2 * Read-Copy Update mechanism for mutual exclusion
4 * This program is free software; you can redistribute it and/or modify
5 * it under the terms of the GNU General Public License as published by
6 * the Free Software Foundation; either version 2 of the License, or
7 * (at your option) any later version.
9 * This program is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 * GNU General Public License for more details.
14 * You should have received a copy of the GNU General Public License
15 * along with this program; if not, you can access it online at
16 * http://www.gnu.org/licenses/gpl-2.0.html.
18 * Copyright IBM Corporation, 2001
20 * 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/compiler.h>
38 #include <linux/atomic.h>
39 #include <linux/irqflags.h>
40 #include <linux/preempt.h>
41 #include <linux/bottom_half.h>
42 #include <linux/lockdep.h>
43 #include <asm/processor.h>
44 #include <linux/cpumask.h>
46 #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
47 #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
48 #define ulong2long(a) (*(long *)(&(a)))
50 /* Exported common interfaces */
51 void call_rcu(struct rcu_head
*head
, rcu_callback_t func
);
52 void rcu_barrier_tasks(void);
53 void synchronize_rcu(void);
55 #ifdef CONFIG_PREEMPT_RCU
57 void __rcu_read_lock(void);
58 void __rcu_read_unlock(void);
61 * Defined as a macro as it is a very low level header included from
62 * areas that don't even know about current. This gives the rcu_read_lock()
63 * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
64 * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
66 #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
68 #else /* #ifdef CONFIG_PREEMPT_RCU */
70 static inline void __rcu_read_lock(void)
72 if (IS_ENABLED(CONFIG_PREEMPT_COUNT
))
76 static inline void __rcu_read_unlock(void)
78 if (IS_ENABLED(CONFIG_PREEMPT_COUNT
))
82 static inline int rcu_preempt_depth(void)
87 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
89 /* Internal to kernel */
91 extern int rcu_scheduler_active __read_mostly
;
92 void rcu_check_callbacks(int user
);
93 void rcu_report_dead(unsigned int cpu
);
94 void rcutree_migrate_callbacks(int cpu
);
96 #ifdef CONFIG_RCU_STALL_COMMON
97 void rcu_sysrq_start(void);
98 void rcu_sysrq_end(void);
99 #else /* #ifdef CONFIG_RCU_STALL_COMMON */
100 static inline void rcu_sysrq_start(void) { }
101 static inline void rcu_sysrq_end(void) { }
102 #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
104 #ifdef CONFIG_NO_HZ_FULL
105 void rcu_user_enter(void);
106 void rcu_user_exit(void);
108 static inline void rcu_user_enter(void) { }
109 static inline void rcu_user_exit(void) { }
110 #endif /* CONFIG_NO_HZ_FULL */
112 #ifdef CONFIG_RCU_NOCB_CPU
113 void rcu_init_nohz(void);
114 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
115 static inline void rcu_init_nohz(void) { }
116 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
119 * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
120 * @a: Code that RCU needs to pay attention to.
122 * RCU read-side critical sections are forbidden in the inner idle loop,
123 * that is, between the rcu_idle_enter() and the rcu_idle_exit() -- RCU
124 * will happily ignore any such read-side critical sections. However,
125 * things like powertop need tracepoints in the inner idle loop.
127 * This macro provides the way out: RCU_NONIDLE(do_something_with_RCU())
128 * will tell RCU that it needs to pay attention, invoke its argument
129 * (in this example, calling the do_something_with_RCU() function),
130 * and then tell RCU to go back to ignoring this CPU. It is permissible
131 * to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is
132 * on the order of a million or so, even on 32-bit systems). It is
133 * not legal to block within RCU_NONIDLE(), nor is it permissible to
134 * transfer control either into or out of RCU_NONIDLE()'s statement.
136 #define RCU_NONIDLE(a) \
138 rcu_irq_enter_irqson(); \
139 do { a; } while (0); \
140 rcu_irq_exit_irqson(); \
144 * Note a quasi-voluntary context switch for RCU-tasks's benefit.
