1 /* SPDX-License-Identifier: GPL-2.0+ */
3 * Read-Copy Update mechanism for mutual exclusion
5 * Copyright IBM Corporation, 2001
7 * Author: Dipankar Sarma <dipankar@in.ibm.com>
9 * Based on the original work by Paul McKenney <paulmck@vnet.ibm.com>
10 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
12 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
13 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
15 * For detailed explanation of Read-Copy Update mechanism see -
16 * http://lse.sourceforge.net/locking/rcupdate.html
20 #ifndef __LINUX_RCUPDATE_H
21 #define __LINUX_RCUPDATE_H
23 #include <linux/types.h>
24 #include <linux/compiler.h>
25 #include <linux/atomic.h>
26 #include <linux/irqflags.h>
27 #include <linux/preempt.h>
28 #include <linux/bottom_half.h>
29 #include <linux/lockdep.h>
30 #include <asm/processor.h>
31 #include <linux/cpumask.h>
33 #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
34 #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
35 #define ulong2long(a) (*(long *)(&(a)))
36 #define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
37 #define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
39 /* Exported common interfaces */
40 void call_rcu(struct rcu_head
*head
, rcu_callback_t func
);
41 void rcu_barrier_tasks(void);
42 void rcu_barrier_tasks_rude(void);
43 void synchronize_rcu(void);
45 #ifdef CONFIG_PREEMPT_RCU
47 void __rcu_read_lock(void);
48 void __rcu_read_unlock(void);
51 * Defined as a macro as it is a very low level header included from
52 * areas that don't even know about current. This gives the rcu_read_lock()
53 * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
54 * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
56 #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
58 #else /* #ifdef CONFIG_PREEMPT_RCU */
60 #ifdef CONFIG_TINY_RCU
61 #define rcu_read_unlock_strict() do { } while (0)
63 void rcu_read_unlock_strict(void);
66 static inline void __rcu_read_lock(void)
71 static inline void __rcu_read_unlock(void)
74 rcu_read_unlock_strict();
77 static inline int rcu_preempt_depth(void)
82 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
84 /* Internal to kernel */
86 extern int rcu_scheduler_active __read_mostly
;
87 void rcu_sched_clock_irq(int user
);
88 void rcu_report_dead(unsigned int cpu
);
89 void rcutree_migrate_callbacks(int cpu
);
91 #ifdef CONFIG_TASKS_RCU_GENERIC
92 void rcu_init_tasks_generic(void);
94 static inline void rcu_init_tasks_generic(void) { }
97 #ifdef CONFIG_RCU_STALL_COMMON
98 void rcu_sysrq_start(void);
99 void rcu_sysrq_end(void);
100 #else /* #ifdef CONFIG_RCU_STALL_COMMON */
101 static inline void rcu_sysrq_start(void) { }
102 static inline void rcu_sysrq_end(void) { }
103 #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
105 #ifdef CONFIG_NO_HZ_FULL
106 void rcu_user_enter(void);
107 void rcu_user_exit(void);
109 static inline void rcu_user_enter(void) { }
110 static inline void rcu_user_exit(void) { }
111 #endif /* CONFIG_NO_HZ_FULL */
113 #ifdef CONFIG_RCU_NOCB_CPU
114 void rcu_init_nohz(void);
115 int rcu_nocb_cpu_offload(int cpu
);
116 int rcu_nocb_cpu_deoffload(int cpu
);
117 void rcu_nocb_flush_deferred_wakeup(void);
118 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
119 static inline void rcu_init_nohz(void) { }
120 static inline int rcu_nocb_cpu_offload(int cpu
) { return -EINVAL
; }
121 static inline int rcu_nocb_cpu_deoffload(int cpu
) { return 0; }
122 static inline void rcu_nocb_flush_deferred_wakeup(void) { }
123 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
126 * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
127 * @a: Code that RCU needs to pay attention to.
129 * RCU read-side critical sections are forbidden in the inner idle loop,
130 * that is, between the rcu_idle_enter() and the rcu_idle_exit() -- RCU
131 * will happily ignore any such read-side critical sections. However,
132 * things like powertop need tracepoints in the inner idle loop.
134 * This macro provides the way out: RCU_NONIDLE(do_something_with_RCU())
135 * will tell RCU that it needs to pay attention, invoke its argument
136 * (in this example, calling the do_something_with_RCU() function),
137 * and then tell RCU to go back to ignoring this CPU. It is permissible
138 * to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is
139 * on the order of a million or so, even on 32-bit systems). It is
140 * not legal to block within RCU_NONIDLE(), nor is it permissible to
141 * transfer control either into or out of RCU_NONIDLE()'s statement.
