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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 void call_rcu(struct rcu_head *head, rcu_callback_t func);
 52 void rcu_barrier_tasks(void);
 53 void synchronize_rcu(void);
 54 
 55 #ifdef CONFIG_PREEMPT_RCU
 56 
 57 void __rcu_read_lock(void);
 58 void __rcu_read_unlock(void);
 59 
 60 /*
 61  * Defined as a macro as it is a very low level header included from
 62  * areas that don't even know about current.  This gives the rcu_read_lock()
 63  * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
 64  * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
 65  */
 66 #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
 67 
 68 #else /* #ifdef CONFIG_PREEMPT_RCU */
 69 
 70 static inline void __rcu_read_lock(void)
 71 {
 72         if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
 73                 preempt_disable();
 74 }
 75 
 76 static inline void __rcu_read_unlock(void)
 77 {
 78         if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
 79                 preempt_enable();
 80 }
 81 
 82 static inline int rcu_preempt_depth(void)
 83 {
 84         return 0;
 85 }
 86 
 87 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 88 
 89 /* Internal to kernel */
 90 void rcu_init(void);
 91 extern int rcu_scheduler_active __read_mostly;
 92 void rcu_check_callbacks(int user);
 93 void rcu_report_dead(unsigned int cpu);
 94 void rcutree_migrate_callbacks(int cpu);
 95 
 96 #ifdef CONFIG_RCU_STALL_COMMON
 97 void rcu_sysrq_start(void);
 98 void rcu_sysrq_end(void);
 99 #else /* #ifdef CONFIG_RCU_STALL_COMMON */
100 static inline void rcu_sysrq_start(void) { }
101 static inline void rcu_sysrq_end(void) { }
102 #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
103 
104 #ifdef CONFIG_NO_HZ_FULL
105 void rcu_user_enter(void);
106 void rcu_user_exit(void);
107 #else
108 static inline void rcu_user_enter(void) { }
109 static inline void rcu_user_exit(void) { }
110 #endif /* CONFIG_NO_HZ_FULL */
111 
112 #ifdef CONFIG_RCU_NOCB_CPU
113 void rcu_init_nohz(void);
114 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
115 static inline void rcu_init_nohz(void) { }
116 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
117 
118 /**
119  * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
120  * @a: Code that RCU needs to pay attention to.
121  *
122  * RCU read-side critical sections are forbidden in the inner idle loop,
123  * that is, between the rcu_idle_enter() and the rcu_idle_exit() -- RCU
124  * will happily ignore any such read-side critical sections.  However,
125  * things like powertop need tracepoints in the inner idle loop.
126  *
127  * This macro provides the way out:  RCU_NONIDLE(do_something_with_RCU())
128  * will tell RCU that it needs to pay attention, invoke its argument
129  * (in this example, calling the do_something_with_RCU() function),
130  * and then tell RCU to go back to ignoring this CPU.  It is permissible
131  * to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is
132  * on the order of a million or so, even on 32-bit systems).  It is
133  * not legal to block within RCU_NONIDLE(), nor is it permissible to
134  * transfer control either into or out of RCU_NONIDLE()'s statement.
135  */
136 #define RCU_NONIDLE(a) \
137         do { \
138                 rcu_irq_enter_irqson(); \
139                 do { a; } while (0); \
140                 rcu_irq_exit_irqson(); \
141         } while (0)
142 
143 /*
144  * Note a quasi-voluntary context switch for RCU-tasks's benefit.
145  * This is a macro rather than an inline function to avoid #include hell.
146  */
147 #ifdef CONFIG_TASKS_RCU
148 #define rcu_tasks_qs(t) \
149         do { \
150                 if (READ_ONCE((t)->rcu_tasks_holdout)) \
151                         WRITE_ONCE((t)->rcu_tasks_holdout, false); \
152         } while (0)
153 #define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t)
154 void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
155 void synchronize_rcu_tasks(void);
156 void exit_tasks_rcu_start(void);
157 void exit_tasks_rcu_finish(void);
158 #else /* #ifdef CONFIG_TASKS_RCU */
159 #define rcu_tasks_qs(t) do { } while (0)
160 #define rcu_note_voluntary_context_switch(t) do { } while (0)
161 #define call_rcu_tasks call_rcu
162 #define synchronize_rcu_tasks synchronize_rcu
163 static inline void exit_tasks_rcu_start(void) { }
164 static inline void exit_tasks_rcu_finish(void) { }
165 #endif /* #else #ifdef CONFIG_TASKS_RCU */
166 
167 /**
168  * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
169  *
170  * This macro resembles cond_resched(), except that it is defined to
171  * report potential quiescent states to RCU-tasks even if the cond_resched()
172  * machinery were to be shut off, as some advocate for PREEMPT kernels.
