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TOMOYO Linux Cross Reference
Linux/include/linux/rcupdate.h

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  1 /*
  2  * Read-Copy Update mechanism for mutual exclusion
  3  *
  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.
  8  *
  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.
 13  *
 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.
 17  *
 18  * Copyright IBM Corporation, 2001
 19  *
 20  * Author: Dipankar Sarma <dipankar@in.ibm.com>
 21  *
 22  * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
 23  * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
 24  * Papers:
 25  * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
 26  * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
 27  *
 28  * For detailed explanation of Read-Copy Update mechanism see -
 29  *              http://lse.sourceforge.net/locking/rcupdate.html
 30  *
 31  */
 32 
 33 #ifndef __LINUX_RCUPDATE_H
 34 #define __LINUX_RCUPDATE_H
 35 
 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>
 45 
 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)))
 49 
 50 /* Exported common interfaces */
 51 
 52 #ifdef CONFIG_PREEMPT_RCU
 53 void call_rcu(struct rcu_head *head, rcu_callback_t func);
 54 #else /* #ifdef CONFIG_PREEMPT_RCU */
 55 #define call_rcu        call_rcu_sched
 56 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 57 
 58 void call_rcu_bh(struct rcu_head *head, rcu_callback_t func);
 59 void call_rcu_sched(struct rcu_head *head, rcu_callback_t func);
 60 void synchronize_sched(void);
 61 void rcu_barrier_tasks(void);
 62 
 63 #ifdef CONFIG_PREEMPT_RCU
 64 
 65 void __rcu_read_lock(void);
 66 void __rcu_read_unlock(void);
 67 void rcu_read_unlock_special(struct task_struct *t);
 68 void synchronize_rcu(void);
 69 
 70 /*
 71  * Defined as a macro as it is a very low level header included from
 72  * areas that don't even know about current.  This gives the rcu_read_lock()
 73  * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
 74  * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
 75  */
 76 #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
 77 
 78 #else /* #ifdef CONFIG_PREEMPT_RCU */
 79 
 80 static inline void __rcu_read_lock(void)
 81 {
 82         if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
 83                 preempt_disable();
 84 }
 85 
 86 static inline void __rcu_read_unlock(void)
 87 {
 88         if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
 89                 preempt_enable();
 90 }
 91 
 92 static inline void synchronize_rcu(void)
 93 {
 94         synchronize_sched();
 95 }
 96 
 97 static inline int rcu_preempt_depth(void)
 98 {
 99         return 0;
100 }
101 
102 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
103 
104 /* Internal to kernel */
105 void rcu_init(void);
106 extern int rcu_scheduler_active __read_mostly;
107 void rcu_sched_qs(void);
108 void rcu_bh_qs(void);
109 void rcu_check_callbacks(int user);
110 void rcu_report_dead(unsigned int cpu);
111 void rcutree_migrate_callbacks(int cpu);
112 
113 #ifdef CONFIG_RCU_STALL_COMMON
114 void rcu_sysrq_start(void);
115 void rcu_sysrq_end(void);
116 #else /* #ifdef CONFIG_RCU_STALL_COMMON */
117 static inline void rcu_sysrq_start(void) { }
118 static inline void rcu_sysrq_end(void) { }
119 #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
120 
121 #ifdef CONFIG_NO_HZ_FULL
122 void rcu_user_enter(void);
123 void rcu_user_exit(void);
124 #else
125 static inline void rcu_user_enter(void) { }
126 static inline void rcu_user_exit(void) { }
127 #endif /* CONFIG_NO_HZ_FULL */
128 
129 #ifdef CONFIG_RCU_NOCB_CPU
130 void rcu_init_nohz(void);
131 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
132 static inline void rcu_init_nohz(void) { }
133 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
134 
135 /**
136  * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
137  * @a: Code that RCU needs to pay attention to.
138  *
139  * RCU, RCU-bh, and RCU-sched read-side critical sections are forbidden
140  * in the inner idle loop, that is, between the rcu_idle_enter() and
141  * the rcu_idle_exit() -- RCU will happily ignore any such read-side
142  * critical sections.  However, things like powertop need tracepoints
143  * in the inner idle loop.
