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)))
37 /* Exported common interfaces */
38 void call_rcu(struct rcu_head *head, rcu_callback_t func);
39 void rcu_barrier_tasks(void);
40 void rcu_barrier_tasks_rude(void);
41 void synchronize_rcu(void);
43 #ifdef CONFIG_PREEMPT_RCU
45 void __rcu_read_lock(void);
46 void __rcu_read_unlock(void);
49 * Defined as a macro as it is a very low level header included from
50 * areas that don't even know about current. This gives the rcu_read_lock()
51 * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
52 * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
54 #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
56 #else /* #ifdef CONFIG_PREEMPT_RCU */
58 #ifdef CONFIG_TINY_RCU
59 #define rcu_read_unlock_strict() do { } while (0)
61 void rcu_read_unlock_strict(void);
64 static inline void __rcu_read_lock(void)
69 static inline void __rcu_read_unlock(void)
72 rcu_read_unlock_strict();
75 static inline int rcu_preempt_depth(void)
80 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
82 /* Internal to kernel */
84 extern int rcu_scheduler_active __read_mostly;
85 void rcu_sched_clock_irq(int user);
86 void rcu_report_dead(unsigned int cpu);
87 void rcutree_migrate_callbacks(int cpu);
89 #ifdef CONFIG_TASKS_RCU_GENERIC
90 void rcu_init_tasks_generic(void);
92 static inline void rcu_init_tasks_generic(void) { }
95 #ifdef CONFIG_RCU_STALL_COMMON
96 void rcu_sysrq_start(void);
97 void rcu_sysrq_end(void);
98 #else /* #ifdef CONFIG_RCU_STALL_COMMON */
99 static inline void rcu_sysrq_start(void) { }
100 static inline void rcu_sysrq_end(void) { }
101 #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
103 #ifdef CONFIG_NO_HZ_FULL
104 void rcu_user_enter(void);
105 void rcu_user_exit(void);
107 static inline void rcu_user_enter(void) { }
108 static inline void rcu_user_exit(void) { }
109 #endif /* CONFIG_NO_HZ_FULL */
111 #ifdef CONFIG_RCU_NOCB_CPU
112 void rcu_init_nohz(void);
113 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
114 static inline void rcu_init_nohz(void) { }
115 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
118 * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
119 * @a: Code that RCU needs to pay attention to.
121 * RCU read-side critical sections are forbidden in the inner idle loop,
122 * that is, between the rcu_idle_enter() and the rcu_idle_exit() -- RCU
123 * will happily ignore any such read-side critical sections. However,
124 * things like powertop need tracepoints in the inner idle loop.
126 * This macro provides the way out: RCU_NONIDLE(do_something_with_RCU())
127 * will tell RCU that it needs to pay attention, invoke its argument
128 * (in this example, calling the do_something_with_RCU() function),
129 * and then tell RCU to go back to ignoring this CPU. It is permissible
130 * to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is
131 * on the order of a million or so, even on 32-bit systems). It is
132 * not legal to block within RCU_NONIDLE(), nor is it permissible to
133 * transfer control either into or out of RCU_NONIDLE()'s statement.
135 #define RCU_NONIDLE(a) \
137 rcu_irq_enter_irqson(); \
138 do { a; } while (0); \
139 rcu_irq_exit_irqson(); \
143 * Note a quasi-voluntary context switch for RCU-tasks's benefit.
144 * This is a macro rather than an inline function to avoid #include hell.