143 #define RCU_NONIDLE(a) \
145 rcu_irq_enter_irqson(); \
146 do { a; } while (0); \
147 rcu_irq_exit_irqson(); \
151 * Note a quasi-voluntary context switch for RCU-tasks's benefit.
152 * This is a macro rather than an inline function to avoid #include hell.
154 #ifdef CONFIG_TASKS_RCU_GENERIC
156 # ifdef CONFIG_TASKS_RCU
157 # define rcu_tasks_classic_qs(t, preempt) \
159 if (!(preempt) && READ_ONCE((t)->rcu_tasks_holdout)) \
160 WRITE_ONCE((t)->rcu_tasks_holdout, false); \
162 void call_rcu_tasks(struct rcu_head
*head
, rcu_callback_t func
);
163 void synchronize_rcu_tasks(void);
165 # define rcu_tasks_classic_qs(t, preempt) do { } while (0)
166 # define call_rcu_tasks call_rcu
167 # define synchronize_rcu_tasks synchronize_rcu
170 # ifdef CONFIG_TASKS_RCU_TRACE
171 # define rcu_tasks_trace_qs(t) \
173 if (!likely(READ_ONCE((t)->trc_reader_checked)) && \
174 !unlikely(READ_ONCE((t)->trc_reader_nesting))) { \
175 smp_store_release(&(t)->trc_reader_checked, true); \
176 smp_mb(); /* Readers partitioned by store. */ \
180 # define rcu_tasks_trace_qs(t) do { } while (0)
183 #define rcu_tasks_qs(t, preempt) \
185 rcu_tasks_classic_qs((t), (preempt)); \
186 rcu_tasks_trace_qs((t)); \
189 # ifdef CONFIG_TASKS_RUDE_RCU
190 void call_rcu_tasks_rude(struct rcu_head
*head
, rcu_callback_t func
);
191 void synchronize_rcu_tasks_rude(void);
194 #define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t, false)
195 void exit_tasks_rcu_start(void);
196 void exit_tasks_rcu_finish(void);
197 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
198 #define rcu_tasks_qs(t, preempt) do { } while (0)
199 #define rcu_note_voluntary_context_switch(t) do { } while (0)
200 #define call_rcu_tasks call_rcu
201 #define synchronize_rcu_tasks synchronize_rcu
202 static inline void exit_tasks_rcu_start(void) { }
203 static inline void exit_tasks_rcu_finish(void) { }
204 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
207 * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
209 * This macro resembles cond_resched(), except that it is defined to
210 * report potential quiescent states to RCU-tasks even if the cond_resched()
211 * machinery were to be shut off, as some advocate for PREEMPTION kernels.
213 #define cond_resched_tasks_rcu_qs() \
215 rcu_tasks_qs(current, false); \
220 * Infrastructure to implement the synchronize_() primitives in
221 * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
224 #if defined(CONFIG_TREE_RCU)
225 #include <linux/rcutree.h>
226 #elif defined(CONFIG_TINY_RCU)
227 #include <linux/rcutiny.h>
229 #error "Unknown RCU implementation specified to kernel configuration"
233 * The init_rcu_head_on_stack() and destroy_rcu_head_on_stack() calls
234 * are needed for dynamic initialization and destruction of rcu_head
235 * on the stack, and init_rcu_head()/destroy_rcu_head() are needed for
236 * dynamic initialization and destruction of statically allocated rcu_head
237 * structures. However, rcu_head structures allocated dynamically in the
238 * heap don't need any initialization.