144  *
145  * This macro provides the way out:  RCU_NONIDLE(do_something_with_RCU())
146  * will tell RCU that it needs to pay attention, invoke its argument
147  * (in this example, calling the do_something_with_RCU() function),
148  * and then tell RCU to go back to ignoring this CPU.  It is permissible
149  * to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is
150  * on the order of a million or so, even on 32-bit systems).  It is
151  * not legal to block within RCU_NONIDLE(), nor is it permissible to
152  * transfer control either into or out of RCU_NONIDLE()'s statement.
153  */
154 #define RCU_NONIDLE(a) \
155         do { \
156                 rcu_irq_enter_irqson(); \
157                 do { a; } while (0); \
158                 rcu_irq_exit_irqson(); \
159         } while (0)
160 
161 /*
162  * Note a voluntary context switch for RCU-tasks benefit.  This is a
163  * macro rather than an inline function to avoid #include hell.
164  */
165 #ifdef CONFIG_TASKS_RCU
166 #define rcu_note_voluntary_context_switch_lite(t) \
167         do { \
168                 if (READ_ONCE((t)->rcu_tasks_holdout)) \
169                         WRITE_ONCE((t)->rcu_tasks_holdout, false); \
170         } while (0)
171 #define rcu_note_voluntary_context_switch(t) \
172         do { \
173                 rcu_all_qs(); \
174                 rcu_note_voluntary_context_switch_lite(t); \
175         } while (0)
176 void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
177 void synchronize_rcu_tasks(void);
178 void exit_tasks_rcu_start(void);
179 void exit_tasks_rcu_finish(void);
180 #else /* #ifdef CONFIG_TASKS_RCU */
181 #define rcu_note_voluntary_context_switch_lite(t)       do { } while (0)
182 #define rcu_note_voluntary_context_switch(t)            rcu_all_qs()
183 #define call_rcu_tasks call_rcu_sched
184 #define synchronize_rcu_tasks synchronize_sched
185 static inline void exit_tasks_rcu_start(void) { }
186 static inline void exit_tasks_rcu_finish(void) { }
187 #endif /* #else #ifdef CONFIG_TASKS_RCU */
188 
189 /**
190  * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
191  *
192  * This macro resembles cond_resched(), except that it is defined to
193  * report potential quiescent states to RCU-tasks even if the cond_resched()
194  * machinery were to be shut off, as some advocate for PREEMPT kernels.
195  */
196 #define cond_resched_tasks_rcu_qs() \
197 do { \
198         if (!cond_resched()) \
199                 rcu_note_voluntary_context_switch_lite(current); \
200 } while (0)
201 
202 /*
203  * Infrastructure to implement the synchronize_() primitives in
204  * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
205  */
206 
207 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU)
208 #include <linux/rcutree.h>
209 #elif defined(CONFIG_TINY_RCU)
210 #include <linux/rcutiny.h>
211 #else
212 #error "Unknown RCU implementation specified to kernel configuration"
213 #endif
214 
215 /*
216  * The init_rcu_head_on_stack() and destroy_rcu_head_on_stack() calls
217  * are needed for dynamic initialization and destruction of rcu_head
218  * on the stack, and init_rcu_head()/destroy_rcu_head() are needed for
219  * dynamic initialization and destruction of statically allocated rcu_head
220  * structures.  However, rcu_head structures allocated dynamically in the
221  * heap don't need any initialization.