146 #ifdef CONFIG_TASKS_RCU_GENERIC
148 # ifdef CONFIG_TASKS_RCU
149 # define rcu_tasks_classic_qs(t, preempt) \
151 if (!(preempt) && READ_ONCE((t)->rcu_tasks_holdout)) \
152 WRITE_ONCE((t)->rcu_tasks_holdout, false); \
154 void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
155 void synchronize_rcu_tasks(void);
157 # define rcu_tasks_classic_qs(t, preempt) do { } while (0)
158 # define call_rcu_tasks call_rcu
159 # define synchronize_rcu_tasks synchronize_rcu
162 # ifdef CONFIG_TASKS_RCU_TRACE
163 # define rcu_tasks_trace_qs(t) \
165 if (!likely(READ_ONCE((t)->trc_reader_checked)) && \
166 !unlikely(READ_ONCE((t)->trc_reader_nesting))) { \
167 smp_store_release(&(t)->trc_reader_checked, true); \
168 smp_mb(); /* Readers partitioned by store. */ \
172 # define rcu_tasks_trace_qs(t) do { } while (0)
175 #define rcu_tasks_qs(t, preempt) \
177 rcu_tasks_classic_qs((t), (preempt)); \
178 rcu_tasks_trace_qs((t)); \
181 # ifdef CONFIG_TASKS_RUDE_RCU
182 void call_rcu_tasks_rude(struct rcu_head *head, rcu_callback_t func);
183 void synchronize_rcu_tasks_rude(void);
186 #define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t, false)
187 void exit_tasks_rcu_start(void);
188 void exit_tasks_rcu_finish(void);
189 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
190 #define rcu_tasks_qs(t, preempt) do { } while (0)
191 #define rcu_note_voluntary_context_switch(t) do { } while (0)
192 #define call_rcu_tasks call_rcu
193 #define synchronize_rcu_tasks synchronize_rcu
194 static inline void exit_tasks_rcu_start(void) { }
195 static inline void exit_tasks_rcu_finish(void) { }
196 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
199 * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
201 * This macro resembles cond_resched(), except that it is defined to
202 * report potential quiescent states to RCU-tasks even if the cond_resched()
203 * machinery were to be shut off, as some advocate for PREEMPTION kernels.
205 #define cond_resched_tasks_rcu_qs() \
207 rcu_tasks_qs(current, false); \
212 * Infrastructure to implement the synchronize_() primitives in
213 * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
216 #if defined(CONFIG_TREE_RCU)
217 #include <linux/rcutree.h>
218 #elif defined(CONFIG_TINY_RCU)
219 #include <linux/rcutiny.h>
221 #error "Unknown RCU implementation specified to kernel configuration"
225 * The init_rcu_head_on_stack() and destroy_rcu_head_on_stack() calls
226 * are needed for dynamic initialization and destruction of rcu_head
227 * on the stack, and init_rcu_head()/destroy_rcu_head() are needed for
228 * dynamic initialization and destruction of statically allocated rcu_head
229 * structures. However, rcu_head structures allocated dynamically in the
230 * heap don't need any initialization.
232 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
233 void init_rcu_head(struct rcu_head *head);
234 void destroy_rcu_head(struct rcu_head *head);
235 void init_rcu_head_on_stack(struct rcu_head *head);
236 void destroy_rcu_head_on_stack(struct rcu_head *head);
237 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
238 static inline void init_rcu_head(struct rcu_head *head) { }
239 static inline void destroy_rcu_head(struct rcu_head *head) { }
240 static inline void init_rcu_head_on_stack(struct rcu_head *head) { }
241 static inline void destroy_rcu_head_on_stack(struct rcu_head *head) { }
242 #endif /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
244 #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
245 bool rcu_lockdep_current_cpu_online(void);
246 #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
247 static inline bool rcu_lockdep_current_cpu_online(void) { return true; }
248 #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
250 extern struct lockdep_map rcu_lock_map;
251 extern struct lockdep_map rcu_bh_lock_map;
252 extern struct lockdep_map rcu_sched_lock_map;
253 extern struct lockdep_map rcu_callback_map;
255 #ifdef CONFIG_DEBUG_LOCK_ALLOC
257 static inline void rcu_lock_acquire(struct lockdep_map *map)
259 lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
262 static inline void rcu_lock_release(struct lockdep_map *map)
264 lock_release(map, _THIS_IP_);
267 int debug_lockdep_rcu_enabled(void);
268 int rcu_read_lock_held(void);
269 int rcu_read_lock_bh_held(void);
270 int rcu_read_lock_sched_held(void);
271 int rcu_read_lock_any_held(void);