145 * This is a macro rather than an inline function to avoid #include hell.
147 #ifdef CONFIG_TASKS_RCU
148 #define rcu_tasks_qs(t) \
150 if (READ_ONCE((t)->rcu_tasks_holdout)) \
151 WRITE_ONCE((t)->rcu_tasks_holdout, false); \
153 #define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t)
154 void call_rcu_tasks(struct rcu_head
*head
, rcu_callback_t func
);
155 void synchronize_rcu_tasks(void);
156 void exit_tasks_rcu_start(void);
157 void exit_tasks_rcu_finish(void);
158 #else /* #ifdef CONFIG_TASKS_RCU */
159 #define rcu_tasks_qs(t) do { } while (0)
160 #define rcu_note_voluntary_context_switch(t) do { } while (0)
161 #define call_rcu_tasks call_rcu
162 #define synchronize_rcu_tasks synchronize_rcu
163 static inline void exit_tasks_rcu_start(void) { }
164 static inline void exit_tasks_rcu_finish(void) { }
165 #endif /* #else #ifdef CONFIG_TASKS_RCU */
168 * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
170 * This macro resembles cond_resched(), except that it is defined to
171 * report potential quiescent states to RCU-tasks even if the cond_resched()
172 * machinery were to be shut off, as some advocate for PREEMPT kernels.
174 #define cond_resched_tasks_rcu_qs() \
176 rcu_tasks_qs(current); \
181 * Infrastructure to implement the synchronize_() primitives in
182 * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
185 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU)
186 #include <linux/rcutree.h>
187 #elif defined(CONFIG_TINY_RCU)
188 #include <linux/rcutiny.h>
190 #error "Unknown RCU implementation specified to kernel configuration"
194 * The init_rcu_head_on_stack() and destroy_rcu_head_on_stack() calls
195 * are needed for dynamic initialization and destruction of rcu_head
196 * on the stack, and init_rcu_head()/destroy_rcu_head() are needed for
197 * dynamic initialization and destruction of statically allocated rcu_head
198 * structures. However, rcu_head structures allocated dynamically in the
199 * heap don't need any initialization.
201 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
202 void init_rcu_head(struct rcu_head
*head
);
203 void destroy_rcu_head(struct rcu_head
*head
);
204 void init_rcu_head_on_stack(struct rcu_head
*head
);
205 void destroy_rcu_head_on_stack(struct rcu_head
*head
);
206 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
207 static inline void init_rcu_head(struct rcu_head
*head
) { }
208 static inline void destroy_rcu_head(struct rcu_head
*head
) { }
209 static inline void init_rcu_head_on_stack(struct rcu_head
*head
) { }
210 static inline void destroy_rcu_head_on_stack(struct rcu_head
*head
) { }
211 #endif /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
213 #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
214 bool rcu_lockdep_current_cpu_online(void);
215 #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
216 static inline bool rcu_lockdep_current_cpu_online(void) { return true; }
217 #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
219 #ifdef CONFIG_DEBUG_LOCK_ALLOC
221 static inline void rcu_lock_acquire(struct lockdep_map
*map
)
223 lock_acquire(map
, 0, 0, 2, 0, NULL
, _THIS_IP_
);
226 static inline void rcu_lock_release(struct lockdep_map
*map
)
228 lock_release(map
, 1, _THIS_IP_
);
231 extern struct lockdep_map rcu_lock_map
;
232 extern struct lockdep_map rcu_bh_lock_map
;
233 extern struct lockdep_map rcu_sched_lock_map
;
234 extern struct lockdep_map rcu_callback_map
;
235 int debug_lockdep_rcu_enabled(void);
236 int rcu_read_lock_held(void);
237 int rcu_read_lock_bh_held(void);
238 int rcu_read_lock_sched_held(void);
240 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
242 # define rcu_lock_acquire(a) do { } while (0)
243 # define rcu_lock_release(a) do { } while (0)
245 static inline int rcu_read_lock_held(void)
250 static inline int rcu_read_lock_bh_held(void)
255 static inline int rcu_read_lock_sched_held(void)
257 return !preemptible();
259 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