222  */
223 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
224 void init_rcu_head(struct rcu_head *head);
225 void destroy_rcu_head(struct rcu_head *head);
226 void init_rcu_head_on_stack(struct rcu_head *head);
227 void destroy_rcu_head_on_stack(struct rcu_head *head);
228 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
229 static inline void init_rcu_head(struct rcu_head *head) { }
230 static inline void destroy_rcu_head(struct rcu_head *head) { }
231 static inline void init_rcu_head_on_stack(struct rcu_head *head) { }
232 static inline void destroy_rcu_head_on_stack(struct rcu_head *head) { }
233 #endif  /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
234 
235 #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
236 bool rcu_lockdep_current_cpu_online(void);
237 #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
238 static inline bool rcu_lockdep_current_cpu_online(void) { return true; }
239 #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
240 
241 #ifdef CONFIG_DEBUG_LOCK_ALLOC
242 
243 static inline void rcu_lock_acquire(struct lockdep_map *map)
244 {
245         lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
246 }
247 
248 static inline void rcu_lock_release(struct lockdep_map *map)
249 {
250         lock_release(map, 1, _THIS_IP_);
251 }
252 
253 extern struct lockdep_map rcu_lock_map;
254 extern struct lockdep_map rcu_bh_lock_map;
255 extern struct lockdep_map rcu_sched_lock_map;
256 extern struct lockdep_map rcu_callback_map;
257 int debug_lockdep_rcu_enabled(void);
258 int rcu_read_lock_held(void);
259 int rcu_read_lock_bh_held(void);
260 int rcu_read_lock_sched_held(void);
261 
262 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
263 
264 # define rcu_lock_acquire(a)            do { } while (0)
265 # define rcu_lock_release(a)            do { } while (0)
266 
267 static inline int rcu_read_lock_held(void)
268 {
269         return 1;
270 }
271 
272 static inline int rcu_read_lock_bh_held(void)
273 {
274         return 1;
275 }
276 
277 static inline int rcu_read_lock_sched_held(void)
278 {
279         return !preemptible();
280 }
281 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
282 
283 #ifdef CONFIG_PROVE_RCU
284 
285 /**
286  * RCU_LOCKDEP_WARN - emit lockdep splat if specified condition is met
287  * @c: condition to check
288  * @s: informative message
289  */
290 #define RCU_LOCKDEP_WARN(c, s)                                          \
291         do {                                                            \
292                 static bool __section(.data.unlikely) __warned;         \
293                 if (debug_lockdep_rcu_enabled() && !__warned && (c)) {  \
294                         __warned = true;                                \
295                         lockdep_rcu_suspicious(__FILE__, __LINE__, s);  \
296                 }                                                       \
297         } while (0)
298 
299 #if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
300 static inline void rcu_preempt_sleep_check(void)
301 {
302         RCU_LOCKDEP_WARN(lock_is_held(&rcu_lock_map),
303                          "Illegal context switch in RCU read-side critical section");
304 }
305 #else /* #ifdef CONFIG_PROVE_RCU */
306 static inline void rcu_preempt_sleep_check(void) { }
307 #endif /* #else #ifdef CONFIG_PROVE_RCU */
308 
309 #define rcu_sleep_check()                                               \
310         do {                                                            \
311                 rcu_preempt_sleep_check();                              \
312                 RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map),        \
313                                  "Illegal context switch in RCU-bh read-side critical section"); \
314                 RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map),     \
315                                  "Illegal context switch in RCU-sched read-side critical section"); \
316         } while (0)
317 
318 #else /* #ifdef CONFIG_PROVE_RCU */
319 
320 #define RCU_LOCKDEP_WARN(c, s) do { } while (0)
321 #define rcu_sleep_check() do { } while (0)
322 
323 #endif /* #else #ifdef CONFIG_PROVE_RCU */
324 
325 /*
326  * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
327  * and rcu_assign_pointer().  Some of these could be folded into their
328  * callers, but they are left separate in order to ease introduction of
329  * multiple flavors of pointers to match the multiple flavors of RCU
330  * (e.g., __rcu_bh, * __rcu_sched, and __srcu), should this make sense in
331  * the future.
332  */
333 
334 #ifdef __CHECKER__
335 #define rcu_dereference_sparse(p, space) \
336         ((void)(((typeof(*p) space *)p) == p))
337 #else /* #ifdef __CHECKER__ */
338 #define rcu_dereference_sparse(p, space)
339 #endif /* #else #ifdef __CHECKER__ */
340 
341 #define __rcu_access_pointer(p, space) \
342 ({ \
343         typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
344         rcu_dereference_sparse(p, space); \
345         ((typeof(*p) __force __kernel *)(_________p1)); \
346 })
347 #define __rcu_dereference_check(p, c, space) \
348 ({ \
349         /* Dependency order vs. p above. */ \
350         typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
351         RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
352         rcu_dereference_sparse(p, space); \
353         ((typeof(*p) __force __kernel *)(________p1)); \
354 })
355 #define __rcu_dereference_protected(p, c, space) \
356 ({ \
357         RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
358         rcu_dereference_sparse(p, space); \
359         ((typeof(*p) __force __kernel *)(p)); \
360 })
361 #define rcu_dereference_raw(p) \
362 ({ \
363         /* Dependency order vs. p above. */ \
364         typeof(p) ________p1 = READ_ONCE(p); \
365         ((typeof(*p) __force __kernel *)(________p1)); \
366 })
367 
368 /**
369  * RCU_INITIALIZER() - statically initialize an RCU-protected global variable
370  * @v: The value to statically initialize with.