273 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
275 # define rcu_lock_acquire(a) do { } while (0)
276 # define rcu_lock_release(a) do { } while (0)
278 static inline int rcu_read_lock_held(void)
283 static inline int rcu_read_lock_bh_held(void)
288 static inline int rcu_read_lock_sched_held(void)
290 return !preemptible();
293 static inline int rcu_read_lock_any_held(void)
295 return !preemptible();
298 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
300 #ifdef CONFIG_PROVE_RCU
303 * RCU_LOCKDEP_WARN - emit lockdep splat if specified condition is met
304 * @c: condition to check
305 * @s: informative message
307 #define RCU_LOCKDEP_WARN(c, s) \
309 static bool __section(".data.unlikely") __warned; \
310 if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \
312 lockdep_rcu_suspicious(__FILE__, __LINE__, s); \
316 #if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
317 static inline void rcu_preempt_sleep_check(void)
319 RCU_LOCKDEP_WARN(lock_is_held(&rcu_lock_map),
320 "Illegal context switch in RCU read-side critical section");
322 #else /* #ifdef CONFIG_PROVE_RCU */
323 static inline void rcu_preempt_sleep_check(void) { }
324 #endif /* #else #ifdef CONFIG_PROVE_RCU */
326 #define rcu_sleep_check() \
328 rcu_preempt_sleep_check(); \
329 RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map), \
330 "Illegal context switch in RCU-bh read-side critical section"); \
331 RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map), \
332 "Illegal context switch in RCU-sched read-side critical section"); \
335 #else /* #ifdef CONFIG_PROVE_RCU */
337 #define RCU_LOCKDEP_WARN(c, s) do { } while (0 && (c))
338 #define rcu_sleep_check() do { } while (0)
340 #endif /* #else #ifdef CONFIG_PROVE_RCU */
343 * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
344 * and rcu_assign_pointer(). Some of these could be folded into their
345 * callers, but they are left separate in order to ease introduction of
346 * multiple pointers markings to match different RCU implementations
347 * (e.g., __srcu), should this make sense in the future.
351 #define rcu_check_sparse(p, space) \
352 ((void)(((typeof(*p) space *)p) == p))
353 #else /* #ifdef __CHECKER__ */
354 #define rcu_check_sparse(p, space)
355 #endif /* #else #ifdef __CHECKER__ */
357 #define __rcu_access_pointer(p, space) \
359 typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
360 rcu_check_sparse(p, space); \
361 ((typeof(*p) __force __kernel *)(_________p1)); \
363 #define __rcu_dereference_check(p, c, space) \
365 /* Dependency order vs. p above. */ \
366 typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
367 RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
368 rcu_check_sparse(p, space); \
369 ((typeof(*p) __force __kernel *)(________p1)); \
371 #define __rcu_dereference_protected(p, c, space) \
373 RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
374 rcu_check_sparse(p, space); \
375 ((typeof(*p) __force __kernel *)(p)); \
377 #define rcu_dereference_raw(p) \
379 /* Dependency order vs. p above. */ \
380 typeof(p) ________p1 = READ_ONCE(p); \
381 ((typeof(*p) __force __kernel *)(________p1)); \
385 * RCU_INITIALIZER() - statically initialize an RCU-protected global variable
386 * @v: The value to statically initialize with.
388 #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
391 * rcu_assign_pointer() - assign to RCU-protected pointer
392 * @p: pointer to assign to
393 * @v: value to assign (publish)
395 * Assigns the specified value to the specified RCU-protected
396 * pointer, ensuring that any concurrent RCU readers will see
397 * any prior initialization.
399 * Inserts memory barriers on architectures that require them
400 * (which is most of them), and also prevents the compiler from
401 * reordering the code that initializes the structure after the pointer
402 * assignment. More importantly, this call documents which pointers
403 * will be dereferenced by RCU read-side code.
405 * In some special cases, you may use RCU_INIT_POINTER() instead
406 * of rcu_assign_pointer(). RCU_INIT_POINTER() is a bit faster due
407 * to the fact that it does not constrain either the CPU or the compiler.
408 * That said, using RCU_INIT_POINTER() when you should have used
409 * rcu_assign_pointer() is a very bad thing that results in
410 * impossible-to-diagnose memory corruption. So please be careful.
411 * See the RCU_INIT_POINTER() comment header for details.