173  */
174 #define cond_resched_tasks_rcu_qs() \
175 do { \
176         rcu_tasks_qs(current); \
177         cond_resched(); \
178 } while (0)
179 
180 /*
181  * Infrastructure to implement the synchronize_() primitives in
182  * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
183  */
184 
185 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU)
186 #include <linux/rcutree.h>
187 #elif defined(CONFIG_TINY_RCU)
188 #include <linux/rcutiny.h>
189 #else
190 #error "Unknown RCU implementation specified to kernel configuration"
191 #endif
192 
193 /*
194  * The init_rcu_head_on_stack() and destroy_rcu_head_on_stack() calls
195  * are needed for dynamic initialization and destruction of rcu_head
196  * on the stack, and init_rcu_head()/destroy_rcu_head() are needed for
197  * dynamic initialization and destruction of statically allocated rcu_head
198  * structures.  However, rcu_head structures allocated dynamically in the
199  * heap don't need any initialization.
200  */
201 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
202 void init_rcu_head(struct rcu_head *head);
203 void destroy_rcu_head(struct rcu_head *head);
204 void init_rcu_head_on_stack(struct rcu_head *head);
205 void destroy_rcu_head_on_stack(struct rcu_head *head);
206 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
207 static inline void init_rcu_head(struct rcu_head *head) { }
208 static inline void destroy_rcu_head(struct rcu_head *head) { }
209 static inline void init_rcu_head_on_stack(struct rcu_head *head) { }
210 static inline void destroy_rcu_head_on_stack(struct rcu_head *head) { }
211 #endif  /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
212 
213 #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
214 bool rcu_lockdep_current_cpu_online(void);
215 #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
216 static inline bool rcu_lockdep_current_cpu_online(void) { return true; }
217 #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
218 
219 #ifdef CONFIG_DEBUG_LOCK_ALLOC
220 
221 static inline void rcu_lock_acquire(struct lockdep_map *map)
222 {
223         lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
224 }
225 
226 static inline void rcu_lock_release(struct lockdep_map *map)
227 {
228         lock_release(map, 1, _THIS_IP_);
229 }
230 
231 extern struct lockdep_map rcu_lock_map;
232 extern struct lockdep_map rcu_bh_lock_map;
233 extern struct lockdep_map rcu_sched_lock_map;
234 extern struct lockdep_map rcu_callback_map;
235 int debug_lockdep_rcu_enabled(void);
236 int rcu_read_lock_held(void);
237 int rcu_read_lock_bh_held(void);
238 int rcu_read_lock_sched_held(void);
239 
240 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
241 
242 # define rcu_lock_acquire(a)            do { } while (0)
243 # define rcu_lock_release(a)            do { } while (0)
244 
245 static inline int rcu_read_lock_held(void)
246 {
247         return 1;
248 }
249 
250 static inline int rcu_read_lock_bh_held(void)
251 {
252         return 1;
253 }
254 
255 static inline int rcu_read_lock_sched_held(void)
256 {
257         return !preemptible();
258 }
259 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
260 
261 #ifdef CONFIG_PROVE_RCU
262 
263 /**
264  * RCU_LOCKDEP_WARN - emit lockdep splat if specified condition is met
265  * @c: condition to check
266  * @s: informative message
267  */
268 #define RCU_LOCKDEP_WARN(c, s)                                          \
269         do {                                                            \
270                 static bool __section(.data.unlikely) __warned;         \
271                 if (debug_lockdep_rcu_enabled() && !__warned && (c)) {  \
272                         __warned = true;                                \
273                         lockdep_rcu_suspicious(__FILE__, __LINE__, s);  \
274                 }                                                       \
275         } while (0)
276 
277 #if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
278 static inline void rcu_preempt_sleep_check(void)
279 {
280         RCU_LOCKDEP_WARN(lock_is_held(&rcu_lock_map),
281                          "Illegal context switch in RCU read-side critical section");
282 }
283 #else /* #ifdef CONFIG_PROVE_RCU */
284 static inline void rcu_preempt_sleep_check(void) { }
285 #endif /* #else #ifdef CONFIG_PROVE_RCU */
286 
287 #define rcu_sleep_check()                                               \
288         do {                                                            \
289                 rcu_preempt_sleep_check();                              \
290                 RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map),        \
291                                  "Illegal context switch in RCU-bh read-side critical section"); \
292                 RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map),     \
293                                  "Illegal context switch in RCU-sched read-side critical section"); \
294         } while (0)
295 
296 #else /* #ifdef CONFIG_PROVE_RCU */
297 
298 #define RCU_LOCKDEP_WARN(c, s) do { } while (0)
299 #define rcu_sleep_check() do { } while (0)
300 
301 #endif /* #else #ifdef CONFIG_PROVE_RCU */
302 
303 /*
304  * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
305  * and rcu_assign_pointer().  Some of these could be folded into their
306  * callers, but they are left separate in order to ease introduction of
307  * multiple pointers markings to match different RCU implementations
308  * (e.g., __srcu), should this make sense in the future.