371  */
372 #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
373 
374 /**
375  * rcu_assign_pointer() - assign to RCU-protected pointer
376  * @p: pointer to assign to
377  * @v: value to assign (publish)
378  *
379  * Assigns the specified value to the specified RCU-protected
380  * pointer, ensuring that any concurrent RCU readers will see
381  * any prior initialization.
382  *
383  * Inserts memory barriers on architectures that require them
384  * (which is most of them), and also prevents the compiler from
385  * reordering the code that initializes the structure after the pointer
386  * assignment.  More importantly, this call documents which pointers
387  * will be dereferenced by RCU read-side code.
388  *
389  * In some special cases, you may use RCU_INIT_POINTER() instead
390  * of rcu_assign_pointer().  RCU_INIT_POINTER() is a bit faster due
391  * to the fact that it does not constrain either the CPU or the compiler.
392  * That said, using RCU_INIT_POINTER() when you should have used
393  * rcu_assign_pointer() is a very bad thing that results in
394  * impossible-to-diagnose memory corruption.  So please be careful.
395  * See the RCU_INIT_POINTER() comment header for details.
396  *
397  * Note that rcu_assign_pointer() evaluates each of its arguments only
398  * once, appearances notwithstanding.  One of the "extra" evaluations
399  * is in typeof() and the other visible only to sparse (__CHECKER__),
400  * neither of which actually execute the argument.  As with most cpp
401  * macros, this execute-arguments-only-once property is important, so
402  * please be careful when making changes to rcu_assign_pointer() and the
403  * other macros that it invokes.
404  */
405 #define rcu_assign_pointer(p, v)                                              \
406 ({                                                                            \
407         uintptr_t _r_a_p__v = (uintptr_t)(v);                                 \
408                                                                               \
409         if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL)        \
410                 WRITE_ONCE((p), (typeof(p))(_r_a_p__v));                      \
411         else                                                                  \
412                 smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
413         _r_a_p__v;                                                            \
414 })
415 
416 /**
417  * rcu_swap_protected() - swap an RCU and a regular pointer
418  * @rcu_ptr: RCU pointer
419  * @ptr: regular pointer
420  * @c: the conditions under which the dereference will take place
421  *
422  * Perform swap(@rcu_ptr, @ptr) where @rcu_ptr is an RCU-annotated pointer and
423  * @c is the argument that is passed to the rcu_dereference_protected() call
424  * used to read that pointer.
425  */
426 #define rcu_swap_protected(rcu_ptr, ptr, c) do {                        \
427         typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));  \
428         rcu_assign_pointer((rcu_ptr), (ptr));                           \
429         (ptr) = __tmp;                                                  \
430 } while (0)
431 
432 /**
433  * rcu_access_pointer() - fetch RCU pointer with no dereferencing
434  * @p: The pointer to read
435  *
436  * Return the value of the specified RCU-protected pointer, but omit the
437  * lockdep checks for being in an RCU read-side critical section.  This is
438  * useful when the value of this pointer is accessed, but the pointer is
439  * not dereferenced, for example, when testing an RCU-protected pointer
440  * against NULL.  Although rcu_access_pointer() may also be used in cases
441  * where update-side locks prevent the value of the pointer from changing,
442  * you should instead use rcu_dereference_protected() for this use case.
443  *
444  * It is also permissible to use rcu_access_pointer() when read-side
445  * access to the pointer was removed at least one grace period ago, as
446  * is the case in the context of the RCU callback that is freeing up
447  * the data, or after a synchronize_rcu() returns.  This can be useful
448  * when tearing down multi-linked structures after a grace period
449  * has elapsed.
450  */
451 #define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
452 
453 /**
454  * rcu_dereference_check() - rcu_dereference with debug checking
455  * @p: The pointer to read, prior to dereferencing
456  * @c: The conditions under which the dereference will take place
457  *
458  * Do an rcu_dereference(), but check that the conditions under which the
459  * dereference will take place are correct.  Typically the conditions
460  * indicate the various locking conditions that should be held at that
461  * point.  The check should return true if the conditions are satisfied.
462  * An implicit check for being in an RCU read-side critical section
463  * (rcu_read_lock()) is included.