413 * Note that rcu_assign_pointer() evaluates each of its arguments only
414 * once, appearances notwithstanding. One of the "extra" evaluations
415 * is in typeof() and the other visible only to sparse (__CHECKER__),
416 * neither of which actually execute the argument. As with most cpp
417 * macros, this execute-arguments-only-once property is important, so
418 * please be careful when making changes to rcu_assign_pointer() and the
419 * other macros that it invokes.
421 #define rcu_assign_pointer(p, v) \
423 uintptr_t _r_a_p__v = (uintptr_t)(v); \
424 rcu_check_sparse(p, __rcu); \
426 if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL) \
427 WRITE_ONCE((p), (typeof(p))(_r_a_p__v)); \
429 smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
433 * rcu_replace_pointer() - replace an RCU pointer, returning its old value
434 * @rcu_ptr: RCU pointer, whose old value is returned
435 * @ptr: regular pointer
436 * @c: the lockdep conditions under which the dereference will take place
438 * Perform a replacement, where @rcu_ptr is an RCU-annotated
439 * pointer and @c is the lockdep argument that is passed to the
440 * rcu_dereference_protected() call used to read that pointer. The old
441 * value of @rcu_ptr is returned, and @rcu_ptr is set to @ptr.
443 #define rcu_replace_pointer(rcu_ptr, ptr, c) \
445 typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c)); \
446 rcu_assign_pointer((rcu_ptr), (ptr)); \
451 * rcu_access_pointer() - fetch RCU pointer with no dereferencing
452 * @p: The pointer to read
454 * Return the value of the specified RCU-protected pointer, but omit the
455 * lockdep checks for being in an RCU read-side critical section. This is
456 * useful when the value of this pointer is accessed, but the pointer is
457 * not dereferenced, for example, when testing an RCU-protected pointer
458 * against NULL. Although rcu_access_pointer() may also be used in cases
459 * where update-side locks prevent the value of the pointer from changing,
460 * you should instead use rcu_dereference_protected() for this use case.
462 * It is also permissible to use rcu_access_pointer() when read-side
463 * access to the pointer was removed at least one grace period ago, as
464 * is the case in the context of the RCU callback that is freeing up
465 * the data, or after a synchronize_rcu() returns. This can be useful
466 * when tearing down multi-linked structures after a grace period
469 #define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
472 * rcu_dereference_check() - rcu_dereference with debug checking
473 * @p: The pointer to read, prior to dereferencing
474 * @c: The conditions under which the dereference will take place
476 * Do an rcu_dereference(), but check that the conditions under which the
477 * dereference will take place are correct. Typically the conditions
478 * indicate the various locking conditions that should be held at that
479 * point. The check should return true if the conditions are satisfied.
480 * An implicit check for being in an RCU read-side critical section
481 * (rcu_read_lock()) is included.
485 * bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
487 * could be used to indicate to lockdep that foo->bar may only be dereferenced
488 * if either rcu_read_lock() is held, or that the lock required to replace
489 * the bar struct at foo->bar is held.
491 * Note that the list of conditions may also include indications of when a lock
492 * need not be held, for example during initialisation or destruction of the
495 * bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
496 * atomic_read(&foo->usage) == 0);
498 * Inserts memory barriers on architectures that require them
499 * (currently only the Alpha), prevents the compiler from refetching
500 * (and from merging fetches), and, more importantly, documents exactly
501 * which pointers are protected by RCU and checks that the pointer is
502 * annotated as __rcu.
504 #define rcu_dereference_check(p, c) \
505 __rcu_dereference_check((p), (c) || rcu_read_lock_held(), __rcu)
508 * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
509 * @p: The pointer to read, prior to dereferencing
510 * @c: The conditions under which the dereference will take place
512 * This is the RCU-bh counterpart to rcu_dereference_check().
514 #define rcu_dereference_bh_check(p, c) \
515 __rcu_dereference_check((p), (c) || rcu_read_lock_bh_held(), __rcu)
518 * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
519 * @p: The pointer to read, prior to dereferencing
520 * @c: The conditions under which the dereference will take place
522 * This is the RCU-sched counterpart to rcu_dereference_check().
524 #define rcu_dereference_sched_check(p, c) \
525 __rcu_dereference_check((p), (c) || rcu_read_lock_sched_held(), \
529 * The tracing infrastructure traces RCU (we want that), but unfortunately
530 * some of the RCU checks causes tracing to lock up the system.