261 #ifdef CONFIG_PROVE_RCU
264 * RCU_LOCKDEP_WARN - emit lockdep splat if specified condition is met
265 * @c: condition to check
266 * @s: informative message
268 #define RCU_LOCKDEP_WARN(c, s) \
270 static bool __section(.data.unlikely) __warned; \
271 if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \
273 lockdep_rcu_suspicious(__FILE__, __LINE__, s); \
277 #if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
278 static inline void rcu_preempt_sleep_check(void)
280 RCU_LOCKDEP_WARN(lock_is_held(&rcu_lock_map
),
281 "Illegal context switch in RCU read-side critical section");
283 #else /* #ifdef CONFIG_PROVE_RCU */
284 static inline void rcu_preempt_sleep_check(void) { }
285 #endif /* #else #ifdef CONFIG_PROVE_RCU */
287 #define rcu_sleep_check() \
289 rcu_preempt_sleep_check(); \
290 RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map), \
291 "Illegal context switch in RCU-bh read-side critical section"); \
292 RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map), \
293 "Illegal context switch in RCU-sched read-side critical section"); \
296 #else /* #ifdef CONFIG_PROVE_RCU */
298 #define RCU_LOCKDEP_WARN(c, s) do { } while (0)
299 #define rcu_sleep_check() do { } while (0)
301 #endif /* #else #ifdef CONFIG_PROVE_RCU */
304 * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
305 * and rcu_assign_pointer(). Some of these could be folded into their
306 * callers, but they are left separate in order to ease introduction of
307 * multiple pointers markings to match different RCU implementations
308 * (e.g., __srcu), should this make sense in the future.
312 #define rcu_dereference_sparse(p, space) \
313 ((void)(((typeof(*p) space *)p) == p))
314 #else /* #ifdef __CHECKER__ */
315 #define rcu_dereference_sparse(p, space)
316 #endif /* #else #ifdef __CHECKER__ */
318 #define __rcu_access_pointer(p, space) \
320 typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
321 rcu_dereference_sparse(p, space); \
322 ((typeof(*p) __force __kernel *)(_________p1)); \
324 #define __rcu_dereference_check(p, c, space) \
326 /* Dependency order vs. p above. */ \
327 typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
328 RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
329 rcu_dereference_sparse(p, space); \
330 ((typeof(*p) __force __kernel *)(________p1)); \
332 #define __rcu_dereference_protected(p, c, space) \
334 RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
335 rcu_dereference_sparse(p, space); \
336 ((typeof(*p) __force __kernel *)(p)); \
338 #define rcu_dereference_raw(p) \
340 /* Dependency order vs. p above. */ \
341 typeof(p) ________p1 = READ_ONCE(p); \
342 ((typeof(*p) __force __kernel *)(________p1)); \
346 * RCU_INITIALIZER() - statically initialize an RCU-protected global variable
347 * @v: The value to statically initialize with.
349 #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
352 * rcu_assign_pointer() - assign to RCU-protected pointer
353 * @p: pointer to assign to
354 * @v: value to assign (publish)
356 * Assigns the specified value to the specified RCU-protected
357 * pointer, ensuring that any concurrent RCU readers will see
358 * any prior initialization.
360 * Inserts memory barriers on architectures that require them
361 * (which is most of them), and also prevents the compiler from
362 * reordering the code that initializes the structure after the pointer
363 * assignment. More importantly, this call documents which pointers
364 * will be dereferenced by RCU read-side code.
366 * In some special cases, you may use RCU_INIT_POINTER() instead
367 * of rcu_assign_pointer(). RCU_INIT_POINTER() is a bit faster due
368 * to the fact that it does not constrain either the CPU or the compiler.
369 * That said, using RCU_INIT_POINTER() when you should have used
370 * rcu_assign_pointer() is a very bad thing that results in
371 * impossible-to-diagnose memory corruption. So please be careful.
372 * See the RCU_INIT_POINTER() comment header for details.