464  *
465  * For example:
466  *
467  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
468  *
469  * could be used to indicate to lockdep that foo->bar may only be dereferenced
470  * if either rcu_read_lock() is held, or that the lock required to replace
471  * the bar struct at foo->bar is held.
472  *
473  * Note that the list of conditions may also include indications of when a lock
474  * need not be held, for example during initialisation or destruction of the
475  * target struct:
476  *
477  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
478  *                                            atomic_read(&foo->usage) == 0);
479  *
480  * Inserts memory barriers on architectures that require them
481  * (currently only the Alpha), prevents the compiler from refetching
482  * (and from merging fetches), and, more importantly, documents exactly
483  * which pointers are protected by RCU and checks that the pointer is
484  * annotated as __rcu.
485  */
486 #define rcu_dereference_check(p, c) \
487         __rcu_dereference_check((p), (c) || rcu_read_lock_held(), __rcu)
488 
489 /**
490  * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
491  * @p: The pointer to read, prior to dereferencing
492  * @c: The conditions under which the dereference will take place
493  *
494  * This is the RCU-bh counterpart to rcu_dereference_check().
495  */
496 #define rcu_dereference_bh_check(p, c) \
497         __rcu_dereference_check((p), (c) || rcu_read_lock_bh_held(), __rcu)
498 
499 /**
500  * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
501  * @p: The pointer to read, prior to dereferencing
502  * @c: The conditions under which the dereference will take place
503  *
504  * This is the RCU-sched counterpart to rcu_dereference_check().
505  */
506 #define rcu_dereference_sched_check(p, c) \
507         __rcu_dereference_check((p), (c) || rcu_read_lock_sched_held(), \
508                                 __rcu)
509 
510 /*
511  * The tracing infrastructure traces RCU (we want that), but unfortunately
512  * some of the RCU checks causes tracing to lock up the system.
513  *
514  * The no-tracing version of rcu_dereference_raw() must not call
515  * rcu_read_lock_held().
516  */
517 #define rcu_dereference_raw_notrace(p) __rcu_dereference_check((p), 1, __rcu)
518 
519 /**
520  * rcu_dereference_protected() - fetch RCU pointer when updates prevented
521  * @p: The pointer to read, prior to dereferencing
522  * @c: The conditions under which the dereference will take place
523  *
524  * Return the value of the specified RCU-protected pointer, but omit
525  * the READ_ONCE().  This is useful in cases where update-side locks
526  * prevent the value of the pointer from changing.  Please note that this
527  * primitive does *not* prevent the compiler from repeating this reference
528  * or combining it with other references, so it should not be used without
529  * protection of appropriate locks.
530  *
531  * This function is only for update-side use.  Using this function
532  * when protected only by rcu_read_lock() will result in infrequent
533  * but very ugly failures.
534  */
535 #define rcu_dereference_protected(p, c) \
536         __rcu_dereference_protected((p), (c), __rcu)
537 
538 
539 /**
540  * rcu_dereference() - fetch RCU-protected pointer for dereferencing
541  * @p: The pointer to read, prior to dereferencing
542  *
543  * This is a simple wrapper around rcu_dereference_check().
544  */
545 #define rcu_dereference(p) rcu_dereference_check(p, 0)
546 
547 /**
548  * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
549  * @p: The pointer to read, prior to dereferencing
550  *
551  * Makes rcu_dereference_check() do the dirty work.
552  */
553 #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
554 
555 /**
556  * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
557  * @p: The pointer to read, prior to dereferencing
558  *
559  * Makes rcu_dereference_check() do the dirty work.
560  */
561 #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
562 
563 /**
564  * rcu_pointer_handoff() - Hand off a pointer from RCU to other mechanism
565  * @p: The pointer to hand off
566  *
567  * This is simply an identity function, but it documents where a pointer
568  * is handed off from RCU to some other synchronization mechanism, for
569  * example, reference counting or locking.  In C11, it would map to
570  * kill_dependency().  It could be used as follows:
571  * ``
572  *      rcu_read_lock();
573  *      p = rcu_dereference(gp);
574  *      long_lived = is_long_lived(p);
575  *      if (long_lived) {
576  *              if (!atomic_inc_not_zero(p->refcnt))
577  *                      long_lived = false;
578  *              else
579  *                      p = rcu_pointer_handoff(p);
580  *      }
581  *      rcu_read_unlock();
582  *``
583  */
584 #define rcu_pointer_handoff(p) (p)
585 
586 /**
587  * rcu_read_lock() - mark the beginning of an RCU read-side critical section
588  *
589  * When synchronize_rcu() is invoked on one CPU while other CPUs
590  * are within RCU read-side critical sections, then the
591  * synchronize_rcu() is guaranteed to block until after all the other
592  * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
593  * on one CPU while other CPUs are within RCU read-side critical
594  * sections, invocation of the corresponding RCU callback is deferred
595  * until after the all the other CPUs exit their critical sections.