532 * The no-tracing version of rcu_dereference_raw() must not call
533 * rcu_read_lock_held().
535 #define rcu_dereference_raw_check(p) __rcu_dereference_check((p), 1, __rcu)
538 * rcu_dereference_protected() - fetch RCU pointer when updates prevented
539 * @p: The pointer to read, prior to dereferencing
540 * @c: The conditions under which the dereference will take place
542 * Return the value of the specified RCU-protected pointer, but omit
543 * the READ_ONCE(). This is useful in cases where update-side locks
544 * prevent the value of the pointer from changing. Please note that this
545 * primitive does *not* prevent the compiler from repeating this reference
546 * or combining it with other references, so it should not be used without
547 * protection of appropriate locks.
549 * This function is only for update-side use. Using this function
550 * when protected only by rcu_read_lock() will result in infrequent
551 * but very ugly failures.
553 #define rcu_dereference_protected(p, c) \
554 __rcu_dereference_protected((p), (c), __rcu)
558 * rcu_dereference() - fetch RCU-protected pointer for dereferencing
559 * @p: The pointer to read, prior to dereferencing
561 * This is a simple wrapper around rcu_dereference_check().
563 #define rcu_dereference(p) rcu_dereference_check(p, 0)
566 * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
567 * @p: The pointer to read, prior to dereferencing
569 * Makes rcu_dereference_check() do the dirty work.
571 #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
574 * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
575 * @p: The pointer to read, prior to dereferencing
577 * Makes rcu_dereference_check() do the dirty work.
579 #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
582 * rcu_pointer_handoff() - Hand off a pointer from RCU to other mechanism
583 * @p: The pointer to hand off
585 * This is simply an identity function, but it documents where a pointer
586 * is handed off from RCU to some other synchronization mechanism, for
587 * example, reference counting or locking. In C11, it would map to
588 * kill_dependency(). It could be used as follows::
591 * p = rcu_dereference(gp);
592 * long_lived = is_long_lived(p);
594 * if (!atomic_inc_not_zero(p->refcnt))
595 * long_lived = false;
597 * p = rcu_pointer_handoff(p);
601 #define rcu_pointer_handoff(p) (p)
604 * rcu_read_lock() - mark the beginning of an RCU read-side critical section
606 * When synchronize_rcu() is invoked on one CPU while other CPUs
607 * are within RCU read-side critical sections, then the
608 * synchronize_rcu() is guaranteed to block until after all the other
609 * CPUs exit their critical sections. Similarly, if call_rcu() is invoked
610 * on one CPU while other CPUs are within RCU read-side critical
611 * sections, invocation of the corresponding RCU callback is deferred
612 * until after the all the other CPUs exit their critical sections.
614 * Note, however, that RCU callbacks are permitted to run concurrently
615 * with new RCU read-side critical sections. One way that this can happen
616 * is via the following sequence of events: (1) CPU 0 enters an RCU
617 * read-side critical section, (2) CPU 1 invokes call_rcu() to register
618 * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
619 * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
620 * callback is invoked. This is legal, because the RCU read-side critical
621 * section that was running concurrently with the call_rcu() (and which
622 * therefore might be referencing something that the corresponding RCU
623 * callback would free up) has completed before the corresponding
624 * RCU callback is invoked.
626 * RCU read-side critical sections may be nested. Any deferred actions
627 * will be deferred until the outermost RCU read-side critical section
630 * You can avoid reading and understanding the next paragraph by
631 * following this rule: don't put anything in an rcu_read_lock() RCU
632 * read-side critical section that would block in a !PREEMPTION kernel.
633 * But if you want the full story, read on!
635 * In non-preemptible RCU implementations (pure TREE_RCU and TINY_RCU),
636 * it is illegal to block while in an RCU read-side critical section.
637 * In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPTION
638 * kernel builds, RCU read-side critical sections may be preempted,
639 * but explicit blocking is illegal. Finally, in preemptible RCU
640 * implementations in real-time (with -rt patchset) kernel builds, RCU
641 * read-side critical sections may be preempted and they may also block, but
642 * only when acquiring spinlocks that are subject to priority inheritance.