309  */
310 
311 #ifdef __CHECKER__
312 #define rcu_dereference_sparse(p, space) \
313         ((void)(((typeof(*p) space *)p) == p))
314 #else /* #ifdef __CHECKER__ */
315 #define rcu_dereference_sparse(p, space)
316 #endif /* #else #ifdef __CHECKER__ */
317 
318 #define __rcu_access_pointer(p, space) \
319 ({ \
320         typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
321         rcu_dereference_sparse(p, space); \
322         ((typeof(*p) __force __kernel *)(_________p1)); \
323 })
324 #define __rcu_dereference_check(p, c, space) \
325 ({ \
326         /* Dependency order vs. p above. */ \
327         typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
328         RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
329         rcu_dereference_sparse(p, space); \
330         ((typeof(*p) __force __kernel *)(________p1)); \
331 })
332 #define __rcu_dereference_protected(p, c, space) \
333 ({ \
334         RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
335         rcu_dereference_sparse(p, space); \
336         ((typeof(*p) __force __kernel *)(p)); \
337 })
338 #define rcu_dereference_raw(p) \
339 ({ \
340         /* Dependency order vs. p above. */ \
341         typeof(p) ________p1 = READ_ONCE(p); \
342         ((typeof(*p) __force __kernel *)(________p1)); \
343 })
344 
345 /**
346  * RCU_INITIALIZER() - statically initialize an RCU-protected global variable
347  * @v: The value to statically initialize with.
348  */
349 #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
350 
351 /**
352  * rcu_assign_pointer() - assign to RCU-protected pointer
353  * @p: pointer to assign to
354  * @v: value to assign (publish)
355  *
356  * Assigns the specified value to the specified RCU-protected
357  * pointer, ensuring that any concurrent RCU readers will see
358  * any prior initialization.
359  *
360  * Inserts memory barriers on architectures that require them
361  * (which is most of them), and also prevents the compiler from
362  * reordering the code that initializes the structure after the pointer
363  * assignment.  More importantly, this call documents which pointers
364  * will be dereferenced by RCU read-side code.
365  *
366  * In some special cases, you may use RCU_INIT_POINTER() instead
367  * of rcu_assign_pointer().  RCU_INIT_POINTER() is a bit faster due
368  * to the fact that it does not constrain either the CPU or the compiler.
369  * That said, using RCU_INIT_POINTER() when you should have used
370  * rcu_assign_pointer() is a very bad thing that results in
371  * impossible-to-diagnose memory corruption.  So please be careful.
372  * See the RCU_INIT_POINTER() comment header for details.
373  *
374  * Note that rcu_assign_pointer() evaluates each of its arguments only
375  * once, appearances notwithstanding.  One of the "extra" evaluations
376  * is in typeof() and the other visible only to sparse (__CHECKER__),
377  * neither of which actually execute the argument.  As with most cpp
378  * macros, this execute-arguments-only-once property is important, so
379  * please be careful when making changes to rcu_assign_pointer() and the
380  * other macros that it invokes.
381  */
382 #define rcu_assign_pointer(p, v)                                              \
383 ({                                                                            \
384         uintptr_t _r_a_p__v = (uintptr_t)(v);                                 \
385                                                                               \
386         if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL)        \
387                 WRITE_ONCE((p), (typeof(p))(_r_a_p__v));                      \
388         else                                                                  \
389                 smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
390         _r_a_p__v;                                                            \
391 })
392 
393 /**
394  * rcu_swap_protected() - swap an RCU and a regular pointer
395  * @rcu_ptr: RCU pointer
396  * @ptr: regular pointer
397  * @c: the conditions under which the dereference will take place
398  *
399  * Perform swap(@rcu_ptr, @ptr) where @rcu_ptr is an RCU-annotated pointer and
400  * @c is the argument that is passed to the rcu_dereference_protected() call
401  * used to read that pointer.