596  *
597  * Note, however, that RCU callbacks are permitted to run concurrently
598  * with new RCU read-side critical sections.  One way that this can happen
599  * is via the following sequence of events: (1) CPU 0 enters an RCU
600  * read-side critical section, (2) CPU 1 invokes call_rcu() to register
601  * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
602  * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
603  * callback is invoked.  This is legal, because the RCU read-side critical
604  * section that was running concurrently with the call_rcu() (and which
605  * therefore might be referencing something that the corresponding RCU
606  * callback would free up) has completed before the corresponding
607  * RCU callback is invoked.
608  *
609  * RCU read-side critical sections may be nested.  Any deferred actions
610  * will be deferred until the outermost RCU read-side critical section
611  * completes.
612  *
613  * You can avoid reading and understanding the next paragraph by
614  * following this rule: don't put anything in an rcu_read_lock() RCU
615  * read-side critical section that would block in a !PREEMPT kernel.
616  * But if you want the full story, read on!
617  *
618  * In non-preemptible RCU implementations (TREE_RCU and TINY_RCU),
619  * it is illegal to block while in an RCU read-side critical section.
620  * In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPT
621  * kernel builds, RCU read-side critical sections may be preempted,
622  * but explicit blocking is illegal.  Finally, in preemptible RCU
623  * implementations in real-time (with -rt patchset) kernel builds, RCU
624  * read-side critical sections may be preempted and they may also block, but
625  * only when acquiring spinlocks that are subject to priority inheritance.
626  */
627 static inline void rcu_read_lock(void)
628 {
629         __rcu_read_lock();
630         __acquire(RCU);
631         rcu_lock_acquire(&rcu_lock_map);
632         RCU_LOCKDEP_WARN(!rcu_is_watching(),
633                          "rcu_read_lock() used illegally while idle");
634 }
635 
636 /*
637  * So where is rcu_write_lock()?  It does not exist, as there is no
638  * way for writers to lock out RCU readers.  This is a feature, not
639  * a bug -- this property is what provides RCU's performance benefits.
640  * Of course, writers must coordinate with each other.  The normal
641  * spinlock primitives work well for this, but any other technique may be
642  * used as well.  RCU does not care how the writers keep out of each
643  * others' way, as long as they do so.
644  */
645 
646 /**
647  * rcu_read_unlock() - marks the end of an RCU read-side critical section.
648  *
649  * In most situations, rcu_read_unlock() is immune from deadlock.
650  * However, in kernels built with CONFIG_RCU_BOOST, rcu_read_unlock()
651  * is responsible for deboosting, which it does via rt_mutex_unlock().
652  * Unfortunately, this function acquires the scheduler's runqueue and
653  * priority-inheritance spinlocks.  This means that deadlock could result
654  * if the caller of rcu_read_unlock() already holds one of these locks or
655  * any lock that is ever acquired while holding them.
656  *
657  * That said, RCU readers are never priority boosted unless they were
658  * preempted.  Therefore, one way to avoid deadlock is to make sure
659  * that preemption never happens within any RCU read-side critical
660  * section whose outermost rcu_read_unlock() is called with one of
661  * rt_mutex_unlock()'s locks held.  Such preemption can be avoided in
662  * a number of ways, for example, by invoking preempt_disable() before
663  * critical section's outermost rcu_read_lock().
664  *
665  * Given that the set of locks acquired by rt_mutex_unlock() might change
666  * at any time, a somewhat more future-proofed approach is to make sure
667  * that that preemption never happens within any RCU read-side critical
668  * section whose outermost rcu_read_unlock() is called with irqs disabled.
669  * This approach relies on the fact that rt_mutex_unlock() currently only
670  * acquires irq-disabled locks.
671  *
672  * The second of these two approaches is best in most situations,
673  * however, the first approach can also be useful, at least to those
674  * developers willing to keep abreast of the set of locks acquired by
675  * rt_mutex_unlock().