374 * Note that rcu_assign_pointer() evaluates each of its arguments only
375 * once, appearances notwithstanding. One of the "extra" evaluations
376 * is in typeof() and the other visible only to sparse (__CHECKER__),
377 * neither of which actually execute the argument. As with most cpp
378 * macros, this execute-arguments-only-once property is important, so
379 * please be careful when making changes to rcu_assign_pointer() and the
380 * other macros that it invokes.
382 #define rcu_assign_pointer(p, v) \
384 uintptr_t _r_a_p__v = (uintptr_t)(v); \
386 if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL) \
387 WRITE_ONCE((p), (typeof(p))(_r_a_p__v)); \
389 smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
394 * rcu_swap_protected() - swap an RCU and a regular pointer
395 * @rcu_ptr: RCU pointer
396 * @ptr: regular pointer
397 * @c: the conditions under which the dereference will take place
399 * Perform swap(@rcu_ptr, @ptr) where @rcu_ptr is an RCU-annotated pointer and
400 * @c is the argument that is passed to the rcu_dereference_protected() call
401 * used to read that pointer.
403 #define rcu_swap_protected(rcu_ptr, ptr, c) do { \
404 typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c)); \
405 rcu_assign_pointer((rcu_ptr), (ptr)); \
410 * rcu_access_pointer() - fetch RCU pointer with no dereferencing
411 * @p: The pointer to read
413 * Return the value of the specified RCU-protected pointer, but omit the
414 * lockdep checks for being in an RCU read-side critical section. This is
415 * useful when the value of this pointer is accessed, but the pointer is
416 * not dereferenced, for example, when testing an RCU-protected pointer
417 * against NULL. Although rcu_access_pointer() may also be used in cases
418 * where update-side locks prevent the value of the pointer from changing,
419 * you should instead use rcu_dereference_protected() for this use case.
421 * It is also permissible to use rcu_access_pointer() when read-side
422 * access to the pointer was removed at least one grace period ago, as
423 * is the case in the context of the RCU callback that is freeing up
424 * the data, or after a synchronize_rcu() returns. This can be useful
425 * when tearing down multi-linked structures after a grace period
428 #define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
431 * rcu_dereference_check() - rcu_dereference with debug checking
432 * @p: The pointer to read, prior to dereferencing
433 * @c: The conditions under which the dereference will take place
435 * Do an rcu_dereference(), but check that the conditions under which the
436 * dereference will take place are correct. Typically the conditions
437 * indicate the various locking conditions that should be held at that
438 * point. The check should return true if the conditions are satisfied.
439 * An implicit check for being in an RCU read-side critical section
440 * (rcu_read_lock()) is included.
444 * bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
446 * could be used to indicate to lockdep that foo->bar may only be dereferenced
447 * if either rcu_read_lock() is held, or that the lock required to replace
448 * the bar struct at foo->bar is held.
450 * Note that the list of conditions may also include indications of when a lock
451 * need not be held, for example during initialisation or destruction of the
454 * bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
455 * atomic_read(&foo->usage) == 0);
457 * Inserts memory barriers on architectures that require them
458 * (currently only the Alpha), prevents the compiler from refetching
459 * (and from merging fetches), and, more importantly, documents exactly
460 * which pointers are protected by RCU and checks that the pointer is
461 * annotated as __rcu.
463 #define rcu_dereference_check(p, c) \
464 __rcu_dereference_check((p), (c) || rcu_read_lock_held(), __rcu)
467 * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
468 * @p: The pointer to read, prior to dereferencing
469 * @c: The conditions under which the dereference will take place
471 * This is the RCU-bh counterpart to rcu_dereference_check().
473 #define rcu_dereference_bh_check(p, c) \
474 __rcu_dereference_check((p), (c) || rcu_read_lock_bh_held(), __rcu)
477 * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
478 * @p: The pointer to read, prior to dereferencing
479 * @c: The conditions under which the dereference will take place
481 * This is the RCU-sched counterpart to rcu_dereference_check().
483 #define rcu_dereference_sched_check(p, c) \
484 __rcu_dereference_check((p), (c) || rcu_read_lock_sched_held(), \
488 * The tracing infrastructure traces RCU (we want that), but unfortunately
489 * some of the RCU checks causes tracing to lock up the system.
