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Linux/include/linux/rcupdate.h

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  1 /* SPDX-License-Identifier: GPL-2.0+ */
  2 /*
  3  * Read-Copy Update mechanism for mutual exclusion
  4  *
  5  * Copyright IBM Corporation, 2001
  6  *
  7  * Author: Dipankar Sarma <dipankar@in.ibm.com>
  8  *
  9  * Based on the original work by Paul McKenney <paulmck@vnet.ibm.com>
 10  * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
 11  * Papers:
 12  * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
 13  * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
 14  *
 15  * For detailed explanation of Read-Copy Update mechanism see -
 16  *              http://lse.sourceforge.net/locking/rcupdate.html
 17  *
 18  */
 19 
 20 #ifndef __LINUX_RCUPDATE_H
 21 #define __LINUX_RCUPDATE_H
 22 
 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>
 32 
 33 #define ULONG_CMP_GE(a, b)      (ULONG_MAX / 2 >= (a) - (b))
 34 #define ULONG_CMP_LT(a, b)      (ULONG_MAX / 2 < (a) - (b))
 35 #define ulong2long(a)           (*(long *)(&(a)))
 36 
 37 /* Exported common interfaces */
 38 void call_rcu(struct rcu_head *head, rcu_callback_t func);
 39 void rcu_barrier_tasks(void);
 40 void synchronize_rcu(void);
 41 
 42 #ifdef CONFIG_PREEMPT_RCU
 43 
 44 void __rcu_read_lock(void);
 45 void __rcu_read_unlock(void);
 46 
 47 /*
 48  * Defined as a macro as it is a very low level header included from
 49  * areas that don't even know about current.  This gives the rcu_read_lock()
 50  * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
 51  * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
 52  */
 53 #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
 54 
 55 #else /* #ifdef CONFIG_PREEMPT_RCU */
 56 
 57 static inline void __rcu_read_lock(void)
 58 {
 59         preempt_disable();
 60 }
 61 
 62 static inline void __rcu_read_unlock(void)
 63 {
 64         preempt_enable();
 65 }
 66 
 67 static inline int rcu_preempt_depth(void)
 68 {
 69         return 0;
 70 }
 71 
 72 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 73 
 74 /* Internal to kernel */
 75 void rcu_init(void);
 76 extern int rcu_scheduler_active __read_mostly;
 77 void rcu_sched_clock_irq(int user);
 78 void rcu_report_dead(unsigned int cpu);
 79 void rcutree_migrate_callbacks(int cpu);
 80 
 81 #ifdef CONFIG_RCU_STALL_COMMON
 82 void rcu_sysrq_start(void);
 83 void rcu_sysrq_end(void);
 84 #else /* #ifdef CONFIG_RCU_STALL_COMMON */
 85 static inline void rcu_sysrq_start(void) { }
 86 static inline void rcu_sysrq_end(void) { }
 87 #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
 88 
 89 #ifdef CONFIG_NO_HZ_FULL
 90 void rcu_user_enter(void);
 91 void rcu_user_exit(void);
 92 #else
 93 static inline void rcu_user_enter(void) { }
 94 static inline void rcu_user_exit(void) { }
 95 #endif /* CONFIG_NO_HZ_FULL */
 96 
 97 #ifdef CONFIG_RCU_NOCB_CPU
 98 void rcu_init_nohz(void);
 99 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
100 static inline void rcu_init_nohz(void) { }
101 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
102 
103 /**
104  * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
105  * @a: Code that RCU needs to pay attention to.
106  *
107  * RCU read-side critical sections are forbidden in the inner idle loop,
108  * that is, between the rcu_idle_enter() and the rcu_idle_exit() -- RCU
109  * will happily ignore any such read-side critical sections.  However,
110  * things like powertop need tracepoints in the inner idle loop.
111  *
112  * This macro provides the way out:  RCU_NONIDLE(do_something_with_RCU())
113  * will tell RCU that it needs to pay attention, invoke its argument
114  * (in this example, calling the do_something_with_RCU() function),
115  * and then tell RCU to go back to ignoring this CPU.  It is permissible
116  * to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is
117  * on the order of a million or so, even on 32-bit systems).  It is
118  * not legal to block within RCU_NONIDLE(), nor is it permissible to
119  * transfer control either into or out of RCU_NONIDLE()'s statement.