240 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
241 void init_rcu_head(struct rcu_head
*head
);
242 void destroy_rcu_head(struct rcu_head
*head
);
243 void init_rcu_head_on_stack(struct rcu_head
*head
);
244 void destroy_rcu_head_on_stack(struct rcu_head
*head
);
245 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
246 static inline void init_rcu_head(struct rcu_head
*head
) { }
247 static inline void destroy_rcu_head(struct rcu_head
*head
) { }
248 static inline void init_rcu_head_on_stack(struct rcu_head
*head
) { }
249 static inline void destroy_rcu_head_on_stack(struct rcu_head
*head
) { }
250 #endif /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
252 #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
253 bool rcu_lockdep_current_cpu_online(void);
254 #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
255 static inline bool rcu_lockdep_current_cpu_online(void) { return true; }
256 #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
258 extern struct lockdep_map rcu_lock_map
;
259 extern struct lockdep_map rcu_bh_lock_map
;
260 extern struct lockdep_map rcu_sched_lock_map
;
261 extern struct lockdep_map rcu_callback_map
;
263 #ifdef CONFIG_DEBUG_LOCK_ALLOC
265 static inline void rcu_lock_acquire(struct lockdep_map
*map
)
267 lock_acquire(map
, 0, 0, 2, 0, NULL
, _THIS_IP_
);
270 static inline void rcu_lock_release(struct lockdep_map
*map
)
272 lock_release(map
, _THIS_IP_
);
275 int debug_lockdep_rcu_enabled(void);
276 int rcu_read_lock_held(void);
277 int rcu_read_lock_bh_held(void);
278 int rcu_read_lock_sched_held(void);
279 int rcu_read_lock_any_held(void);
281 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
283 # define rcu_lock_acquire(a) do { } while (0)
284 # define rcu_lock_release(a) do { } while (0)
286 static inline int rcu_read_lock_held(void)
291 static inline int rcu_read_lock_bh_held(void)
296 static inline int rcu_read_lock_sched_held(void)
298 return !preemptible();
301 static inline int rcu_read_lock_any_held(void)
303 return !preemptible();
306 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
308 #ifdef CONFIG_PROVE_RCU
311 * RCU_LOCKDEP_WARN - emit lockdep splat if specified condition is met
312 * @c: condition to check
313 * @s: informative message
315 #define RCU_LOCKDEP_WARN(c, s) \
317 static bool __section(".data.unlikely") __warned; \
318 if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \
320 lockdep_rcu_suspicious(__FILE__, __LINE__, s); \
324 #if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
325 static inline void rcu_preempt_sleep_check(void)
327 RCU_LOCKDEP_WARN(lock_is_held(&rcu_lock_map
),
328 "Illegal context switch in RCU read-side critical section");
330 #else /* #ifdef CONFIG_PROVE_RCU */
331 static inline void rcu_preempt_sleep_check(void) { }
332 #endif /* #else #ifdef CONFIG_PROVE_RCU */
334 #define rcu_sleep_check() \
336 rcu_preempt_sleep_check(); \
337 if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
338 RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map), \
339 "Illegal context switch in RCU-bh read-side critical section"); \
340 RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map), \
341 "Illegal context switch in RCU-sched read-side critical section"); \
344 #else /* #ifdef CONFIG_PROVE_RCU */
346 #define RCU_LOCKDEP_WARN(c, s) do { } while (0 && (c))
347 #define rcu_sleep_check() do { } while (0)
349 #endif /* #else #ifdef CONFIG_PROVE_RCU */
352 * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
353 * and rcu_assign_pointer(). Some of these could be folded into their
354 * callers, but they are left separate in order to ease introduction of
355 * multiple pointers markings to match different RCU implementations
356 * (e.g., __srcu), should this make sense in the future.
360 #define rcu_check_sparse(p, space) \
361 ((void)(((typeof(*p) space *)p) == p))
362 #else /* #ifdef __CHECKER__ */
363 #define rcu_check_sparse(p, space)
364 #endif /* #else #ifdef __CHECKER__ */
366 #define __rcu_access_pointer(p, space) \
368 typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
369 rcu_check_sparse(p, space); \
370 ((typeof(*p) __force __kernel *)(_________p1)); \
372 #define __rcu_dereference_check(p, c, space) \
374 /* Dependency order vs. p above. */ \
375 typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
376 RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
377 rcu_check_sparse(p, space); \
378 ((typeof(*p) __force __kernel *)(________p1)); \
380 #define __rcu_dereference_protected(p, c, space) \
382 RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
383 rcu_check_sparse(p, space); \
384 ((typeof(*p) __force __kernel *)(p)); \
386 #define rcu_dereference_raw(p) \
388 /* Dependency order vs. p above. */ \
389 typeof(p) ________p1 = READ_ONCE(p); \
390 ((typeof(*p) __force __kernel *)(________p1)); \
394 * RCU_INITIALIZER() - statically initialize an RCU-protected global variable
395 * @v: The value to statically initialize with.
397 #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
400 * rcu_assign_pointer() - assign to RCU-protected pointer
401 * @p: pointer to assign to
402 * @v: value to assign (publish)
404 * Assigns the specified value to the specified RCU-protected
405 * pointer, ensuring that any concurrent RCU readers will see
406 * any prior initialization.
