_suspicious(__FILE__, __LINE__, s); \
428 #if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
429 static inline void rcu_preempt_sleep_check(void)
431 rcu_lockdep_assert(!lock_is_held(&rcu_lock_map),
432 "Illegal context switch in RCU read-side "
435 #else /* #ifdef CONFIG_PROVE_RCU */
436 static inline void rcu_preempt_sleep_check(void)
439 #endif /* #else #ifdef CONFIG_PROVE_RCU */
441 #define rcu_sleep_check() \
443 rcu_preempt_sleep_check(); \
444 rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map), \
445 "Illegal context switch in RCU-bh" \
446 " read-side critical section"); \
447 rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map), \
448 "Illegal context switch in RCU-sched"\
449 " read-side critical section"); \
452 #else /* #ifdef CONFIG_PROVE_RCU */
454 #define rcu_lockdep_assert(c, s) do { } while (0)
455 #define rcu_sleep_check() do { } while (0)
457 #endif /* #else #ifdef CONFIG_PROVE_RCU */
460 * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
461 * and rcu_assign_pointer(). Some of these could be folded into their
462 * callers, but they are left separate in order to ease introduction of
463 * multiple flavors of pointers to match the multiple flavors of RCU
464 * (e.g., __rcu_bh, * __rcu_sched, and __srcu), should this make sense in
469 #define rcu_dereference_sparse(p, space) \
470 ((void)(((typeof(*p) space *)p) == p))
471 #else /* #ifdef __CHECKER__ */
472 #define rcu_dereference_sparse(p, space)
473 #endif /* #else #ifdef __CHECKER__ */
475 #define __rcu_access_pointer(p, space) \
477 typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
478 rcu_dereference_sparse(p, space); \
479 ((typeof(*p) __force __kernel *)(_________p1)); \
481 #define __rcu_dereference_check(p, c, space) \
483 typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
484 rcu_lockdep_assert(c, "suspicious rcu_dereference_check()" \
486 rcu_dereference_sparse(p, space); \
487 smp_read_barrier_depends(); \
488 ((typeof(*p) __force __kernel *)(_________p1)); \
490 #define __rcu_dereference_protected(p, c, space) \
492 rcu_lockdep_assert(c, "suspicious rcu_dereference_protected()" \
494 rcu_dereference_sparse(p, space); \
495 ((typeof(*p) __force __kernel *)(p)); \
498 #define __rcu_access_index(p, space) \
500 typeof(p) _________p1 = ACCESS_ONCE(p); \
501 rcu_dereference_sparse(p, space); \
504 #define __rcu_dereference_index_check(p, c) \
506 typeof(p) _________p1 = ACCESS_ONCE(p); \
507 rcu_lockdep_assert(c, \
508 "suspicious rcu_dereference_index_check()" \
510 smp_read_barrier_depends(); \
513 #define __rcu_assign_pointer(p, v, space) \
516 (p) = (typeof(*v) __force space *)(v); \
521 * rcu_access_pointer() - fetch RCU pointer with no dereferencing
522 * @p: The pointer to read
524 * Return the value of the specified RCU-protected pointer, but omit the
525 * smp_read_barrier_depends() and keep the ACCESS_ONCE(). This is useful
526 * when the value of this pointer is accessed, but the pointer is not
527 * dereferenced, for example, when testing an RCU-protected pointer against
528 * NULL. Although rcu_access_pointer() may also be used in cases where
529 * update-side locks prevent the value of the pointer from changing, you
530 * should instead use rcu_dereference_protected() for this use case.
532 * It is also permissible to use rcu_access_pointer() when read-side
533 * access to the pointer was removed at least one grace period ago, as
534 * is the case in the context of the RCU callback that is freeing up
535 * the data, or after a synchronize_rcu() returns. This can be useful
536 * when tearing down multi-linked structures after a grace period
539 #define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
542 * rcu_dereference_check() - rcu_dereference with debug checking
543 * @p: The pointer to read, prior to dereferencing
544 * @c: The conditions under which the dereference will take place
546 * Do an rcu_dereference(), but check that the conditions under which the
547 * dereference will take place are correct. Typically the conditions
548 * indicate the various locking conditions that should be held at that
549 * point. The check should return true if the conditions are satisfied.
550 * An implicit check for being in an RCU read-side critical section
551 * (rcu_read_lock()) is included.
555 * bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
557 * could be used to indicate to lockdep that foo->bar may only be dereferenced
558 * if either rcu_read_lock() is held, or that the lock required to replace
559 * the bar struct at foo->bar is held.
561 * Note that the list of conditions may also include indications of when a lock
562 * need not be held, for example during initialisation or destruction of the
565 * bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
566 * atomic_read(&foo->usage) == 0);
568 * Inserts memory barriers on architectures that require them
569 * (currently only the Alpha), prevents the compiler from refetching
570 * (and from merging fetches), and, more importantly, documents exactly
571 * which pointers are protected by RCU and checks that the pointer is
572 * annotated as __rcu.