120  */
121 #define RCU_NONIDLE(a) \
122         do { \
123                 rcu_irq_enter_irqson(); \
124                 do { a; } while (0); \
125                 rcu_irq_exit_irqson(); \
126         } while (0)
127 
128 /*
129  * Note a quasi-voluntary context switch for RCU-tasks's benefit.
130  * This is a macro rather than an inline function to avoid #include hell.
131  */
132 #ifdef CONFIG_TASKS_RCU
133 #define rcu_tasks_qs(t) \
134         do { \
135                 if (READ_ONCE((t)->rcu_tasks_holdout)) \
136                         WRITE_ONCE((t)->rcu_tasks_holdout, false); \
137         } while (0)
138 #define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t)
139 void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
140 void synchronize_rcu_tasks(void);
141 void exit_tasks_rcu_start(void);
142 void exit_tasks_rcu_finish(void);
143 #else /* #ifdef CONFIG_TASKS_RCU */
144 #define rcu_tasks_qs(t) do { } while (0)
145 #define rcu_note_voluntary_context_switch(t) do { } while (0)
146 #define call_rcu_tasks call_rcu
147 #define synchronize_rcu_tasks synchronize_rcu
148 static inline void exit_tasks_rcu_start(void) { }
149 static inline void exit_tasks_rcu_finish(void) { }
150 #endif /* #else #ifdef CONFIG_TASKS_RCU */
151 
152 /**
153  * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
154  *
155  * This macro resembles cond_resched(), except that it is defined to
156  * report potential quiescent states to RCU-tasks even if the cond_resched()
157  * machinery were to be shut off, as some advocate for PREEMPT kernels.
158  */
159 #define cond_resched_tasks_rcu_qs() \
160 do { \
161         rcu_tasks_qs(current); \
162         cond_resched(); \
163 } while (0)
164 
165 /*
166  * Infrastructure to implement the synchronize_() primitives in
167  * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
168  */
169 
170 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU)
171 #include <linux/rcutree.h>
172 #elif defined(CONFIG_TINY_RCU)
173 #include <linux/rcutiny.h>
174 #else
175 #error "Unknown RCU implementation specified to kernel configuration"
176 #endif
177 
178 /*
179  * The init_rcu_head_on_stack() and destroy_rcu_head_on_stack() calls
180  * are needed for dynamic initialization and destruction of rcu_head
181  * on the stack, and init_rcu_head()/destroy_rcu_head() are needed for
182  * dynamic initialization and destruction of statically allocated rcu_head
183  * structures.  However, rcu_head structures allocated dynamically in the
184  * heap don't need any initialization.
185  */
186 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
187 void init_rcu_head(struct rcu_head *head);
188 void destroy_rcu_head(struct rcu_head *head);
189 void init_rcu_head_on_stack(struct rcu_head *head);
190 void destroy_rcu_head_on_stack(struct rcu_head *head);
191 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
192 static inline void init_rcu_head(struct rcu_head *head) { }
193 static inline void destroy_rcu_head(struct rcu_head *head) { }
194 static inline void init_rcu_head_on_stack(struct rcu_head *head) { }
195 static inline void destroy_rcu_head_on_stack(struct rcu_head *head) { }
196 #endif  /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
197 
198 #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
199 bool rcu_lockdep_current_cpu_online(void);
200 #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
201 static inline bool rcu_lockdep_current_cpu_online(void) { return true; }
202 #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
203 
204 #ifdef CONFIG_DEBUG_LOCK_ALLOC
205 
206 static inline void rcu_lock_acquire(struct lockdep_map *map)
207 {
208         lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
209 }
210 
211 static inline void rcu_lock_release(struct lockdep_map *map)
212 {
213         lock_release(map, 1, _THIS_IP_);
214 }
215 
216 extern struct lockdep_map rcu_lock_map;
217 extern struct lockdep_map rcu_bh_lock_map;
218 extern struct lockdep_map rcu_sched_lock_map;
219 extern struct lockdep_map rcu_callback_map;
220 int debug_lockdep_rcu_enabled(void);
221 int rcu_read_lock_held(void);
222 int rcu_read_lock_bh_held(void);
223 int rcu_read_lock_sched_held(void);
224 
225 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
226 
227 # define rcu_lock_acquire(a)            do { } while (0)
228 # define rcu_lock_release(a)            do { } while (0)
229 
230 static inline int rcu_read_lock_held(void)
231 {
232         return 1;
233 }
234 
235 static inline int rcu_read_lock_bh_held(void)
236 {
237         return 1;
238 }
239 
240 static inline int rcu_read_lock_sched_held(void)
241 {