491 * The no-tracing version of rcu_dereference_raw() must not call
492 * rcu_read_lock_held().
494 #define rcu_dereference_raw_notrace(p) __rcu_dereference_check((p), 1, __rcu)
497 * rcu_dereference_protected() - fetch RCU pointer when updates prevented
498 * @p: The pointer to read, prior to dereferencing
499 * @c: The conditions under which the dereference will take place
501 * Return the value of the specified RCU-protected pointer, but omit
502 * the READ_ONCE(). This is useful in cases where update-side locks
503 * prevent the value of the pointer from changing. Please note that this
504 * primitive does *not* prevent the compiler from repeating this reference
505 * or combining it with other references, so it should not be used without
506 * protection of appropriate locks.
508 * This function is only for update-side use. Using this function
509 * when protected only by rcu_read_lock() will result in infrequent
510 * but very ugly failures.
512 #define rcu_dereference_protected(p, c) \
513 __rcu_dereference_protected((p), (c), __rcu)
517 * rcu_dereference() - fetch RCU-protected pointer for dereferencing
518 * @p: The pointer to read, prior to dereferencing
520 * This is a simple wrapper around rcu_dereference_check().
522 #define rcu_dereference(p) rcu_dereference_check(p, 0)
525 * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
526 * @p: The pointer to read, prior to dereferencing
528 * Makes rcu_dereference_check() do the dirty work.
530 #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
533 * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
534 * @p: The pointer to read, prior to dereferencing
536 * Makes rcu_dereference_check() do the dirty work.
538 #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
541 * rcu_pointer_handoff() - Hand off a pointer from RCU to other mechanism
542 * @p: The pointer to hand off
544 * This is simply an identity function, but it documents where a pointer
545 * is handed off from RCU to some other synchronization mechanism, for
546 * example, reference counting or locking. In C11, it would map to
547 * kill_dependency(). It could be used as follows::
550 * p = rcu_dereference(gp);
551 * long_lived = is_long_lived(p);
553 * if (!atomic_inc_not_zero(p->refcnt))
554 * long_lived = false;
556 * p = rcu_pointer_handoff(p);
560 #define rcu_pointer_handoff(p) (p)
563 * rcu_read_lock() - mark the beginning of an RCU read-side critical section
565 * When synchronize_rcu() is invoked on one CPU while other CPUs
566 * are within RCU read-side critical sections, then the
567 * synchronize_rcu() is guaranteed to block until after all the other
568 * CPUs exit their critical sections. Similarly, if call_rcu() is invoked
569 * on one CPU while other CPUs are within RCU read-side critical
570 * sections, invocation of the corresponding RCU callback is deferred
571 * until after the all the other CPUs exit their critical sections.
573 * Note, however, that RCU callbacks are permitted to run concurrently
574 * with new RCU read-side critical sections. One way that this can happen
575 * is via the following sequence of events: (1) CPU 0 enters an RCU
576 * read-side critical section, (2) CPU 1 invokes call_rcu() to register
577 * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
578 * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
579 * callback is invoked. This is legal, because the RCU read-side critical
580 * section that was running concurrently with the call_rcu() (and which
581 * therefore might be referencing something that the corresponding RCU
582 * callback would free up) has completed before the corresponding
583 * RCU callback is invoked.
585 * RCU read-side critical sections may be nested. Any deferred actions
586 * will be deferred until the outermost RCU read-side critical section
589 * You can avoid reading and understanding the next paragraph by
590 * following this rule: don't put anything in an rcu_read_lock() RCU
591 * read-side critical section that would block in a !PREEMPT kernel.
592 * But if you want the full story, read on!
594 * In non-preemptible RCU implementations (TREE_RCU and TINY_RCU),
595 * it is illegal to block while in an RCU read-side critical section.