408 * Inserts memory barriers on architectures that require them
409 * (which is most of them), and also prevents the compiler from
410 * reordering the code that initializes the structure after the pointer
411 * assignment. More importantly, this call documents which pointers
412 * will be dereferenced by RCU read-side code.
414 * In some special cases, you may use RCU_INIT_POINTER() instead
415 * of rcu_assign_pointer(). RCU_INIT_POINTER() is a bit faster due
416 * to the fact that it does not constrain either the CPU or the compiler.
417 * That said, using RCU_INIT_POINTER() when you should have used
418 * rcu_assign_pointer() is a very bad thing that results in
419 * impossible-to-diagnose memory corruption. So please be careful.
420 * See the RCU_INIT_POINTER() comment header for details.
422 * Note that rcu_assign_pointer() evaluates each of its arguments only
423 * once, appearances notwithstanding. One of the "extra" evaluations
424 * is in typeof() and the other visible only to sparse (__CHECKER__),
425 * neither of which actually execute the argument. As with most cpp
426 * macros, this execute-arguments-only-once property is important, so
427 * please be careful when making changes to rcu_assign_pointer() and the
428 * other macros that it invokes.
430 #define rcu_assign_pointer(p, v) \
432 uintptr_t _r_a_p__v = (uintptr_t)(v); \
433 rcu_check_sparse(p, __rcu); \
435 if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL) \
436 WRITE_ONCE((p), (typeof(p))(_r_a_p__v)); \
438 smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
442 * rcu_replace_pointer() - replace an RCU pointer, returning its old value
443 * @rcu_ptr: RCU pointer, whose old value is returned
444 * @ptr: regular pointer
445 * @c: the lockdep conditions under which the dereference will take place
447 * Perform a replacement, where @rcu_ptr is an RCU-annotated
448 * pointer and @c is the lockdep argument that is passed to the
449 * rcu_dereference_protected() call used to read that pointer. The old
450 * value of @rcu_ptr is returned, and @rcu_ptr is set to @ptr.
452 #define rcu_replace_pointer(rcu_ptr, ptr, c) \
454 typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c)); \
455 rcu_assign_pointer((rcu_ptr), (ptr)); \
460 * rcu_access_pointer() - fetch RCU pointer with no dereferencing
461 * @p: The pointer to read
463 * Return the value of the specified RCU-protected pointer, but omit the
464 * lockdep checks for being in an RCU read-side critical section. This is
465 * useful when the value of this pointer is accessed, but the pointer is
466 * not dereferenced, for example, when testing an RCU-protected pointer
467 * against NULL. Although rcu_access_pointer() may also be used in cases
468 * where update-side locks prevent the value of the pointer from changing,
469 * you should instead use rcu_dereference_protected() for this use case.
471 * It is also permissible to use rcu_access_pointer() when read-side
472 * access to the pointer was removed at least one grace period ago, as
473 * is the case in the context of the RCU callback that is freeing up
474 * the data, or after a synchronize_rcu() returns. This can be useful
475 * when tearing down multi-linked structures after a grace period
478 #define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
481 * rcu_dereference_check() - rcu_dereference with debug checking
482 * @p: The pointer to read, prior to dereferencing
483 * @c: The conditions under which the dereference will take place
485 * Do an rcu_dereference(), but check that the conditions under which the
486 * dereference will take place are correct. Typically the conditions
487 * indicate the various locking conditions that should be held at that
488 * point. The check should return true if the conditions are satisfied.
489 * An implicit check for being in an RCU read-side critical section
490 * (rcu_read_lock()) is included.
494 * bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
496 * could be used to indicate to lockdep that foo->bar may only be dereferenced
497 * if either rcu_read_lock() is held, or that the lock required to replace
498 * the bar struct at foo->bar is held.
500 * Note that the list of conditions may also include indications of when a lock
501 * need not be held, for example during initialisation or destruction of the
504 * bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
505 * atomic_read(&foo->usage) == 0);
507 * Inserts memory barriers on architectures that require them
508 * (currently only the Alpha), prevents the compiler from refetching
509 * (and from merging fetches), and, more importantly, documents exactly
510 * which pointers are protected by RCU and checks that the pointer is
511 * annotated as __rcu.