644 static __always_inline void rcu_read_lock(void)
648 rcu_lock_acquire(&rcu_lock_map);
649 RCU_LOCKDEP_WARN(!rcu_is_watching(),
650 "rcu_read_lock() used illegally while idle");
654 * So where is rcu_write_lock()? It does not exist, as there is no
655 * way for writers to lock out RCU readers. This is a feature, not
656 * a bug -- this property is what provides RCU's performance benefits.
657 * Of course, writers must coordinate with each other. The normal
658 * spinlock primitives work well for this, but any other technique may be
659 * used as well. RCU does not care how the writers keep out of each
660 * others' way, as long as they do so.
664 * rcu_read_unlock() - marks the end of an RCU read-side critical section.
666 * In most situations, rcu_read_unlock() is immune from deadlock.
667 * However, in kernels built with CONFIG_RCU_BOOST, rcu_read_unlock()
668 * is responsible for deboosting, which it does via rt_mutex_unlock().
669 * Unfortunately, this function acquires the scheduler's runqueue and
670 * priority-inheritance spinlocks. This means that deadlock could result
671 * if the caller of rcu_read_unlock() already holds one of these locks or
672 * any lock that is ever acquired while holding them.
674 * That said, RCU readers are never priority boosted unless they were
675 * preempted. Therefore, one way to avoid deadlock is to make sure
676 * that preemption never happens within any RCU read-side critical
677 * section whose outermost rcu_read_unlock() is called with one of
678 * rt_mutex_unlock()'s locks held. Such preemption can be avoided in
679 * a number of ways, for example, by invoking preempt_disable() before
680 * critical section's outermost rcu_read_lock().
682 * Given that the set of locks acquired by rt_mutex_unlock() might change
683 * at any time, a somewhat more future-proofed approach is to make sure
684 * that that preemption never happens within any RCU read-side critical
685 * section whose outermost rcu_read_unlock() is called with irqs disabled.
686 * This approach relies on the fact that rt_mutex_unlock() currently only
687 * acquires irq-disabled locks.
689 * The second of these two approaches is best in most situations,
690 * however, the first approach can also be useful, at least to those
691 * developers willing to keep abreast of the set of locks acquired by
694 * See rcu_read_lock() for more information.
696 static inline void rcu_read_unlock(void)
698 RCU_LOCKDEP_WARN(!rcu_is_watching(),
699 "rcu_read_unlock() used illegally while idle");
702 rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
706 * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
708 * This is equivalent of rcu_read_lock(), but also disables softirqs.
709 * Note that anything else that disables softirqs can also serve as
710 * an RCU read-side critical section.
712 * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
713 * must occur in the same context, for example, it is illegal to invoke
714 * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
715 * was invoked from some other task.
717 static inline void rcu_read_lock_bh(void)
721 rcu_lock_acquire(&rcu_bh_lock_map);
722 RCU_LOCKDEP_WARN(!rcu_is_watching(),
723 "rcu_read_lock_bh() used illegally while idle");
727 * rcu_read_unlock_bh() - marks the end of a softirq-only RCU critical section
729 * See rcu_read_lock_bh() for more information.
731 static inline void rcu_read_unlock_bh(void)
733 RCU_LOCKDEP_WARN(!rcu_is_watching(),
734 "rcu_read_unlock_bh() used illegally while idle");
735 rcu_lock_release(&rcu_bh_lock_map);
741 * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
743 * This is equivalent of rcu_read_lock(), but disables preemption.
744 * Read-side critical sections can also be introduced by anything else
745 * that disables preemption, including local_irq_disable() and friends.
747 * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
748 * must occur in the same context, for example, it is illegal to invoke
749 * rcu_read_unlock_sched() from process context if the matching
750 * rcu_read_lock_sched() was invoked from an NMI handler.
752 static inline void rcu_read_lock_sched(void)
755 __acquire(RCU_SCHED);
756 rcu_lock_acquire(&rcu_sched_lock_map);
757 RCU_LOCKDEP_WARN(!rcu_is_watching(),
758 "rcu_read_lock_sched() used illegally while idle");
761 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
762 static inline notrace void rcu_read_lock_sched_notrace(void)
764 preempt_disable_notrace();
765 __acquire(RCU_SCHED);
769 * rcu_read_unlock_sched() - marks the end of a RCU-classic critical section
771 * See rcu_read_lock_sched() for more information.