242         return !preemptible();
243 }
244 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
245 
246 #ifdef CONFIG_PROVE_RCU
247 
248 /**
249  * RCU_LOCKDEP_WARN - emit lockdep splat if specified condition is met
250  * @c: condition to check
251  * @s: informative message
252  */
253 #define RCU_LOCKDEP_WARN(c, s)                                          \
254         do {                                                            \
255                 static bool __section(.data.unlikely) __warned;         \
256                 if (debug_lockdep_rcu_enabled() && !__warned && (c)) {  \
257                         __warned = true;                                \
258                         lockdep_rcu_suspicious(__FILE__, __LINE__, s);  \
259                 }                                                       \
260         } while (0)
261 
262 #if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
263 static inline void rcu_preempt_sleep_check(void)
264 {
265         RCU_LOCKDEP_WARN(lock_is_held(&rcu_lock_map),
266                          "Illegal context switch in RCU read-side critical section");
267 }
268 #else /* #ifdef CONFIG_PROVE_RCU */
269 static inline void rcu_preempt_sleep_check(void) { }
270 #endif /* #else #ifdef CONFIG_PROVE_RCU */
271 
272 #define rcu_sleep_check()                                               \
273         do {                                                            \
274                 rcu_preempt_sleep_check();                              \
275                 RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map),        \
276                                  "Illegal context switch in RCU-bh read-side critical section"); \
277                 RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map),     \
278                                  "Illegal context switch in RCU-sched read-side critical section"); \
279         } while (0)
280 
281 #else /* #ifdef CONFIG_PROVE_RCU */
282 
283 #define RCU_LOCKDEP_WARN(c, s) do { } while (0)
284 #define rcu_sleep_check() do { } while (0)
285 
286 #endif /* #else #ifdef CONFIG_PROVE_RCU */
287 
288 /*
289  * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
290  * and rcu_assign_pointer().  Some of these could be folded into their
291  * callers, but they are left separate in order to ease introduction of
292  * multiple pointers markings to match different RCU implementations
293  * (e.g., __srcu), should this make sense in the future.
294  */
295 
296 #ifdef __CHECKER__
297 #define rcu_check_sparse(p, space) \
298         ((void)(((typeof(*p) space *)p) == p))
299 #else /* #ifdef __CHECKER__ */
300 #define rcu_check_sparse(p, space)
301 #endif /* #else #ifdef __CHECKER__ */
302 
303 #define __rcu_access_pointer(p, space) \
304 ({ \
305         typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
306         rcu_check_sparse(p, space); \
307         ((typeof(*p) __force __kernel *)(_________p1)); \
308 })
309 #define __rcu_dereference_check(p, c, space) \
310 ({ \
311         /* Dependency order vs. p above. */ \
312         typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
313         RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
314         rcu_check_sparse(p, space); \
315         ((typeof(*p) __force __kernel *)(________p1)); \
316 })
317 #define __rcu_dereference_protected(p, c, space) \
318 ({ \
319         RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
320         rcu_check_sparse(p, space); \
321         ((typeof(*p) __force __kernel *)(p)); \
322 })
323 #define rcu_dereference_raw(p) \
324 ({ \
325         /* Dependency order vs. p above. */ \
326         typeof(p) ________p1 = READ_ONCE(p); \
327         ((typeof(*p) __force __kernel *)(________p1)); \
328 })
329 
330 /**
331  * RCU_INITIALIZER() - statically initialize an RCU-protected global variable
332  * @v: The value to statically initialize with.
333  */
334 #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
335 
336 /**
337  * rcu_assign_pointer() - assign to RCU-protected pointer
338  * @p: pointer to assign to
339  * @v: value to assign (publish)
340  *
341  * Assigns the specified value to the specified RCU-protected
342  * pointer, ensuring that any concurrent RCU readers will see
343  * any prior initialization.
344  *
345  * Inserts memory barriers on architectures that require them
346  * (which is most of them), and also prevents the compiler from
347  * reordering the code that initializes the structure after the pointer
348  * assignment.  More importantly, this call documents which pointers
349  * will be dereferenced by RCU read-side code.