513 #define rcu_dereference_check(p, c) \
514 __rcu_dereference_check((p), (c) || rcu_read_lock_held(), __rcu)
517 * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
518 * @p: The pointer to read, prior to dereferencing
519 * @c: The conditions under which the dereference will take place
521 * This is the RCU-bh counterpart to rcu_dereference_check().
523 #define rcu_dereference_bh_check(p, c) \
524 __rcu_dereference_check((p), (c) || rcu_read_lock_bh_held(), __rcu)
527 * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
528 * @p: The pointer to read, prior to dereferencing
529 * @c: The conditions under which the dereference will take place
531 * This is the RCU-sched counterpart to rcu_dereference_check().
533 #define rcu_dereference_sched_check(p, c) \
534 __rcu_dereference_check((p), (c) || rcu_read_lock_sched_held(), \
538 * The tracing infrastructure traces RCU (we want that), but unfortunately
539 * some of the RCU checks causes tracing to lock up the system.
541 * The no-tracing version of rcu_dereference_raw() must not call
542 * rcu_read_lock_held().
544 #define rcu_dereference_raw_check(p) __rcu_dereference_check((p), 1, __rcu)
547 * rcu_dereference_protected() - fetch RCU pointer when updates prevented
548 * @p: The pointer to read, prior to dereferencing
549 * @c: The conditions under which the dereference will take place
551 * Return the value of the specified RCU-protected pointer, but omit
552 * the READ_ONCE(). This is useful in cases where update-side locks
553 * prevent the value of the pointer from changing. Please note that this
554 * primitive does *not* prevent the compiler from repeating this reference
555 * or combining it with other references, so it should not be used without
556 * protection of appropriate locks.
558 * This function is only for update-side use. Using this function
559 * when protected only by rcu_read_lock() will result in infrequent
560 * but very ugly failures.
562 #define rcu_dereference_protected(p, c) \
563 __rcu_dereference_protected((p), (c), __rcu)
567 * rcu_dereference() - fetch RCU-protected pointer for dereferencing
568 * @p: The pointer to read, prior to dereferencing
570 * This is a simple wrapper around rcu_dereference_check().
572 #define rcu_dereference(p) rcu_dereference_check(p, 0)
575 * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
576 * @p: The pointer to read, prior to dereferencing
578 * Makes rcu_dereference_check() do the dirty work.
580 #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
583 * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
584 * @p: The pointer to read, prior to dereferencing
586 * Makes rcu_dereference_check() do the dirty work.
588 #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
591 * rcu_pointer_handoff() - Hand off a pointer from RCU to other mechanism
592 * @p: The pointer to hand off
594 * This is simply an identity function, but it documents where a pointer
595 * is handed off from RCU to some other synchronization mechanism, for
596 * example, reference counting or locking. In C11, it would map to
597 * kill_dependency(). It could be used as follows::
600 * p = rcu_dereference(gp);
601 * long_lived = is_long_lived(p);
603 * if (!atomic_inc_not_zero(p->refcnt))
604 * long_lived = false;
606 * p = rcu_pointer_handoff(p);
610 #define rcu_pointer_handoff(p) (p)
613 * rcu_read_lock() - mark the beginning of an RCU read-side critical section
615 * When synchronize_rcu() is invoked on one CPU while other CPUs
616 * are within RCU read-side critical sections, then the
617 * synchronize_rcu() is guaranteed to block until after all the other
618 * CPUs exit their critical sections. Similarly, if call_rcu() is invoked
619 * on one CPU while other CPUs are within RCU read-side critical
620 * sections, invocation of the corresponding RCU callback is deferred
621 * until after the all the other CPUs exit their critical sections.
623 * Note, however, that RCU callbacks are permitted to run concurrently
624 * with new RCU read-side critical sections. One way that this can happen
625 * is via the following sequence of events: (1) CPU 0 enters an RCU
626 * read-side critical section, (2) CPU 1 invokes call_rcu() to register
627 * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
628 * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
629 * callback is invoked. This is legal, because the RCU read-side critical
630 * section that was running concurrently with the call_rcu() (and which
631 * therefore might be referencing something that the corresponding RCU
632 * callback would free up) has completed before the corresponding
633 * RCU callback is invoked.
635 * RCU read-side critical sections may be nested. Any deferred actions
636 * will be deferred until the outermost RCU read-side critical section
639 * You can avoid reading and understanding the next paragraph by
640 * following this rule: don't put anything in an rcu_read_lock() RCU
641 * read-side critical section that would block in a !PREEMPTION kernel.