402  */
403 #define rcu_swap_protected(rcu_ptr, ptr, c) do {                        \
404         typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));  \
405         rcu_assign_pointer((rcu_ptr), (ptr));                           \
406         (ptr) = __tmp;                                                  \
407 } while (0)
408 
409 /**
410  * rcu_access_pointer() - fetch RCU pointer with no dereferencing
411  * @p: The pointer to read
412  *
413  * Return the value of the specified RCU-protected pointer, but omit the
414  * lockdep checks for being in an RCU read-side critical section.  This is
415  * useful when the value of this pointer is accessed, but the pointer is
416  * not dereferenced, for example, when testing an RCU-protected pointer
417  * against NULL.  Although rcu_access_pointer() may also be used in cases
418  * where update-side locks prevent the value of the pointer from changing,
419  * you should instead use rcu_dereference_protected() for this use case.
420  *
421  * It is also permissible to use rcu_access_pointer() when read-side
422  * access to the pointer was removed at least one grace period ago, as
423  * is the case in the context of the RCU callback that is freeing up
424  * the data, or after a synchronize_rcu() returns.  This can be useful
425  * when tearing down multi-linked structures after a grace period
426  * has elapsed.
427  */
428 #define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
429 
430 /**
431  * rcu_dereference_check() - rcu_dereference with debug checking
432  * @p: The pointer to read, prior to dereferencing
433  * @c: The conditions under which the dereference will take place
434  *
435  * Do an rcu_dereference(), but check that the conditions under which the
436  * dereference will take place are correct.  Typically the conditions
437  * indicate the various locking conditions that should be held at that
438  * point.  The check should return true if the conditions are satisfied.
439  * An implicit check for being in an RCU read-side critical section
440  * (rcu_read_lock()) is included.
441  *
442  * For example:
443  *
444  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
445  *
446  * could be used to indicate to lockdep that foo->bar may only be dereferenced
447  * if either rcu_read_lock() is held, or that the lock required to replace
448  * the bar struct at foo->bar is held.
449  *
450  * Note that the list of conditions may also include indications of when a lock
451  * need not be held, for example during initialisation or destruction of the
452  * target struct:
453  *
454  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
455  *                                            atomic_read(&foo->usage) == 0);
456  *
457  * Inserts memory barriers on architectures that require them
458  * (currently only the Alpha), prevents the compiler from refetching
459  * (and from merging fetches), and, more importantly, documents exactly
460  * which pointers are protected by RCU and checks that the pointer is
461  * annotated as __rcu.
462  */
463 #define rcu_dereference_check(p, c) \
464         __rcu_dereference_check((p), (c) || rcu_read_lock_held(), __rcu)
465 
466 /**
467  * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
468  * @p: The pointer to read, prior to dereferencing
469  * @c: The conditions under which the dereference will take place
470  *
471  * This is the RCU-bh counterpart to rcu_dereference_check().
472  */
473 #define rcu_dereference_bh_check(p, c) \
474         __rcu_dereference_check((p), (c) || rcu_read_lock_bh_held(), __rcu)
475 
476 /**
477  * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
478  * @p: The pointer to read, prior to dereferencing
479  * @c: The conditions under which the dereference will take place
480  *
481  * This is the RCU-sched counterpart to rcu_dereference_check().
482  */
483 #define rcu_dereference_sched_check(p, c) \
484         __rcu_dereference_check((p), (c) || rcu_read_lock_sched_held(), \
485                                 __rcu)
486 
487 /*
488  * The tracing infrastructure traces RCU (we want that), but unfortunately
489  * some of the RCU checks causes tracing to lock up the system.
490  *
491  * The no-tracing version of rcu_dereference_raw() must not call
492  * rcu_read_lock_held().
493  */
494 #define rcu_dereference_raw_notrace(p) __rcu_dereference_check((p), 1, __rcu)
495 
496 /**
497  * rcu_dereference_protected() - fetch RCU pointer when updates prevented
498  * @p: The pointer to read, prior to dereferencing
499  * @c: The conditions under which the dereference will take place
500  *
501  * Return the value of the specified RCU-protected pointer, but omit
502  * the READ_ONCE().  This is useful in cases where update-side locks
503  * prevent the value of the pointer from changing.  Please note that this
504  * primitive does *not* prevent the compiler from repeating this reference
505  * or combining it with other references, so it should not be used without
506  * protection of appropriate locks.