350  *
351  * In some special cases, you may use RCU_INIT_POINTER() instead
352  * of rcu_assign_pointer().  RCU_INIT_POINTER() is a bit faster due
353  * to the fact that it does not constrain either the CPU or the compiler.
354  * That said, using RCU_INIT_POINTER() when you should have used
355  * rcu_assign_pointer() is a very bad thing that results in
356  * impossible-to-diagnose memory corruption.  So please be careful.
357  * See the RCU_INIT_POINTER() comment header for details.
358  *
359  * Note that rcu_assign_pointer() evaluates each of its arguments only
360  * once, appearances notwithstanding.  One of the "extra" evaluations
361  * is in typeof() and the other visible only to sparse (__CHECKER__),
362  * neither of which actually execute the argument.  As with most cpp
363  * macros, this execute-arguments-only-once property is important, so
364  * please be careful when making changes to rcu_assign_pointer() and the
365  * other macros that it invokes.
366  */
367 #define rcu_assign_pointer(p, v)                                              \
368 do {                                                                          \
369         uintptr_t _r_a_p__v = (uintptr_t)(v);                                 \
370         rcu_check_sparse(p, __rcu);                                           \
371                                                                               \
372         if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL)        \
373                 WRITE_ONCE((p), (typeof(p))(_r_a_p__v));                      \
374         else                                                                  \
375                 smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
376 } while (0)
377 
378 /**
379  * rcu_swap_protected() - swap an RCU and a regular pointer
380  * @rcu_ptr: RCU pointer
381  * @ptr: regular pointer
382  * @c: the conditions under which the dereference will take place
383  *
384  * Perform swap(@rcu_ptr, @ptr) where @rcu_ptr is an RCU-annotated pointer and
385  * @c is the argument that is passed to the rcu_dereference_protected() call
386  * used to read that pointer.
387  */
388 #define rcu_swap_protected(rcu_ptr, ptr, c) do {                        \
389         typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));  \
390         rcu_assign_pointer((rcu_ptr), (ptr));                           \
391         (ptr) = __tmp;                                                  \
392 } while (0)
393 
394 /**
395  * rcu_access_pointer() - fetch RCU pointer with no dereferencing
396  * @p: The pointer to read
397  *
398  * Return the value of the specified RCU-protected pointer, but omit the
399  * lockdep checks for being in an RCU read-side critical section.  This is
400  * useful when the value of this pointer is accessed, but the pointer is
401  * not dereferenced, for example, when testing an RCU-protected pointer
402  * against NULL.  Although rcu_access_pointer() may also be used in cases
403  * where update-side locks prevent the value of the pointer from changing,
404  * you should instead use rcu_dereference_protected() for this use case.
405  *
406  * It is also permissible to use rcu_access_pointer() when read-side
407  * access to the pointer was removed at least one grace period ago, as
408  * is the case in the context of the RCU callback that is freeing up
409  * the data, or after a synchronize_rcu() returns.  This can be useful
410  * when tearing down multi-linked structures after a grace period
411  * has elapsed.
412  */
413 #define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
414 
415 /**
416  * rcu_dereference_check() - rcu_dereference with debug checking
417  * @p: The pointer to read, prior to dereferencing
418  * @c: The conditions under which the dereference will take place
419  *
420  * Do an rcu_dereference(), but check that the conditions under which the
421  * dereference will take place are correct.  Typically the conditions
422  * indicate the various locking conditions that should be held at that
423  * point.  The check should return true if the conditions are satisfied.
424  * An implicit check for being in an RCU read-side critical section
425  * (rcu_read_lock()) is included.
426  *
427  * For example:
428  *
429  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
430  *
431  * could be used to indicate to lockdep that foo->bar may only be dereferenced
432  * if either rcu_read_lock() is held, or that the lock required to replace
433  * the bar struct at foo->bar is held.
434  *
435  * Note that the list of conditions may also include indications of when a lock
436  * need not be held, for example during initialisation or destruction of the
437  * target struct:
438  *
439  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
440  *                                            atomic_read(&foo->usage) == 0);
441  *
442  * Inserts memory barriers on architectures that require them
443  * (currently only the Alpha), prevents the compiler from refetching
444  * (and from merging fetches), and, more importantly, documents exactly
445  * which pointers are protected by RCU and checks that the pointer is
446  * annotated as __rcu.