642 * But if you want the full story, read on!
644 * In non-preemptible RCU implementations (pure TREE_RCU and TINY_RCU),
645 * it is illegal to block while in an RCU read-side critical section.
646 * In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPTION
647 * kernel builds, RCU read-side critical sections may be preempted,
648 * but explicit blocking is illegal. Finally, in preemptible RCU
649 * implementations in real-time (with -rt patchset) kernel builds, RCU
650 * read-side critical sections may be preempted and they may also block, but
651 * only when acquiring spinlocks that are subject to priority inheritance.
653 static __always_inline
void rcu_read_lock(void)
657 rcu_lock_acquire(&rcu_lock_map
);
658 RCU_LOCKDEP_WARN(!rcu_is_watching(),
659 "rcu_read_lock() used illegally while idle");
663 * So where is rcu_write_lock()? It does not exist, as there is no
664 * way for writers to lock out RCU readers. This is a feature, not
665 * a bug -- this property is what provides RCU's performance benefits.
666 * Of course, writers must coordinate with each other. The normal
667 * spinlock primitives work well for this, but any other technique may be
668 * used as well. RCU does not care how the writers keep out of each
669 * others' way, as long as they do so.
673 * rcu_read_unlock() - marks the end of an RCU read-side critical section.
675 * In most situations, rcu_read_unlock() is immune from deadlock.
676 * However, in kernels built with CONFIG_RCU_BOOST, rcu_read_unlock()
677 * is responsible for deboosting, which it does via rt_mutex_unlock().
678 * Unfortunately, this function acquires the scheduler's runqueue and
679 * priority-inheritance spinlocks. This means that deadlock could result
680 * if the caller of rcu_read_unlock() already holds one of these locks or
681 * any lock that is ever acquired while holding them.
683 * That said, RCU readers are never priority boosted unless they were
684 * preempted. Therefore, one way to avoid deadlock is to make sure
685 * that preemption never happens within any RCU read-side critical
686 * section whose outermost rcu_read_unlock() is called with one of
687 * rt_mutex_unlock()'s locks held. Such preemption can be avoided in
688 * a number of ways, for example, by invoking preempt_disable() before
689 * critical section's outermost rcu_read_lock().
691 * Given that the set of locks acquired by rt_mutex_unlock() might change
692 * at any time, a somewhat more future-proofed approach is to make sure
693 * that that preemption never happens within any RCU read-side critical
694 * section whose outermost rcu_read_unlock() is called with irqs disabled.
695 * This approach relies on the fact that rt_mutex_unlock() currently only
696 * acquires irq-disabled locks.
698 * The second of these two approaches is best in most situations,
699 * however, the first approach can also be useful, at least to those
700 * developers willing to keep abreast of the set of locks acquired by
703 * See rcu_read_lock() for more information.
705 static inline void rcu_read_unlock(void)
707 RCU_LOCKDEP_WARN(!rcu_is_watching(),
708 "rcu_read_unlock() used illegally while idle");
711 rcu_lock_release(&rcu_lock_map
); /* Keep acq info for rls diags. */
715 * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
717 * This is equivalent of rcu_read_lock(), but also disables softirqs.
718 * Note that anything else that disables softirqs can also serve as
719 * an RCU read-side critical section.
721 * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
722 * must occur in the same context, for example, it is illegal to invoke
723 * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
724 * was invoked from some other task.
726 static inline void rcu_read_lock_bh(void)
730 rcu_lock_acquire(&rcu_bh_lock_map
);
731 RCU_LOCKDEP_WARN(!rcu_is_watching(),
732 "rcu_read_lock_bh() used illegally while idle");
736 * rcu_read_unlock_bh() - marks the end of a softirq-only RCU critical section
738 * See rcu_read_lock_bh() for more information.
740 static inline void rcu_read_unlock_bh(void)
742 RCU_LOCKDEP_WARN(!rcu_is_watching(),
743 "rcu_read_unlock_bh() used illegally while idle");
744 rcu_lock_release(&rcu_bh_lock_map
);
750 * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
752 * This is equivalent of rcu_read_lock(), but disables preemption.
753 * Read-side critical sections can also be introduced by anything else
754 * that disables preemption, including local_irq_disable() and friends.
756 * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
757 * must occur in the same context, for example, it is illegal to invoke
758 * rcu_read_unlock_sched() from process context if the matching
759 * rcu_read_lock_sched() was invoked from an NMI handler.