507  *
508  * This function is only for update-side use.  Using this function
509  * when protected only by rcu_read_lock() will result in infrequent
510  * but very ugly failures.
511  */
512 #define rcu_dereference_protected(p, c) \
513         __rcu_dereference_protected((p), (c), __rcu)
514 
515 
516 /**
517  * rcu_dereference() - fetch RCU-protected pointer for dereferencing
518  * @p: The pointer to read, prior to dereferencing
519  *
520  * This is a simple wrapper around rcu_dereference_check().
521  */
522 #define rcu_dereference(p) rcu_dereference_check(p, 0)
523 
524 /**
525  * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
526  * @p: The pointer to read, prior to dereferencing
527  *
528  * Makes rcu_dereference_check() do the dirty work.
529  */
530 #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
531 
532 /**
533  * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
534  * @p: The pointer to read, prior to dereferencing
535  *
536  * Makes rcu_dereference_check() do the dirty work.
537  */
538 #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
539 
540 /**
541  * rcu_pointer_handoff() - Hand off a pointer from RCU to other mechanism
542  * @p: The pointer to hand off
543  *
544  * This is simply an identity function, but it documents where a pointer
545  * is handed off from RCU to some other synchronization mechanism, for
546  * example, reference counting or locking.  In C11, it would map to
547  * kill_dependency().  It could be used as follows::
548  *
549  *      rcu_read_lock();
550  *      p = rcu_dereference(gp);
551  *      long_lived = is_long_lived(p);
552  *      if (long_lived) {
553  *              if (!atomic_inc_not_zero(p->refcnt))
554  *                      long_lived = false;
555  *              else
556  *                      p = rcu_pointer_handoff(p);
557  *      }
558  *      rcu_read_unlock();
559  */
560 #define rcu_pointer_handoff(p) (p)
561 
562 /**
563  * rcu_read_lock() - mark the beginning of an RCU read-side critical section
564  *
565  * When synchronize_rcu() is invoked on one CPU while other CPUs
566  * are within RCU read-side critical sections, then the
567  * synchronize_rcu() is guaranteed to block until after all the other
568  * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
569  * on one CPU while other CPUs are within RCU read-side critical
570  * sections, invocation of the corresponding RCU callback is deferred
571  * until after the all the other CPUs exit their critical sections.
572  *
573  * Note, however, that RCU callbacks are permitted to run concurrently
574  * with new RCU read-side critical sections.  One way that this can happen
575  * is via the following sequence of events: (1) CPU 0 enters an RCU
576  * read-side critical section, (2) CPU 1 invokes call_rcu() to register
577  * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
578  * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
579  * callback is invoked.  This is legal, because the RCU read-side critical
580  * section that was running concurrently with the call_rcu() (and which
581  * therefore might be referencing something that the corresponding RCU
582  * callback would free up) has completed before the corresponding
583  * RCU callback is invoked.
584  *
585  * RCU read-side critical sections may be nested.  Any deferred actions
586  * will be deferred until the outermost RCU read-side critical section
587  * completes.
588  *
589  * You can avoid reading and understanding the next paragraph by
590  * following this rule: don't put anything in an rcu_read_lock() RCU
591  * read-side critical section that would block in a !PREEMPT kernel.
592  * But if you want the full story, read on!
593  *
594  * In non-preemptible RCU implementations (TREE_RCU and TINY_RCU),
595  * it is illegal to block while in an RCU read-side critical section.
596  * In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPT
597  * kernel builds, RCU read-side critical sections may be preempted,
598  * but explicit blocking is illegal.  Finally, in preemptible RCU
599  * implementations in real-time (with -rt patchset) kernel builds, RCU
600  * read-side critical sections may be preempted and they may also block, but
601  * only when acquiring spinlocks that are subject to priority inheritance.
602  */
603 static inline void rcu_read_lock(void)
604 {
605         __rcu_read_lock();
606         __acquire(RCU);
607         rcu_lock_acquire(&rcu_lock_map);
608         RCU_LOCKDEP_WARN(!rcu_is_watching(),
609                          "rcu_read_lock() used illegally while idle");
610 }
611 
612 /*
613  * So where is rcu_write_lock()?  It does not exist, as there is no
614  * way for writers to lock out RCU readers.  This is a feature, not
615  * a bug -- this property is what provides RCU's performance benefits.