596 * In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPT
597 * kernel builds, RCU read-side critical sections may be preempted,
598 * but explicit blocking is illegal. Finally, in preemptible RCU
599 * implementations in real-time (with -rt patchset) kernel builds, RCU
600 * read-side critical sections may be preempted and they may also block, but
601 * only when acquiring spinlocks that are subject to priority inheritance.
603 static inline void rcu_read_lock(void)
607 rcu_lock_acquire(&rcu_lock_map
);
608 RCU_LOCKDEP_WARN(!rcu_is_watching(),
609 "rcu_read_lock() used illegally while idle");
613 * So where is rcu_write_lock()? It does not exist, as there is no
614 * way for writers to lock out RCU readers. This is a feature, not
615 * a bug -- this property is what provides RCU's performance benefits.
616 * Of course, writers must coordinate with each other. The normal
617 * spinlock primitives work well for this, but any other technique may be
618 * used as well. RCU does not care how the writers keep out of each
619 * others' way, as long as they do so.
623 * rcu_read_unlock() - marks the end of an RCU read-side critical section.
625 * In most situations, rcu_read_unlock() is immune from deadlock.
626 * However, in kernels built with CONFIG_RCU_BOOST, rcu_read_unlock()
627 * is responsible for deboosting, which it does via rt_mutex_unlock().
628 * Unfortunately, this function acquires the scheduler's runqueue and
629 * priority-inheritance spinlocks. This means that deadlock could result
630 * if the caller of rcu_read_unlock() already holds one of these locks or
631 * any lock that is ever acquired while holding them.
633 * That said, RCU readers are never priority boosted unless they were
634 * preempted. Therefore, one way to avoid deadlock is to make sure
635 * that preemption never happens within any RCU read-side critical
636 * section whose outermost rcu_read_unlock() is called with one of
637 * rt_mutex_unlock()'s locks held. Such preemption can be avoided in
638 * a number of ways, for example, by invoking preempt_disable() before
639 * critical section's outermost rcu_read_lock().
641 * Given that the set of locks acquired by rt_mutex_unlock() might change
642 * at any time, a somewhat more future-proofed approach is to make sure
643 * that that preemption never happens within any RCU read-side critical
644 * section whose outermost rcu_read_unlock() is called with irqs disabled.
645 * This approach relies on the fact that rt_mutex_unlock() currently only
646 * acquires irq-disabled locks.
648 * The second of these two approaches is best in most situations,
649 * however, the first approach can also be useful, at least to those
650 * developers willing to keep abreast of the set of locks acquired by
653 * See rcu_read_lock() for more information.
655 static inline void rcu_read_unlock(void)
657 RCU_LOCKDEP_WARN(!rcu_is_watching(),
658 "rcu_read_unlock() used illegally while idle");
661 rcu_lock_release(&rcu_lock_map
); /* Keep acq info for rls diags. */
665 * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
667 * This is equivalent of rcu_read_lock(), but also disables softirqs.
668 * Note that anything else that disables softirqs can also serve as
669 * an RCU read-side critical section.
671 * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
672 * must occur in the same context, for example, it is illegal to invoke
673 * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
674 * was invoked from some other task.
676 static inline void rcu_read_lock_bh(void)
680 rcu_lock_acquire(&rcu_bh_lock_map
);
681 RCU_LOCKDEP_WARN(!rcu_is_watching(),
682 "rcu_read_lock_bh() used illegally while idle");
686 * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
688 * See rcu_read_lock_bh() for more information.
690 static inline void rcu_read_unlock_bh(void)
692 RCU_LOCKDEP_WARN(!rcu_is_watching(),
693 "rcu_read_unlock_bh() used illegally while idle");
694 rcu_lock_release(&rcu_bh_lock_map
);
700 * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
702 * This is equivalent of rcu_read_lock(), but disables preemption.
703 * Read-side critical sections can also be introduced by anything else
704 * that disables preemption, including local_irq_disable() and friends.
706 * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
707 * must occur in the same context, for example, it is illegal to invoke
708 * rcu_read_unlock_sched() from process context if the matching
709 * rcu_read_lock_sched() was invoked from an NMI handler.