447  */
448 #define rcu_dereference_check(p, c) \
449         __rcu_dereference_check((p), (c) || rcu_read_lock_held(), __rcu)
450 
451 /**
452  * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
453  * @p: The pointer to read, prior to dereferencing
454  * @c: The conditions under which the dereference will take place
455  *
456  * This is the RCU-bh counterpart to rcu_dereference_check().
457  */
458 #define rcu_dereference_bh_check(p, c) \
459         __rcu_dereference_check((p), (c) || rcu_read_lock_bh_held(), __rcu)
460 
461 /**
462  * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
463  * @p: The pointer to read, prior to dereferencing
464  * @c: The conditions under which the dereference will take place
465  *
466  * This is the RCU-sched counterpart to rcu_dereference_check().
467  */
468 #define rcu_dereference_sched_check(p, c) \
469         __rcu_dereference_check((p), (c) || rcu_read_lock_sched_held(), \
470                                 __rcu)
471 
472 /*
473  * The tracing infrastructure traces RCU (we want that), but unfortunately
474  * some of the RCU checks causes tracing to lock up the system.
475  *
476  * The no-tracing version of rcu_dereference_raw() must not call
477  * rcu_read_lock_held().
478  */
479 #define rcu_dereference_raw_notrace(p) __rcu_dereference_check((p), 1, __rcu)
480 
481 /**
482  * rcu_dereference_protected() - fetch RCU pointer when updates prevented
483  * @p: The pointer to read, prior to dereferencing
484  * @c: The conditions under which the dereference will take place
485  *
486  * Return the value of the specified RCU-protected pointer, but omit
487  * the READ_ONCE().  This is useful in cases where update-side locks
488  * prevent the value of the pointer from changing.  Please note that this
489  * primitive does *not* prevent the compiler from repeating this reference
490  * or combining it with other references, so it should not be used without
491  * protection of appropriate locks.
492  *
493  * This function is only for update-side use.  Using this function
494  * when protected only by rcu_read_lock() will result in infrequent
495  * but very ugly failures.
496  */
497 #define rcu_dereference_protected(p, c) \
498         __rcu_dereference_protected((p), (c), __rcu)
499 
500 
501 /**
502  * rcu_dereference() - fetch RCU-protected pointer for dereferencing
503  * @p: The pointer to read, prior to dereferencing
504  *
505  * This is a simple wrapper around rcu_dereference_check().
506  */
507 #define rcu_dereference(p) rcu_dereference_check(p, 0)
508 
509 /**
510  * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
511  * @p: The pointer to read, prior to dereferencing
512  *
513  * Makes rcu_dereference_check() do the dirty work.
514  */
515 #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
516 
517 /**
518  * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
519  * @p: The pointer to read, prior to dereferencing
520  *
521  * Makes rcu_dereference_check() do the dirty work.
522  */
523 #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
524 
525 /**
526  * rcu_pointer_handoff() - Hand off a pointer from RCU to other mechanism
527  * @p: The pointer to hand off
528  *
529  * This is simply an identity function, but it documents where a pointer
530  * is handed off from RCU to some other synchronization mechanism, for
531  * example, reference counting or locking.  In C11, it would map to
532  * kill_dependency().  It could be used as follows::
533  *
534  *      rcu_read_lock();
535  *      p = rcu_dereference(gp);
536  *      long_lived = is_long_lived(p);
537  *      if (long_lived) {
538  *              if (!atomic_inc_not_zero(p->refcnt))
539  *                      long_lived = false;
540  *              else
541  *                      p = rcu_pointer_handoff(p);
542  *      }
543  *      rcu_read_unlock();
544  */
545 #define rcu_pointer_handoff(p) (p)
546 
547 /**
548  * rcu_read_lock() - mark the beginning of an RCU read-side critical section
549  *
550  * When synchronize_rcu() is invoked on one CPU while other CPUs
551  * are within RCU read-side critical sections, then the
552  * synchronize_rcu() is guaranteed to block until after all the other
553  * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
554  * on one CPU while other CPUs are within RCU read-side critical
555  * sections, invocation of the corresponding RCU callback is deferred
556  * until after the all the other CPUs exit their critical sections.