574 #define rcu_dereference_check(p, c) \
575 __rcu_dereference_check((p), rcu_read_lock_held() || (c), __rcu)
578 * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
579 * @p: The pointer to read, prior to dereferencing
580 * @c: The conditions under which the dereference will take place
582 * This is the RCU-bh counterpart to rcu_dereference_check().
584 #define rcu_dereference_bh_check(p, c) \
585 __rcu_dereference_check((p), rcu_read_lock_bh_held() || (c), __rcu)
588 * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
589 * @p: The pointer to read, prior to dereferencing
590 * @c: The conditions under which the dereference will take place
592 * This is the RCU-sched counterpart to rcu_dereference_check().
594 #define rcu_dereference_sched_check(p, c) \
595 __rcu_dereference_check((p), rcu_read_lock_sched_held() || (c), \
598 #define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ needed? @@@*/
601 * rcu_access_index() - fetch RCU index with no dereferencing
602 * @p: The index to read
604 * Return the value of the specified RCU-protected index, but omit the
605 * smp_read_barrier_depends() and keep the ACCESS_ONCE(). This is useful
606 * when the value of this index is accessed, but the index is not
607 * dereferenced, for example, when testing an RCU-protected index against
608 * -1. Although rcu_access_index() may also be used in cases where
609 * update-side locks prevent the value of the index from changing, you
610 * should instead use rcu_dereference_index_protected() for this use case.
612 #define rcu_access_index(p) __rcu_access_index((p), __rcu)
615 * rcu_dereference_index_check() - rcu_dereference for indices with debug checking
616 * @p: The pointer to read, prior to dereferencing
617 * @c: The conditions under which the dereference will take place
619 * Similar to rcu_dereference_check(), but omits the sparse checking.
620 * This allows rcu_dereference_index_check() to be used on integers,
621 * which can then be used as array indices. Attempting to use
622 * rcu_dereference_check() on an integer will give compiler warnings
623 * because the sparse address-space mechanism relies on dereferencing
624 * the RCU-protected pointer. Dereferencing integers is not something
625 * that even gcc will put up with.
627 * Note that this function does not implicitly check for RCU read-side
628 * critical sections. If this function gains lots of uses, it might
629 * make sense to provide versions for each flavor of RCU, but it does
630 * not make sense as of early 2010.
632 #define rcu_dereference_index_check(p, c) \
633 __rcu_dereference_index_check((p), (c))
636 * rcu_dereference_protected() - fetch RCU pointer when updates prevented
637 * @p: The pointer to read, prior to dereferencing
638 * @c: The conditions under which the dereference will take place
640 * Return the value of the specified RCU-protected pointer, but omit
641 * both the smp_read_barrier_depends() and the ACCESS_ONCE(). This
642 * is useful in cases where update-side locks prevent the value of the
643 * pointer from changing. Please note that this primitive does -not-
644 * prevent the compiler from repeating this reference or combining it
645 * with other references, so it should not be used without protection
646 * of appropriate locks.
648 * This function is only for update-side use. Using this function
649 * when protected only by rcu_read_lock() will result in infrequent
650 * but very ugly failures.
652 #define rcu_dereference_protected(p, c) \
653 __rcu_dereference_protected((p), (c), __rcu)
657 * rcu_dereference() - fetch RCU-protected pointer for dereferencing
658 * @p: The pointer to read, prior to dereferencing
660 * This is a simple wrapper around rcu_dereference_check().
662 #define rcu_dereference(p) rcu_dereference_check(p, 0)
665 * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
666 * @p: The pointer to read, prior to dereferencing
668 * Makes rcu_dereference_check() do the dirty work.
670 #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
673 * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
674 * @p: The pointer to read, prior to dereferencing
676 * Makes rcu_dereference_check() do the dirty work.
678 #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
681 * rcu_read_lock() - mark the beginning of an RCU read-side critical section
683 * When synchronize_rcu() is invoked on one CPU while other CPUs
684 * are within RCU read-side critical sections, then the
685 * synchronize_rcu() is guaranteed to block until after all the other
686 * CPUs exit their critical sections. Similarly, if call_rcu() is invoked
687 * on one CPU while other CPUs are within RCU read-side critical
688 * sections, invocation of the corresponding RCU callback is deferred
689 * until after the all the other CPUs exit their critical sections.