676  *
677  * See rcu_read_lock() for more information.
678  */
679 static inline void rcu_read_unlock(void)
680 {
681         RCU_LOCKDEP_WARN(!rcu_is_watching(),
682                          "rcu_read_unlock() used illegally while idle");
683         __release(RCU);
684         __rcu_read_unlock();
685         rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
686 }
687 
688 /**
689  * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
690  *
691  * This is equivalent of rcu_read_lock(), but to be used when updates
692  * are being done using call_rcu_bh() or synchronize_rcu_bh(). Since
693  * both call_rcu_bh() and synchronize_rcu_bh() consider completion of a
694  * softirq handler to be a quiescent state, a process in RCU read-side
695  * critical section must be protected by disabling softirqs. Read-side
696  * critical sections in interrupt context can use just rcu_read_lock(),
697  * though this should at least be commented to avoid confusing people
698  * reading the code.
699  *
700  * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
701  * must occur in the same context, for example, it is illegal to invoke
702  * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
703  * was invoked from some other task.
704  */
705 static inline void rcu_read_lock_bh(void)
706 {
707         local_bh_disable();
708         __acquire(RCU_BH);
709         rcu_lock_acquire(&rcu_bh_lock_map);
710         RCU_LOCKDEP_WARN(!rcu_is_watching(),
711                          "rcu_read_lock_bh() used illegally while idle");
712 }
713 
714 /*
715  * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
716  *
717  * See rcu_read_lock_bh() for more information.
718  */
719 static inline void rcu_read_unlock_bh(void)
720 {
721         RCU_LOCKDEP_WARN(!rcu_is_watching(),
722                          "rcu_read_unlock_bh() used illegally while idle");
723         rcu_lock_release(&rcu_bh_lock_map);
724         __release(RCU_BH);
725         local_bh_enable();
726 }
727 
728 /**
729  * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
730  *
731  * This is equivalent of rcu_read_lock(), but to be used when updates
732  * are being done using call_rcu_sched() or synchronize_rcu_sched().
733  * Read-side critical sections can also be introduced by anything that
734  * disables preemption, including local_irq_disable() and friends.
735  *
736  * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
737  * must occur in the same context, for example, it is illegal to invoke
738  * rcu_read_unlock_sched() from process context if the matching
739  * rcu_read_lock_sched() was invoked from an NMI handler.
740  */
741 static inline void rcu_read_lock_sched(void)
742 {
743         preempt_disable();
744         __acquire(RCU_SCHED);
745         rcu_lock_acquire(&rcu_sched_lock_map);
746         RCU_LOCKDEP_WARN(!rcu_is_watching(),
747                          "rcu_read_lock_sched() used illegally while idle");
748 }
749 
750 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
751 static inline notrace void rcu_read_lock_sched_notrace(void)
752 {
753         preempt_disable_notrace();
754         __acquire(RCU_SCHED);
755 }
756 
757 /*
758  * rcu_read_unlock_sched - marks the end of a RCU-classic critical section
759  *
760  * See rcu_read_lock_sched for more information.
761  */
762 static inline void rcu_read_unlock_sched(void)
763 {
764         RCU_LOCKDEP_WARN(!rcu_is_watching(),
765                          "rcu_read_unlock_sched() used illegally while idle");
766         rcu_lock_release(&rcu_sched_lock_map);
767         __release(RCU_SCHED);
768         preempt_enable();
769 }
770 
771 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
772 static inline notrace void rcu_read_unlock_sched_notrace(void)
773 {
774         __release(RCU_SCHED);
775         preempt_enable_notrace();
776 }
777 
778 /**
779  * RCU_INIT_POINTER() - initialize an RCU protected pointer
780  * @p: The pointer to be initialized.
781  * @v: The value to initialized the pointer to.
782  *
783  * Initialize an RCU-protected pointer in special cases where readers
784  * do not need ordering constraints on the CPU or the compiler.  These
785  * special cases are:
786  *
787  * 1.   This use of RCU_INIT_POINTER() is NULLing out the pointer *or*
788  * 2.   The caller has taken whatever steps are required to prevent
789  *      RCU readers from concurrently accessing this pointer *or*
790  * 3.   The referenced data structure has already been exposed to
791  *      readers either at compile time or via rcu_assign_pointer() *and*
792  *
793  *      a.      You have not made *any* reader-visible changes to
794  *              this structure since then *or*
795  *      b.      It is OK for readers accessing this structure from its
796  *              new location to see the old state of the structure.  (For
797  *              example, the changes were to statistical counters or to
798  *              other state where exact synchronization is not required.)