616  * Of course, writers must coordinate with each other.  The normal
617  * spinlock primitives work well for this, but any other technique may be
618  * used as well.  RCU does not care how the writers keep out of each
619  * others' way, as long as they do so.
620  */
621 
622 /**
623  * rcu_read_unlock() - marks the end of an RCU read-side critical section.
624  *
625  * In most situations, rcu_read_unlock() is immune from deadlock.
626  * However, in kernels built with CONFIG_RCU_BOOST, rcu_read_unlock()
627  * is responsible for deboosting, which it does via rt_mutex_unlock().
628  * Unfortunately, this function acquires the scheduler's runqueue and
629  * priority-inheritance spinlocks.  This means that deadlock could result
630  * if the caller of rcu_read_unlock() already holds one of these locks or
631  * any lock that is ever acquired while holding them.
632  *
633  * That said, RCU readers are never priority boosted unless they were
634  * preempted.  Therefore, one way to avoid deadlock is to make sure
635  * that preemption never happens within any RCU read-side critical
636  * section whose outermost rcu_read_unlock() is called with one of
637  * rt_mutex_unlock()'s locks held.  Such preemption can be avoided in
638  * a number of ways, for example, by invoking preempt_disable() before
639  * critical section's outermost rcu_read_lock().
640  *
641  * Given that the set of locks acquired by rt_mutex_unlock() might change
642  * at any time, a somewhat more future-proofed approach is to make sure
643  * that that preemption never happens within any RCU read-side critical
644  * section whose outermost rcu_read_unlock() is called with irqs disabled.
645  * This approach relies on the fact that rt_mutex_unlock() currently only
646  * acquires irq-disabled locks.
647  *
648  * The second of these two approaches is best in most situations,
649  * however, the first approach can also be useful, at least to those
650  * developers willing to keep abreast of the set of locks acquired by
651  * rt_mutex_unlock().
652  *
653  * See rcu_read_lock() for more information.
654  */
655 static inline void rcu_read_unlock(void)
656 {
657         RCU_LOCKDEP_WARN(!rcu_is_watching(),
658                          "rcu_read_unlock() used illegally while idle");
659         __release(RCU);
660         __rcu_read_unlock();
661         rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
662 }
663 
664 /**
665  * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
666  *
667  * This is equivalent of rcu_read_lock(), but also disables softirqs.
668  * Note that anything else that disables softirqs can also serve as
669  * an RCU read-side critical section.
670  *
671  * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
672  * must occur in the same context, for example, it is illegal to invoke
673  * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
674  * was invoked from some other task.
675  */
676 static inline void rcu_read_lock_bh(void)
677 {
678         local_bh_disable();
679         __acquire(RCU_BH);
680         rcu_lock_acquire(&rcu_bh_lock_map);
681         RCU_LOCKDEP_WARN(!rcu_is_watching(),
682                          "rcu_read_lock_bh() used illegally while idle");
683 }
684 
685 /*
686  * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
687  *
688  * See rcu_read_lock_bh() for more information.
689  */
690 static inline void rcu_read_unlock_bh(void)
691 {
692         RCU_LOCKDEP_WARN(!rcu_is_watching(),
693                          "rcu_read_unlock_bh() used illegally while idle");
694         rcu_lock_release(&rcu_bh_lock_map);
695         __release(RCU_BH);
696         local_bh_enable();
697 }
698 
699 /**
700  * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
701  *
702  * This is equivalent of rcu_read_lock(), but disables preemption.
703  * Read-side critical sections can also be introduced by anything else
704  * that disables preemption, including local_irq_disable() and friends.
705  *
706  * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
707  * must occur in the same context, for example, it is illegal to invoke
708  * rcu_read_unlock_sched() from process context if the matching
709  * rcu_read_lock_sched() was invoked from an NMI handler.
710  */
711 static inline void rcu_read_lock_sched(void)
712 {
713         preempt_disable();
714         __acquire(RCU_SCHED);
715         rcu_lock_acquire(&rcu_sched_lock_map);
716         RCU_LOCKDEP_WARN(!rcu_is_watching(),
717                          "rcu_read_lock_sched() used illegally while idle");
718 }
719 
720 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
721 static inline notrace void rcu_read_lock_sched_notrace(void)
722 {
723         preempt_disable_notrace();
724         __acquire(RCU_SCHED);
725 }
726 
727 /*
728  * rcu_read_unlock_sched - marks the end of a RCU-classic critical section
729  *
730  * See rcu_read_lock_sched for more information.