773 static inline void rcu_read_unlock_sched(void)
775 RCU_LOCKDEP_WARN(!rcu_is_watching(),
776 "rcu_read_unlock_sched() used illegally while idle");
777 rcu_lock_release(&rcu_sched_lock_map);
778 __release(RCU_SCHED);
782 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
783 static inline notrace void rcu_read_unlock_sched_notrace(void)
785 __release(RCU_SCHED);
786 preempt_enable_notrace();
790 * RCU_INIT_POINTER() - initialize an RCU protected pointer
791 * @p: The pointer to be initialized.
792 * @v: The value to initialized the pointer to.
794 * Initialize an RCU-protected pointer in special cases where readers
795 * do not need ordering constraints on the CPU or the compiler. These
798 * 1. This use of RCU_INIT_POINTER() is NULLing out the pointer *or*
799 * 2. The caller has taken whatever steps are required to prevent
800 * RCU readers from concurrently accessing this pointer *or*
801 * 3. The referenced data structure has already been exposed to
802 * readers either at compile time or via rcu_assign_pointer() *and*
804 * a. You have not made *any* reader-visible changes to
805 * this structure since then *or*
806 * b. It is OK for readers accessing this structure from its
807 * new location to see the old state of the structure. (For
808 * example, the changes were to statistical counters or to
809 * other state where exact synchronization is not required.)
811 * Failure to follow these rules governing use of RCU_INIT_POINTER() will
812 * result in impossible-to-diagnose memory corruption. As in the structures
813 * will look OK in crash dumps, but any concurrent RCU readers might
814 * see pre-initialized values of the referenced data structure. So
815 * please be very careful how you use RCU_INIT_POINTER()!!!
817 * If you are creating an RCU-protected linked structure that is accessed
818 * by a single external-to-structure RCU-protected pointer, then you may
819 * use RCU_INIT_POINTER() to initialize the internal RCU-protected
820 * pointers, but you must use rcu_assign_pointer() to initialize the
821 * external-to-structure pointer *after* you have completely initialized
822 * the reader-accessible portions of the linked structure.
824 * Note that unlike rcu_assign_pointer(), RCU_INIT_POINTER() provides no
825 * ordering guarantees for either the CPU or the compiler.
827 #define RCU_INIT_POINTER(p, v) \
829 rcu_check_sparse(p, __rcu); \
830 WRITE_ONCE(p, RCU_INITIALIZER(v)); \
834 * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer
835 * @p: The pointer to be initialized.
836 * @v: The value to initialized the pointer to.
838 * GCC-style initialization for an RCU-protected pointer in a structure field.
840 #define RCU_POINTER_INITIALIZER(p, v) \
841 .p = RCU_INITIALIZER(v)
844 * Does the specified offset indicate that the corresponding rcu_head
845 * structure can be handled by kvfree_rcu()?
847 #define __is_kvfree_rcu_offset(offset) ((offset) < 4096)
850 * Helper macro for kfree_rcu() to prevent argument-expansion eyestrain.
852 #define __kvfree_rcu(head, offset) \
854 BUILD_BUG_ON(!__is_kvfree_rcu_offset(offset)); \
855 kvfree_call_rcu(head, (rcu_callback_t)(unsigned long)(offset)); \
859 * kfree_rcu() - kfree an object after a grace period.
860 * @ptr: pointer to kfree
861 * @rhf: the name of the struct rcu_head within the type of @ptr.
863 * Many rcu callbacks functions just call kfree() on the base structure.
864 * These functions are trivial, but their size adds up, and furthermore
865 * when they are used in a kernel module, that module must invoke the
866 * high-latency rcu_barrier() function at module-unload time.
868 * The kfree_rcu() function handles this issue. Rather than encoding a
869 * function address in the embedded rcu_head structure, kfree_rcu() instead
870 * encodes the offset of the rcu_head structure within the base structure.