761 static inline void rcu_read_lock_sched(void)
764 __acquire(RCU_SCHED
);
765 rcu_lock_acquire(&rcu_sched_lock_map
);
766 RCU_LOCKDEP_WARN(!rcu_is_watching(),
767 "rcu_read_lock_sched() used illegally while idle");
770 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
771 static inline notrace
void rcu_read_lock_sched_notrace(void)
773 preempt_disable_notrace();
774 __acquire(RCU_SCHED
);
778 * rcu_read_unlock_sched() - marks the end of a RCU-classic critical section
780 * See rcu_read_lock_sched() for more information.
782 static inline void rcu_read_unlock_sched(void)
784 RCU_LOCKDEP_WARN(!rcu_is_watching(),
785 "rcu_read_unlock_sched() used illegally while idle");
786 rcu_lock_release(&rcu_sched_lock_map
);
787 __release(RCU_SCHED
);
791 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
792 static inline notrace
void rcu_read_unlock_sched_notrace(void)
794 __release(RCU_SCHED
);
795 preempt_enable_notrace();
799 * RCU_INIT_POINTER() - initialize an RCU protected pointer
800 * @p: The pointer to be initialized.
801 * @v: The value to initialized the pointer to.
803 * Initialize an RCU-protected pointer in special cases where readers
804 * do not need ordering constraints on the CPU or the compiler. These
807 * 1. This use of RCU_INIT_POINTER() is NULLing out the pointer *or*
808 * 2. The caller has taken whatever steps are required to prevent
809 * RCU readers from concurrently accessing this pointer *or*
810 * 3. The referenced data structure has already been exposed to
811 * readers either at compile time or via rcu_assign_pointer() *and*
813 * a. You have not made *any* reader-visible changes to
814 * this structure since then *or*
815 * b. It is OK for readers accessing this structure from its
816 * new location to see the old state of the structure. (For
817 * example, the changes were to statistical counters or to
818 * other state where exact synchronization is not required.)
820 * Failure to follow these rules governing use of RCU_INIT_POINTER() will
821 * result in impossible-to-diagnose memory corruption. As in the structures
822 * will look OK in crash dumps, but any concurrent RCU readers might
823 * see pre-initialized values of the referenced data structure. So
824 * please be very careful how you use RCU_INIT_POINTER()!!!
826 * If you are creating an RCU-protected linked structure that is accessed
827 * by a single external-to-structure RCU-protected pointer, then you may
828 * use RCU_INIT_POINTER() to initialize the internal RCU-protected
829 * pointers, but you must use rcu_assign_pointer() to initialize the
830 * external-to-structure pointer *after* you have completely initialized
831 * the reader-accessible portions of the linked structure.
833 * Note that unlike rcu_assign_pointer(), RCU_INIT_POINTER() provides no
834 * ordering guarantees for either the CPU or the compiler.
836 #define RCU_INIT_POINTER(p, v) \
838 rcu_check_sparse(p, __rcu); \
839 WRITE_ONCE(p, RCU_INITIALIZER(v)); \
843 * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer
844 * @p: The pointer to be initialized.
845 * @v: The value to initialized the pointer to.
847 * GCC-style initialization for an RCU-protected pointer in a structure field.
849 #define RCU_POINTER_INITIALIZER(p, v) \
850 .p = RCU_INITIALIZER(v)
853 * Does the specified offset indicate that the corresponding rcu_head
854 * structure can be handled by kvfree_rcu()?
856 #define __is_kvfree_rcu_offset(offset) ((offset) < 4096)
859 * kfree_rcu() - kfree an object after a grace period.
860 * @ptr: pointer to kfree for both single- and double-argument invocations.
861 * @rhf: the name of the struct rcu_head within the type of @ptr,
862 * but only for double-argument invocations.
864 * Many rcu callbacks functions just call kfree() on the base structure.
865 * These functions are trivial, but their size adds up, and furthermore
866 * when they are used in a kernel module, that module must invoke the
867 * high-latency rcu_barrier() function at module-unload time.
869 * The kfree_rcu() function handles this issue. Rather than encoding a
870 * function address in the embedded rcu_head structure, kfree_rcu() instead
871 * encodes the offset of the rcu_head structure within the base structure.
872 * Because the functions are not allowed in the low-order 4096 bytes of
873 * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
874 * If the offset is larger than 4095 bytes, a compile-time error will
875 * be generated in kvfree_rcu_arg_2(). If this error is triggered, you can
876 * either fall back to use of call_rcu() or rearrange the structure to
877 * position the rcu_head structure into the first 4096 bytes.