731  */
732 static inline void rcu_read_unlock_sched(void)
733 {
734         RCU_LOCKDEP_WARN(!rcu_is_watching(),
735                          "rcu_read_unlock_sched() used illegally while idle");
736         rcu_lock_release(&rcu_sched_lock_map);
737         __release(RCU_SCHED);
738         preempt_enable();
739 }
740 
741 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
742 static inline notrace void rcu_read_unlock_sched_notrace(void)
743 {
744         __release(RCU_SCHED);
745         preempt_enable_notrace();
746 }
747 
748 /**
749  * RCU_INIT_POINTER() - initialize an RCU protected pointer
750  * @p: The pointer to be initialized.
751  * @v: The value to initialized the pointer to.
752  *
753  * Initialize an RCU-protected pointer in special cases where readers
754  * do not need ordering constraints on the CPU or the compiler.  These
755  * special cases are:
756  *
757  * 1.   This use of RCU_INIT_POINTER() is NULLing out the pointer *or*
758  * 2.   The caller has taken whatever steps are required to prevent
759  *      RCU readers from concurrently accessing this pointer *or*
760  * 3.   The referenced data structure has already been exposed to
761  *      readers either at compile time or via rcu_assign_pointer() *and*
762  *
763  *      a.      You have not made *any* reader-visible changes to
764  *              this structure since then *or*
765  *      b.      It is OK for readers accessing this structure from its
766  *              new location to see the old state of the structure.  (For
767  *              example, the changes were to statistical counters or to
768  *              other state where exact synchronization is not required.)
769  *
770  * Failure to follow these rules governing use of RCU_INIT_POINTER() will
771  * result in impossible-to-diagnose memory corruption.  As in the structures
772  * will look OK in crash dumps, but any concurrent RCU readers might
773  * see pre-initialized values of the referenced data structure.  So
774  * please be very careful how you use RCU_INIT_POINTER()!!!
775  *
776  * If you are creating an RCU-protected linked structure that is accessed
777  * by a single external-to-structure RCU-protected pointer, then you may
778  * use RCU_INIT_POINTER() to initialize the internal RCU-protected
779  * pointers, but you must use rcu_assign_pointer() to initialize the
780  * external-to-structure pointer *after* you have completely initialized
781  * the reader-accessible portions of the linked structure.
782  *
783  * Note that unlike rcu_assign_pointer(), RCU_INIT_POINTER() provides no
784  * ordering guarantees for either the CPU or the compiler.
785  */
786 #define RCU_INIT_POINTER(p, v) \
787         do { \
788                 rcu_dereference_sparse(p, __rcu); \
789                 WRITE_ONCE(p, RCU_INITIALIZER(v)); \
790         } while (0)
791 
792 /**
793  * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer
794  * @p: The pointer to be initialized.
795  * @v: The value to initialized the pointer to.
796  *
797  * GCC-style initialization for an RCU-protected pointer in a structure field.
798  */
799 #define RCU_POINTER_INITIALIZER(p, v) \
800                 .p = RCU_INITIALIZER(v)
801 
802 /*
803  * Does the specified offset indicate that the corresponding rcu_head
804  * structure can be handled by kfree_rcu()?
805  */
806 #define __is_kfree_rcu_offset(offset) ((offset) < 4096)
807 
808 /*
809  * Helper macro for kfree_rcu() to prevent argument-expansion eyestrain.
810  */
811 #define __kfree_rcu(head, offset) \
812         do { \
813                 BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \
814                 kfree_call_rcu(head, (rcu_callback_t)(unsigned long)(offset)); \
815         } while (0)
816 
817 /**
818  * kfree_rcu() - kfree an object after a grace period.
819  * @ptr:        pointer to kfree
820  * @rcu_head:   the name of the struct rcu_head within the type of @ptr.
821  *
822  * Many rcu callbacks functions just call kfree() on the base structure.
823  * These functions are trivial, but their size adds up, and furthermore
824  * when they are used in a kernel module, that module must invoke the
825  * high-latency rcu_barrier() function at module-unload time.
826  *
827  * The kfree_rcu() function handles this issue.  Rather than encoding a
828  * function address in the embedded rcu_head structure, kfree_rcu() instead
829  * encodes the offset of the rcu_head structure within the base structure.