691 * Note, however, that RCU callbacks are permitted to run concurrently
692 * with new RCU read-side critical sections. One way that this can happen
693 * is via the following sequence of events: (1) CPU 0 enters an RCU
694 * read-side critical section, (2) CPU 1 invokes call_rcu() to register
695 * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
696 * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
697 * callback is invoked. This is legal, because the RCU read-side critical
698 * section that was running concurrently with the call_rcu() (and which
699 * therefore might be referencing something that the corresponding RCU
700 * callback would free up) has completed before the corresponding
701 * RCU callback is invoked.
703 * RCU read-side critical sections may be nested. Any deferred actions
704 * will be deferred until the outermost RCU read-side critical section
707 * You can avoid reading and understanding the next paragraph by
708 * following this rule: don't put anything in an rcu_read_lock() RCU
709 * read-side critical section that would block in a !PREEMPT kernel.
710 * But if you want the full story, read on!
712 * In non-preemptible RCU implementations (TREE_RCU and TINY_RCU), it
713 * is illegal to block while in an RCU read-side critical section. In
714 * preemptible RCU implementations (TREE_PREEMPT_RCU and TINY_PREEMPT_RCU)
715 * in CONFIG_PREEMPT kernel builds, RCU read-side critical sections may
716 * be preempted, but explicit blocking is illegal. Finally, in preemptible
717 * RCU implementations in real-time (CONFIG_PREEMPT_RT) kernel builds,
718 * RCU read-side critical sections may be preempted and they may also
719 * block, but only when acquiring spinlocks that are subject to priority
722 static inline void rcu_read_lock(void)
726 rcu_lock_acquire(&rcu_lock_map);
727 rcu_lockdep_assert(!rcu_is_cpu_idle(),
728 "rcu_read_lock() used illegally while idle");
732 * So where is rcu_write_lock()? It does not exist, as there is no
733 * way for writers to lock out RCU readers. This is a feature, not
734 * a bug -- this property is what provides RCU's performance benefits.
735 * Of course, writers must coordinate with each other. The normal
736 * spinlock primitives work well for this, but any other technique may be
737 * used as well. RCU does not care how the writers keep out of each
738 * others' way, as long as they do so.
742 * rcu_read_unlock() - marks the end of an RCU read-side critical section.
744 * See rcu_read_lock() for more information.
746 static inline void rcu_read_unlock(void)
748 rcu_lockdep_assert(!rcu_is_cpu_idle(),
749 "rcu_read_unlock() used illegally while idle");
750 rcu_lock_release(&rcu_lock_map);
756 * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
758 * This is equivalent of rcu_read_lock(), but to be used when updates
759 * are being done using call_rcu_bh() or synchronize_rcu_bh(). Since
760 * both call_rcu_bh() and synchronize_rcu_bh() consider completion of a
761 * softirq handler to be a quiescent state, a process in RCU read-side
762 * critical section must be protected by disabling softirqs. Read-side
763 * critical sections in interrupt context can use just rcu_read_lock(),
764 * though this should at least be commented to avoid confusing people
767 * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
768 * must occur in the same context, for example, it is illegal to invoke
769 * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
770 * was invoked from some other task.
772 static inline void rcu_read_lock_bh(void)
776 rcu_lock_acquire(&rcu_bh_lock_map);
777 rcu_lockdep_assert(!rcu_is_cpu_idle(),
778 "rcu_read_lock_bh() used illegally while idle");
782 * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
784 * See rcu_read_lock_bh() for more information.
786 static inline void rcu_read_unlock_bh(void)
788 rcu_lockdep_assert(!rcu_is_cpu_idle(),
789 "rcu_read_unlock_bh() used illegally while idle");
790 rcu_lock_release(&rcu_bh_lock_map);
796 * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
798 * This is equivalent of rcu_read_lock(), but to be used when updates
799 * are being done using call_rcu_sched() or synchronize_rcu_sched().
800 * Read-side critical sections can also be introduced by anything that
801 * disables preemption, including local_irq_disable() and friends.
803 * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
804 * must occur in the same context, for example, it is illegal to invoke
805 * rcu_read_unlock_sched() from process context if the matching
806 * rcu_read_lock_sched() was invoked from an NMI handler.
808 static inline void rcu_read_lock_sched(void)
811 __acquire(RCU_SCHED);
812 rcu_lock_acquire(&rcu_sched_lock_map);
813 rcu_lockdep_assert(!rcu_is_cpu_idle(),
814 "rcu_read_lock_sched() used illegally while idle");
817 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
818 static inline notrace void rcu_read_lock_sched_notrace(void)
820 preempt_disable_notrace();
821 __acquire(RCU_SCHED);
825 * rcu_read_unlock_sched - marks the end of a RCU-classic critical section
827 * See rcu_read_lock_sched for more information.