557  *
558  * Note, however, that RCU callbacks are permitted to run concurrently
559  * with new RCU read-side critical sections.  One way that this can happen
560  * is via the following sequence of events: (1) CPU 0 enters an RCU
561  * read-side critical section, (2) CPU 1 invokes call_rcu() to register
562  * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
563  * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
564  * callback is invoked.  This is legal, because the RCU read-side critical
565  * section that was running concurrently with the call_rcu() (and which
566  * therefore might be referencing something that the corresponding RCU
567  * callback would free up) has completed before the corresponding
568  * RCU callback is invoked.
569  *
570  * RCU read-side critical sections may be nested.  Any deferred actions
571  * will be deferred until the outermost RCU read-side critical section
572  * completes.
573  *
574  * You can avoid reading and understanding the next paragraph by
575  * following this rule: don't put anything in an rcu_read_lock() RCU
576  * read-side critical section that would block in a !PREEMPT kernel.
577  * But if you want the full story, read on!
578  *
579  * In non-preemptible RCU implementations (TREE_RCU and TINY_RCU),
580  * it is illegal to block while in an RCU read-side critical section.
581  * In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPT
582  * kernel builds, RCU read-side critical sections may be preempted,
583  * but explicit blocking is illegal.  Finally, in preemptible RCU
584  * implementations in real-time (with -rt patchset) kernel builds, RCU
585  * read-side critical sections may be preempted and they may also block, but
586  * only when acquiring spinlocks that are subject to priority inheritance.
587  */
588 static __always_inline void rcu_read_lock(void)
589 {
590         __rcu_read_lock();
591         __acquire(RCU);
592         rcu_lock_acquire(&rcu_lock_map);
593         RCU_LOCKDEP_WARN(!rcu_is_watching(),
594                          "rcu_read_lock() used illegally while idle");
595 }
596 
597 /*
598  * So where is rcu_write_lock()?  It does not exist, as there is no
599  * way for writers to lock out RCU readers.  This is a feature, not
600  * a bug -- this property is what provides RCU's performance benefits.
601  * Of course, writers must coordinate with each other.  The normal
602  * spinlock primitives work well for this, but any other technique may be
603  * used as well.  RCU does not care how the writers keep out of each
604  * others' way, as long as they do so.
605  */
606 
607 /**
608  * rcu_read_unlock() - marks the end of an RCU read-side critical section.
609  *
610  * In most situations, rcu_read_unlock() is immune from deadlock.
611  * However, in kernels built with CONFIG_RCU_BOOST, rcu_read_unlock()
612  * is responsible for deboosting, which it does via rt_mutex_unlock().
613  * Unfortunately, this function acquires the scheduler's runqueue and
614  * priority-inheritance spinlocks.  This means that deadlock could result
615  * if the caller of rcu_read_unlock() already holds one of these locks or
616  * any lock that is ever acquired while holding them.
617  *
618  * That said, RCU readers are never priority boosted unless they were
619  * preempted.  Therefore, one way to avoid deadlock is to make sure
620  * that preemption never happens within any RCU read-side critical
621  * section whose outermost rcu_read_unlock() is called with one of
622  * rt_mutex_unlock()'s locks held.  Such preemption can be avoided in
623  * a number of ways, for example, by invoking preempt_disable() before
624  * critical section's outermost rcu_read_lock().
625  *
626  * Given that the set of locks acquired by rt_mutex_unlock() might change
627  * at any time, a somewhat more future-proofed approach is to make sure
628  * that that preemption never happens within any RCU read-side critical
629  * section whose outermost rcu_read_unlock() is called with irqs disabled.
630  * This approach relies on the fact that rt_mutex_unlock() currently only
631  * acquires irq-disabled locks.
632  *
633  * The second of these two approaches is best in most situations,
634  * however, the first approach can also be useful, at least to those
635  * developers willing to keep abreast of the set of locks acquired by
636  * rt_mutex_unlock().
637  *
638  * See rcu_read_lock() for more information.
639  */
640 static inline void rcu_read_unlock(void)
641 {
642         RCU_LOCKDEP_WARN(!rcu_is_watching(),
643                          "rcu_read_unlock() used illegally while idle");
644         __release(RCU);
645         __rcu_read_unlock();
646         rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
647 }
648 
649 /**
650  * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
651  *
652  * This is equivalent of rcu_read_lock(), but also disables softirqs.
653  * Note that anything else that disables softirqs can also serve as
654  * an RCU read-side critical section.
655  *
656  * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
657  * must occur in the same context, for example, it is illegal to invoke
658  * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
659  * was invoked from some other task.