871 * Because the functions are not allowed in the low-order 4096 bytes of
872 * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
873 * If the offset is larger than 4095 bytes, a compile-time error will
874 * be generated in __kvfree_rcu(). If this error is triggered, you can
875 * either fall back to use of call_rcu() or rearrange the structure to
876 * position the rcu_head structure into the first 4096 bytes.
878 * Note that the allowable offset might decrease in the future, for example,
879 * to allow something like kmem_cache_free_rcu().
881 * The BUILD_BUG_ON check must not involve any function calls, hence the
882 * checks are done in macros here.
884 #define kfree_rcu(ptr, rhf) \
886 typeof (ptr) ___p = (ptr); \
889 __kvfree_rcu(&((___p)->rhf), offsetof(typeof(*(ptr)), rhf)); \
893 * kvfree_rcu() - kvfree an object after a grace period.
895 * This macro consists of one or two arguments and it is
896 * based on whether an object is head-less or not. If it
897 * has a head then a semantic stays the same as it used
900 * kvfree_rcu(ptr, rhf);
902 * where @ptr is a pointer to kvfree(), @rhf is the name
903 * of the rcu_head structure within the type of @ptr.
905 * When it comes to head-less variant, only one argument
906 * is passed and that is just a pointer which has to be
907 * freed after a grace period. Therefore the semantic is
911 * where @ptr is a pointer to kvfree().
913 * Please note, head-less way of freeing is permitted to
914 * use from a context that has to follow might_sleep()
915 * annotation. Otherwise, please switch and embed the
916 * rcu_head structure within the type of @ptr.
918 #define kvfree_rcu(...) KVFREE_GET_MACRO(__VA_ARGS__, \
919 kvfree_rcu_arg_2, kvfree_rcu_arg_1)(__VA_ARGS__)
921 #define KVFREE_GET_MACRO(_1, _2, NAME, ...) NAME
922 #define kvfree_rcu_arg_2(ptr, rhf) kfree_rcu(ptr, rhf)
923 #define kvfree_rcu_arg_1(ptr) \
925 typeof(ptr) ___p = (ptr); \
928 kvfree_call_rcu(NULL, (rcu_callback_t) (___p)); \
932 * Place this after a lock-acquisition primitive to guarantee that
933 * an UNLOCK+LOCK pair acts as a full barrier. This guarantee applies
934 * if the UNLOCK and LOCK are executed by the same CPU or if the
935 * UNLOCK and LOCK operate on the same lock variable.
937 #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE
938 #define smp_mb__after_unlock_lock() smp_mb() /* Full ordering for lock. */
939 #else /* #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
940 #define smp_mb__after_unlock_lock() do { } while (0)
941 #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
944 /* Has the specified rcu_head structure been handed to call_rcu()? */
947 * rcu_head_init - Initialize rcu_head for rcu_head_after_call_rcu()
948 * @rhp: The rcu_head structure to initialize.
950 * If you intend to invoke rcu_head_after_call_rcu() to test whether a
951 * given rcu_head structure has already been passed to call_rcu(), then
952 * you must also invoke this rcu_head_init() function on it just after
953 * allocating that structure. Calls to this function must not race with
954 * calls to call_rcu(), rcu_head_after_call_rcu(), or callback invocation.
956 static inline void rcu_head_init(struct rcu_head *rhp)
958 rhp->func = (rcu_callback_t)~0L;
962 * rcu_head_after_call_rcu() - Has this rcu_head been passed to call_rcu()?
963 * @rhp: The rcu_head structure to test.
964 * @f: The function passed to call_rcu() along with @rhp.
966 * Returns @true if the @rhp has been passed to call_rcu() with @func,
967 * and @false otherwise. Emits a warning in any other case, including
968 * the case where @rhp has already been invoked after a grace period.
969 * Calls to this function must not race with callback invocation. One way
970 * to avoid such races is to enclose the call to rcu_head_after_call_rcu()
971 * in an RCU read-side critical section that includes a read-side fetch
972 * of the pointer to the structure containing @rhp.
975 rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
977 rcu_callback_t func = READ_ONCE(rhp->func);
981 WARN_ON_ONCE(func != (rcu_callback_t)~0L);
985 /* kernel/ksysfs.c definitions */
986 extern int rcu_expedited;
987 extern int rcu_normal;
989 #endif /* __LINUX_RCUPDATE_H */