829 static inline void rcu_read_unlock_sched(void)
831 rcu_lockdep_assert(!rcu_is_cpu_idle(),
832 "rcu_read_unlock_sched() used illegally while idle");
833 rcu_lock_release(&rcu_sched_lock_map);
834 __release(RCU_SCHED);
838 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
839 static inline notrace void rcu_read_unlock_sched_notrace(void)
841 __release(RCU_SCHED);
842 preempt_enable_notrace();
846 * rcu_assign_pointer() - assign to RCU-protected pointer
847 * @p: pointer to assign to
848 * @v: value to assign (publish)
850 * Assigns the specified value to the specified RCU-protected
851 * pointer, ensuring that any concurrent RCU readers will see
852 * any prior initialization. Returns the value assigned.
854 * Inserts memory barriers on architectures that require them
855 * (which is most of them), and also prevents the compiler from
856 * reordering the code that initializes the structure after the pointer
857 * assignment. More importantly, this call documents which pointers
858 * will be dereferenced by RCU read-side code.
860 * In some special cases, you may use RCU_INIT_POINTER() instead
861 * of rcu_assign_pointer(). RCU_INIT_POINTER() is a bit faster due
862 * to the fact that it does not constrain either the CPU or the compiler.
863 * That said, using RCU_INIT_POINTER() when you should have used
864 * rcu_assign_pointer() is a very bad thing that results in
865 * impossible-to-diagnose memory corruption. So please be careful.
866 * See the RCU_INIT_POINTER() comment header for details.
868 #define rcu_assign_pointer(p, v) \
869 __rcu_assign_pointer((p), (v), __rcu)
872 * RCU_INIT_POINTER() - initialize an RCU protected pointer
874 * Initialize an RCU-protected pointer in special cases where readers
875 * do not need ordering constraints on the CPU or the compiler. These
878 * 1. This use of RCU_INIT_POINTER() is NULLing out the pointer -or-
879 * 2. The caller has taken whatever steps are required to prevent
880 * RCU readers from concurrently accessing this pointer -or-
881 * 3. The referenced data structure has already been exposed to
882 * readers either at compile time or via rcu_assign_pointer() -and-
883 * a. You have not made -any- reader-visible changes to
884 * this structure since then -or-
885 * b. It is OK for readers accessing this structure from its
886 * new location to see the old state of the structure. (For
887 * example, the changes were to statistical counters or to
888 * other state where exact synchronization is not required.)
890 * Failure to follow these rules governing use of RCU_INIT_POINTER() will
891 * result in impossible-to-diagnose memory corruption. As in the structures
892 * will look OK in crash dumps, but any concurrent RCU readers might
893 * see pre-initialized values of the referenced data structure. So
894 * please be very careful how you use RCU_INIT_POINTER()!!!
896 * If you are creating an RCU-protected linked structure that is accessed
897 * by a single external-to-structure RCU-protected pointer, then you may
898 * use RCU_INIT_POINTER() to initialize the internal RCU-protected
899 * pointers, but you must use rcu_assign_pointer() to initialize the
900 * external-to-structure pointer -after- you have completely initialized
901 * the reader-accessible portions of the linked structure.
903 #define RCU_INIT_POINTER(p, v) \
904 p = (typeof(*v) __force __rcu *)(v)
906 static __always_inline bool __is_kfree_rcu_offset(unsigned long offset)
908 return offset < 4096;
911 static __always_inline
912 void __kfree_rcu(struct rcu_head *head, unsigned long offset)
914 typedef void (*rcu_callback)(struct rcu_head *);
916 BUILD_BUG_ON(!__builtin_constant_p(offset));
918 /* See the kfree_rcu() header comment. */
919 BUILD_BUG_ON(!__is_kfree_rcu_offset(offset));
921 kfree_call_rcu(head, (rcu_callback)offset);
925 * kfree_rcu() - kfree an object after a grace period.
926 * @ptr: pointer to kfree
927 * @rcu_head: the name of the struct rcu_head within the type of @ptr.
929 * Many rcu callbacks functions just call kfree() on the base structure.
930 * These functions are trivial, but their size adds up, and furthermore
931 * when they are used in a kernel module, that module must invoke the
932 * high-latency rcu_barrier() function at module-unload time.
934 * The kfree_rcu() function handles this issue. Rather than encoding a
935 * function address in the embedded rcu_head structure, kfree_rcu() instead
936 * encodes the offset of the rcu_head structure within the base structure.
937 * Because the functions are not allowed in the low-order 4096 bytes of
938 * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
939 * If the offset is larger than 4095 bytes, a compile-time error will
940 * be generated in __kfree_rcu(). If this error is triggered, you can
941 * either fall back to use of call_rcu() or rearrange the structure to
942 * position the rcu_head structure into the first 4096 bytes.
944 * Note that the allowable offset might decrease in the future, for example,
945 * to allow something like kmem_cache_free_rcu().
947 #define kfree_rcu(ptr, rcu_head) \
948 __kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
950 #endif /* __LINUX_RCUPDATE_H */