660  */
661 static inline void rcu_read_lock_bh(void)
662 {
663         local_bh_disable();
664         __acquire(RCU_BH);
665         rcu_lock_acquire(&rcu_bh_lock_map);
666         RCU_LOCKDEP_WARN(!rcu_is_watching(),
667                          "rcu_read_lock_bh() used illegally while idle");
668 }
669 
670 /*
671  * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
672  *
673  * See rcu_read_lock_bh() for more information.
674  */
675 static inline void rcu_read_unlock_bh(void)
676 {
677         RCU_LOCKDEP_WARN(!rcu_is_watching(),
678                          "rcu_read_unlock_bh() used illegally while idle");
679         rcu_lock_release(&rcu_bh_lock_map);
680         __release(RCU_BH);
681         local_bh_enable();
682 }
683 
684 /**
685  * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
686  *
687  * This is equivalent of rcu_read_lock(), but disables preemption.
688  * Read-side critical sections can also be introduced by anything else
689  * that disables preemption, including local_irq_disable() and friends.
690  *
691  * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
692  * must occur in the same context, for example, it is illegal to invoke
693  * rcu_read_unlock_sched() from process context if the matching
694  * rcu_read_lock_sched() was invoked from an NMI handler.
695  */
696 static inline void rcu_read_lock_sched(void)
697 {
698         preempt_disable();
699         __acquire(RCU_SCHED);
700         rcu_lock_acquire(&rcu_sched_lock_map);
701         RCU_LOCKDEP_WARN(!rcu_is_watching(),
702                          "rcu_read_lock_sched() used illegally while idle");
703 }
704 
705 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
706 static inline notrace void rcu_read_lock_sched_notrace(void)
707 {
708         preempt_disable_notrace();
709         __acquire(RCU_SCHED);
710 }
711 
712 /*
713  * rcu_read_unlock_sched - marks the end of a RCU-classic critical section
714  *
715  * See rcu_read_lock_sched for more information.
716  */
717 static inline void rcu_read_unlock_sched(void)
718 {
719         RCU_LOCKDEP_WARN(!rcu_is_watching(),
720                          "rcu_read_unlock_sched() used illegally while idle");
721         rcu_lock_release(&rcu_sched_lock_map);
722         __release(RCU_SCHED);
723         preempt_enable();
724 }
725 
726 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
727 static inline notrace void rcu_read_unlock_sched_notrace(void)
728 {
729         __release(RCU_SCHED);
730         preempt_enable_notrace();
731 }
732 
733 /**
734  * RCU_INIT_POINTER() - initialize an RCU protected pointer
735  * @p: The pointer to be initialized.
736  * @v: The value to initialized the pointer to.
737  *
738  * Initialize an RCU-protected pointer in special cases where readers
739  * do not need ordering constraints on the CPU or the compiler.  These
740  * special cases are:
741  *
742  * 1.   This use of RCU_INIT_POINTER() is NULLing out the pointer *or*
743  * 2.   The caller has taken whatever steps are required to prevent
744  *      RCU readers from concurrently accessing this pointer *or*
745  * 3.   The referenced data structure has already been exposed to
746  *      readers either at compile time or via rcu_assign_pointer() *and*
747  *
748  *      a.      You have not made *any* reader-visible changes to
749  *              this structure since then *or*
750  *      b.      It is OK for readers accessing this structure from its
751  *              new location to see the old state of the structure.  (For
752  *              example, the changes were to statistical counters or to
753  *              other state where exact synchronization is not required.)
754  *
755  * Failure to follow these rules governing use of RCU_INIT_POINTER() will
756  * result in impossible-to-diagnose memory corruption.  As in the structures
757  * will look OK in crash dumps, but any concurrent RCU readers might
758  * see pre-initialized values of the referenced data structure.  So
759  * please be very careful how you use RCU_INIT_POINTER()!!!
760  *
761  * If you are creating an RCU-protected linked structure that is accessed
762  * by a single external-to-structure RCU-protected pointer, then you may
763  * use RCU_INIT_POINTER() to initialize the internal RCU-protected
764  * pointers, but you must use rcu_assign_pointer() to initialize the
765  * external-to-structure pointer *after* you have completely initialized
766  * the reader-accessible portions of the linked structure.
767  *
768  * Note that unlike rcu_assign_pointer(), RCU_INIT_POINTER() provides no
769  * ordering guarantees for either the CPU or the compiler.