830  * Because the functions are not allowed in the low-order 4096 bytes of
831  * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
832  * If the offset is larger than 4095 bytes, a compile-time error will
833  * be generated in __kfree_rcu().  If this error is triggered, you can
834  * either fall back to use of call_rcu() or rearrange the structure to
835  * position the rcu_head structure into the first 4096 bytes.
836  *
837  * Note that the allowable offset might decrease in the future, for example,
838  * to allow something like kmem_cache_free_rcu().
839  *
840  * The BUILD_BUG_ON check must not involve any function calls, hence the
841  * checks are done in macros here.
842  */
843 #define kfree_rcu(ptr, rcu_head)                                        \
844         __kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
845 
846 
847 /*
848  * Place this after a lock-acquisition primitive to guarantee that
849  * an UNLOCK+LOCK pair acts as a full barrier.  This guarantee applies
850  * if the UNLOCK and LOCK are executed by the same CPU or if the
851  * UNLOCK and LOCK operate on the same lock variable.
852  */
853 #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE
854 #define smp_mb__after_unlock_lock()     smp_mb()  /* Full ordering for lock. */
855 #else /* #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
856 #define smp_mb__after_unlock_lock()     do { } while (0)
857 #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
858 
859 
860 /* Has the specified rcu_head structure been handed to call_rcu()? */
861 
862 /*
863  * rcu_head_init - Initialize rcu_head for rcu_head_after_call_rcu()
864  * @rhp: The rcu_head structure to initialize.
865  *
866  * If you intend to invoke rcu_head_after_call_rcu() to test whether a
867  * given rcu_head structure has already been passed to call_rcu(), then
868  * you must also invoke this rcu_head_init() function on it just after
869  * allocating that structure.  Calls to this function must not race with
870  * calls to call_rcu(), rcu_head_after_call_rcu(), or callback invocation.
871  */
872 static inline void rcu_head_init(struct rcu_head *rhp)
873 {
874         rhp->func = (rcu_callback_t)~0L;
875 }
876 
877 /*
878  * rcu_head_after_call_rcu - Has this rcu_head been passed to call_rcu()?
879  * @rhp: The rcu_head structure to test.
880  * @func: The function passed to call_rcu() along with @rhp.
881  *
882  * Returns @true if the @rhp has been passed to call_rcu() with @func,
883  * and @false otherwise.  Emits a warning in any other case, including
884  * the case where @rhp has already been invoked after a grace period.
885  * Calls to this function must not race with callback invocation.  One way
886  * to avoid such races is to enclose the call to rcu_head_after_call_rcu()
887  * in an RCU read-side critical section that includes a read-side fetch
888  * of the pointer to the structure containing @rhp.
889  */
890 static inline bool
891 rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
892 {
893         if (READ_ONCE(rhp->func) == f)
894                 return true;
895         WARN_ON_ONCE(READ_ONCE(rhp->func) != (rcu_callback_t)~0L);
896         return false;
897 }
898 
899 
900 /* Transitional pre-consolidation compatibility definitions. */
901 
902 static inline void synchronize_rcu_bh(void)
903 {
904         synchronize_rcu();
905 }
906 
907 static inline void synchronize_rcu_bh_expedited(void)
908 {
909         synchronize_rcu_expedited();
910 }
911 
912 static inline void call_rcu_bh(struct rcu_head *head, rcu_callback_t func)
913 {
914         call_rcu(head, func);
915 }
916 
917 static inline void rcu_barrier_bh(void)
918 {
919         rcu_barrier();
920 }
921 
922 static inline void synchronize_sched(void)
923 {
924         synchronize_rcu();
925 }
926 
927 static inline void synchronize_sched_expedited(void)
928 {
929         synchronize_rcu_expedited();
930 }
931 
932 static inline void call_rcu_sched(struct rcu_head *head, rcu_callback_t func)
933 {
934         call_rcu(head, func);
935 }
936 
937 static inline void rcu_barrier_sched(void)
938 {
939         rcu_barrier();
940 }
941 
942 static inline unsigned long get_state_synchronize_sched(void)
943 {
944         return get_state_synchronize_rcu();
945 }
946 
947 static inline void cond_synchronize_sched(unsigned long oldstate)
948 {
949         cond_synchronize_rcu(oldstate);
950 }
951 
952 #endif /* __LINUX_RCUPDATE_H */
953 

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