879 * Note that the allowable offset might decrease in the future, for example,
880 * to allow something like kmem_cache_free_rcu().
882 * The BUILD_BUG_ON check must not involve any function calls, hence the
883 * checks are done in macros here.
885 #define kfree_rcu(ptr, rhf...) kvfree_rcu(ptr, ## rhf)
888 * kvfree_rcu() - kvfree an object after a grace period.
890 * This macro consists of one or two arguments and it is
891 * based on whether an object is head-less or not. If it
892 * has a head then a semantic stays the same as it used
895 * kvfree_rcu(ptr, rhf);
897 * where @ptr is a pointer to kvfree(), @rhf is the name
898 * of the rcu_head structure within the type of @ptr.
900 * When it comes to head-less variant, only one argument
901 * is passed and that is just a pointer which has to be
902 * freed after a grace period. Therefore the semantic is
906 * where @ptr is a pointer to kvfree().
908 * Please note, head-less way of freeing is permitted to
909 * use from a context that has to follow might_sleep()
910 * annotation. Otherwise, please switch and embed the
911 * rcu_head structure within the type of @ptr.
913 #define kvfree_rcu(...) KVFREE_GET_MACRO(__VA_ARGS__, \
914 kvfree_rcu_arg_2, kvfree_rcu_arg_1)(__VA_ARGS__)
916 #define KVFREE_GET_MACRO(_1, _2, NAME, ...) NAME
917 #define kvfree_rcu_arg_2(ptr, rhf) \
919 typeof (ptr) ___p = (ptr); \
922 BUILD_BUG_ON(!__is_kvfree_rcu_offset(offsetof(typeof(*(ptr)), rhf))); \
923 kvfree_call_rcu(&((___p)->rhf), (rcu_callback_t)(unsigned long) \
924 (offsetof(typeof(*(ptr)), rhf))); \
928 #define kvfree_rcu_arg_1(ptr) \
930 typeof(ptr) ___p = (ptr); \
933 kvfree_call_rcu(NULL, (rcu_callback_t) (___p)); \
937 * Place this after a lock-acquisition primitive to guarantee that
938 * an UNLOCK+LOCK pair acts as a full barrier. This guarantee applies
939 * if the UNLOCK and LOCK are executed by the same CPU or if the
940 * UNLOCK and LOCK operate on the same lock variable.
942 #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE
943 #define smp_mb__after_unlock_lock() smp_mb() /* Full ordering for lock. */
944 #else /* #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
945 #define smp_mb__after_unlock_lock() do { } while (0)
946 #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
949 /* Has the specified rcu_head structure been handed to call_rcu()? */
952 * rcu_head_init - Initialize rcu_head for rcu_head_after_call_rcu()
953 * @rhp: The rcu_head structure to initialize.
955 * If you intend to invoke rcu_head_after_call_rcu() to test whether a
956 * given rcu_head structure has already been passed to call_rcu(), then
957 * you must also invoke this rcu_head_init() function on it just after
958 * allocating that structure. Calls to this function must not race with
959 * calls to call_rcu(), rcu_head_after_call_rcu(), or callback invocation.
961 static inline void rcu_head_init(struct rcu_head
*rhp
)
963 rhp
->func
= (rcu_callback_t
)~0L;
967 * rcu_head_after_call_rcu() - Has this rcu_head been passed to call_rcu()?
968 * @rhp: The rcu_head structure to test.
969 * @f: The function passed to call_rcu() along with @rhp.
971 * Returns @true if the @rhp has been passed to call_rcu() with @func,
972 * and @false otherwise. Emits a warning in any other case, including
973 * the case where @rhp has already been invoked after a grace period.
974 * Calls to this function must not race with callback invocation. One way
975 * to avoid such races is to enclose the call to rcu_head_after_call_rcu()
976 * in an RCU read-side critical section that includes a read-side fetch
977 * of the pointer to the structure containing @rhp.
980 rcu_head_after_call_rcu(struct rcu_head
*rhp
, rcu_callback_t f
)
982 rcu_callback_t func
= READ_ONCE(rhp
->func
);
986 WARN_ON_ONCE(func
!= (rcu_callback_t
)~0L);
990 /* kernel/ksysfs.c definitions */
991 extern int rcu_expedited
;
992 extern int rcu_normal
;
994 #endif /* __LINUX_RCUPDATE_H */