711 static inline void rcu_read_lock_sched(void)
714 __acquire(RCU_SCHED
);
715 rcu_lock_acquire(&rcu_sched_lock_map
);
716 RCU_LOCKDEP_WARN(!rcu_is_watching(),
717 "rcu_read_lock_sched() used illegally while idle");
720 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
721 static inline notrace
void rcu_read_lock_sched_notrace(void)
723 preempt_disable_notrace();
724 __acquire(RCU_SCHED
);
728 * rcu_read_unlock_sched - marks the end of a RCU-classic critical section
730 * See rcu_read_lock_sched for more information.
732 static inline void rcu_read_unlock_sched(void)
734 RCU_LOCKDEP_WARN(!rcu_is_watching(),
735 "rcu_read_unlock_sched() used illegally while idle");
736 rcu_lock_release(&rcu_sched_lock_map
);
737 __release(RCU_SCHED
);
741 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
742 static inline notrace
void rcu_read_unlock_sched_notrace(void)
744 __release(RCU_SCHED
);
745 preempt_enable_notrace();
749 * RCU_INIT_POINTER() - initialize an RCU protected pointer
750 * @p: The pointer to be initialized.
751 * @v: The value to initialized the pointer to.
753 * Initialize an RCU-protected pointer in special cases where readers
754 * do not need ordering constraints on the CPU or the compiler. These
757 * 1. This use of RCU_INIT_POINTER() is NULLing out the pointer *or*
758 * 2. The caller has taken whatever steps are required to prevent
759 * RCU readers from concurrently accessing this pointer *or*
760 * 3. The referenced data structure has already been exposed to
761 * readers either at compile time or via rcu_assign_pointer() *and*
763 * a. You have not made *any* reader-visible changes to
764 * this structure since then *or*
765 * b. It is OK for readers accessing this structure from its
766 * new location to see the old state of the structure. (For
767 * example, the changes were to statistical counters or to
768 * other state where exact synchronization is not required.)
770 * Failure to follow these rules governing use of RCU_INIT_POINTER() will
771 * result in impossible-to-diagnose memory corruption. As in the structures
772 * will look OK in crash dumps, but any concurrent RCU readers might
773 * see pre-initialized values of the referenced data structure. So
774 * please be very careful how you use RCU_INIT_POINTER()!!!
776 * If you are creating an RCU-protected linked structure that is accessed
777 * by a single external-to-structure RCU-protected pointer, then you may
778 * use RCU_INIT_POINTER() to initialize the internal RCU-protected
779 * pointers, but you must use rcu_assign_pointer() to initialize the
780 * external-to-structure pointer *after* you have completely initialized
781 * the reader-accessible portions of the linked structure.
783 * Note that unlike rcu_assign_pointer(), RCU_INIT_POINTER() provides no
784 * ordering guarantees for either the CPU or the compiler.
786 #define RCU_INIT_POINTER(p, v) \
788 rcu_dereference_sparse(p, __rcu); \
789 WRITE_ONCE(p, RCU_INITIALIZER(v)); \
793 * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer
794 * @p: The pointer to be initialized.
795 * @v: The value to initialized the pointer to.
797 * GCC-style initialization for an RCU-protected pointer in a structure field.
799 #define RCU_POINTER_INITIALIZER(p, v) \
800 .p = RCU_INITIALIZER(v)
803 * Does the specified offset indicate that the corresponding rcu_head
804 * structure can be handled by kfree_rcu()?
806 #define __is_kfree_rcu_offset(offset) ((offset) < 4096)
809 * Helper macro for kfree_rcu() to prevent argument-expansion eyestrain.
811 #define __kfree_rcu(head, offset) \
813 BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \
814 kfree_call_rcu(head, (rcu_callback_t)(unsigned long)(offset)); \
818 * kfree_rcu() - kfree an object after a grace period.
819 * @ptr: pointer to kfree
820 * @rcu_head: the name of the struct rcu_head within the type of @ptr.
822 * Many rcu callbacks functions just call kfree() on the base structure.
823 * These functions are trivial, but their size adds up, and furthermore
824 * when they are used in a kernel module, that module must invoke the
825 * high-latency rcu_barrier() function at module-unload time.