799  *
800  * Failure to follow these rules governing use of RCU_INIT_POINTER() will
801  * result in impossible-to-diagnose memory corruption.  As in the structures
802  * will look OK in crash dumps, but any concurrent RCU readers might
803  * see pre-initialized values of the referenced data structure.  So
804  * please be very careful how you use RCU_INIT_POINTER()!!!
805  *
806  * If you are creating an RCU-protected linked structure that is accessed
807  * by a single external-to-structure RCU-protected pointer, then you may
808  * use RCU_INIT_POINTER() to initialize the internal RCU-protected
809  * pointers, but you must use rcu_assign_pointer() to initialize the
810  * external-to-structure pointer *after* you have completely initialized
811  * the reader-accessible portions of the linked structure.
812  *
813  * Note that unlike rcu_assign_pointer(), RCU_INIT_POINTER() provides no
814  * ordering guarantees for either the CPU or the compiler.
815  */
816 #define RCU_INIT_POINTER(p, v) \
817         do { \
818                 rcu_dereference_sparse(p, __rcu); \
819                 WRITE_ONCE(p, RCU_INITIALIZER(v)); \
820         } while (0)
821 
822 /**
823  * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer
824  * @p: The pointer to be initialized.
825  * @v: The value to initialized the pointer to.
826  *
827  * GCC-style initialization for an RCU-protected pointer in a structure field.
828  */
829 #define RCU_POINTER_INITIALIZER(p, v) \
830                 .p = RCU_INITIALIZER(v)
831 
832 /*
833  * Does the specified offset indicate that the corresponding rcu_head
834  * structure can be handled by kfree_rcu()?
835  */
836 #define __is_kfree_rcu_offset(offset) ((offset) < 4096)
837 
838 /*
839  * Helper macro for kfree_rcu() to prevent argument-expansion eyestrain.
840  */
841 #define __kfree_rcu(head, offset) \
842         do { \
843                 BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \
844                 kfree_call_rcu(head, (rcu_callback_t)(unsigned long)(offset)); \
845         } while (0)
846 
847 /**
848  * kfree_rcu() - kfree an object after a grace period.
849  * @ptr:        pointer to kfree
850  * @rcu_head:   the name of the struct rcu_head within the type of @ptr.
851  *
852  * Many rcu callbacks functions just call kfree() on the base structure.
853  * These functions are trivial, but their size adds up, and furthermore
854  * when they are used in a kernel module, that module must invoke the
855  * high-latency rcu_barrier() function at module-unload time.
856  *
857  * The kfree_rcu() function handles this issue.  Rather than encoding a
858  * function address in the embedded rcu_head structure, kfree_rcu() instead
859  * encodes the offset of the rcu_head structure within the base structure.
860  * Because the functions are not allowed in the low-order 4096 bytes of
861  * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
862  * If the offset is larger than 4095 bytes, a compile-time error will
863  * be generated in __kfree_rcu().  If this error is triggered, you can
864  * either fall back to use of call_rcu() or rearrange the structure to
865  * position the rcu_head structure into the first 4096 bytes.
866  *
867  * Note that the allowable offset might decrease in the future, for example,
868  * to allow something like kmem_cache_free_rcu().
869  *
870  * The BUILD_BUG_ON check must not involve any function calls, hence the
871  * checks are done in macros here.
872  */
873 #define kfree_rcu(ptr, rcu_head)                                        \
874         __kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
875 
876 
877 /*
878  * Place this after a lock-acquisition primitive to guarantee that
879  * an UNLOCK+LOCK pair acts as a full barrier.  This guarantee applies
880  * if the UNLOCK and LOCK are executed by the same CPU or if the
881  * UNLOCK and LOCK operate on the same lock variable.
882  */
883 #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE
884 #define smp_mb__after_unlock_lock()     smp_mb()  /* Full ordering for lock. */
885 #else /* #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
886 #define smp_mb__after_unlock_lock()     do { } while (0)
887 #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
888 
889 
890 #endif /* __LINUX_RCUPDATE_H */
891 

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