827 * The kfree_rcu() function handles this issue. Rather than encoding a
828 * function address in the embedded rcu_head structure, kfree_rcu() instead
829 * encodes the offset of the rcu_head structure within the base structure.
830 * Because the functions are not allowed in the low-order 4096 bytes of
831 * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
832 * If the offset is larger than 4095 bytes, a compile-time error will
833 * be generated in __kfree_rcu(). If this error is triggered, you can
834 * either fall back to use of call_rcu() or rearrange the structure to
835 * position the rcu_head structure into the first 4096 bytes.
837 * Note that the allowable offset might decrease in the future, for example,
838 * to allow something like kmem_cache_free_rcu().
840 * The BUILD_BUG_ON check must not involve any function calls, hence the
841 * checks are done in macros here.
843 #define kfree_rcu(ptr, rcu_head) \
844 __kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
848 * Place this after a lock-acquisition primitive to guarantee that
849 * an UNLOCK+LOCK pair acts as a full barrier. This guarantee applies
850 * if the UNLOCK and LOCK are executed by the same CPU or if the
851 * UNLOCK and LOCK operate on the same lock variable.
853 #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE
854 #define smp_mb__after_unlock_lock() smp_mb() /* Full ordering for lock. */
855 #else /* #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
856 #define smp_mb__after_unlock_lock() do { } while (0)
857 #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
860 /* Has the specified rcu_head structure been handed to call_rcu()? */
863 * rcu_head_init - Initialize rcu_head for rcu_head_after_call_rcu()
864 * @rhp: The rcu_head structure to initialize.
866 * If you intend to invoke rcu_head_after_call_rcu() to test whether a
867 * given rcu_head structure has already been passed to call_rcu(), then
868 * you must also invoke this rcu_head_init() function on it just after
869 * allocating that structure. Calls to this function must not race with
870 * calls to call_rcu(), rcu_head_after_call_rcu(), or callback invocation.
872 static inline void rcu_head_init(struct rcu_head
*rhp
)
874 rhp
->func
= (rcu_callback_t
)~0L;
878 * rcu_head_after_call_rcu - Has this rcu_head been passed to call_rcu()?
879 * @rhp: The rcu_head structure to test.
880 * @func: The function passed to call_rcu() along with @rhp.
882 * Returns @true if the @rhp has been passed to call_rcu() with @func,
883 * and @false otherwise. Emits a warning in any other case, including
884 * the case where @rhp has already been invoked after a grace period.
885 * Calls to this function must not race with callback invocation. One way
886 * to avoid such races is to enclose the call to rcu_head_after_call_rcu()
887 * in an RCU read-side critical section that includes a read-side fetch
888 * of the pointer to the structure containing @rhp.
891 rcu_head_after_call_rcu(struct rcu_head
*rhp
, rcu_callback_t f
)
893 if (READ_ONCE(rhp
->func
) == f
)
895 WARN_ON_ONCE(READ_ONCE(rhp
->func
) != (rcu_callback_t
)~0L);
900 /* Transitional pre-consolidation compatibility definitions. */
902 static inline void synchronize_rcu_bh(void)
907 static inline void synchronize_rcu_bh_expedited(void)
909 synchronize_rcu_expedited();
912 static inline void call_rcu_bh(struct rcu_head
*head
, rcu_callback_t func
)
914 call_rcu(head
, func
);
917 static inline void rcu_barrier_bh(void)
922 static inline void synchronize_sched(void)
927 static inline void synchronize_sched_expedited(void)
929 synchronize_rcu_expedited();
932 static inline void call_rcu_sched(struct rcu_head
*head
, rcu_callback_t func
)
934 call_rcu(head
, func
);
937 static inline void rcu_barrier_sched(void)
942 static inline unsigned long get_state_synchronize_sched(void)
944 return get_state_synchronize_rcu();
947 static inline void cond_synchronize_sched(unsigned long oldstate
)
949 cond_synchronize_rcu(oldstate
);
952 #endif /* __LINUX_RCUPDATE_H */