770  */
771 #define RCU_INIT_POINTER(p, v) \
772         do { \
773                 rcu_check_sparse(p, __rcu); \
774                 WRITE_ONCE(p, RCU_INITIALIZER(v)); \
775         } while (0)
776 
777 /**
778  * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer
779  * @p: The pointer to be initialized.
780  * @v: The value to initialized the pointer to.
781  *
782  * GCC-style initialization for an RCU-protected pointer in a structure field.
783  */
784 #define RCU_POINTER_INITIALIZER(p, v) \
785                 .p = RCU_INITIALIZER(v)
786 
787 /*
788  * Does the specified offset indicate that the corresponding rcu_head
789  * structure can be handled by kfree_rcu()?
790  */
791 #define __is_kfree_rcu_offset(offset) ((offset) < 4096)
792 
793 /*
794  * Helper macro for kfree_rcu() to prevent argument-expansion eyestrain.
795  */
796 #define __kfree_rcu(head, offset) \
797         do { \
798                 BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \
799                 kfree_call_rcu(head, (rcu_callback_t)(unsigned long)(offset)); \
800         } while (0)
801 
802 /**
803  * kfree_rcu() - kfree an object after a grace period.
804  * @ptr:        pointer to kfree
805  * @rhf:        the name of the struct rcu_head within the type of @ptr.
806  *
807  * Many rcu callbacks functions just call kfree() on the base structure.
808  * These functions are trivial, but their size adds up, and furthermore
809  * when they are used in a kernel module, that module must invoke the
810  * high-latency rcu_barrier() function at module-unload time.
811  *
812  * The kfree_rcu() function handles this issue.  Rather than encoding a
813  * function address in the embedded rcu_head structure, kfree_rcu() instead
814  * encodes the offset of the rcu_head structure within the base structure.
815  * Because the functions are not allowed in the low-order 4096 bytes of
816  * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
817  * If the offset is larger than 4095 bytes, a compile-time error will
818  * be generated in __kfree_rcu().  If this error is triggered, you can
819  * either fall back to use of call_rcu() or rearrange the structure to
820  * position the rcu_head structure into the first 4096 bytes.
821  *
822  * Note that the allowable offset might decrease in the future, for example,
823  * to allow something like kmem_cache_free_rcu().
824  *
825  * The BUILD_BUG_ON check must not involve any function calls, hence the
826  * checks are done in macros here.
827  */
828 #define kfree_rcu(ptr, rhf)                                             \
829 do {                                                                    \
830         typeof (ptr) ___p = (ptr);                                      \
831                                                                         \
832         if (___p)                                                       \
833                 __kfree_rcu(&((___p)->rhf), offsetof(typeof(*(ptr)), rhf)); \
834 } while (0)
835 
836 /*
837  * Place this after a lock-acquisition primitive to guarantee that
838  * an UNLOCK+LOCK pair acts as a full barrier.  This guarantee applies
839  * if the UNLOCK and LOCK are executed by the same CPU or if the
840  * UNLOCK and LOCK operate on the same lock variable.
841  */
842 #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE
843 #define smp_mb__after_unlock_lock()     smp_mb()  /* Full ordering for lock. */
844 #else /* #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
845 #define smp_mb__after_unlock_lock()     do { } while (0)
846 #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
847 
848 
849 /* Has the specified rcu_head structure been handed to call_rcu()? */
850 
851 /**
852  * rcu_head_init - Initialize rcu_head for rcu_head_after_call_rcu()
853  * @rhp: The rcu_head structure to initialize.
854  *
855  * If you intend to invoke rcu_head_after_call_rcu() to test whether a
856  * given rcu_head structure has already been passed to call_rcu(), then
857  * you must also invoke this rcu_head_init() function on it just after
858  * allocating that structure.  Calls to this function must not race with
859  * calls to call_rcu(), rcu_head_after_call_rcu(), or callback invocation.
860  */
861 static inline void rcu_head_init(struct rcu_head *rhp)
862 {
863         rhp->func = (rcu_callback_t)~0L;
864 }
865 
866 /**
867  * rcu_head_after_call_rcu - Has this rcu_head been passed to call_rcu()?
868  * @rhp: The rcu_head structure to test.
869  * @f: The function passed to call_rcu() along with @rhp.
870  *
871  * Returns @true if the @rhp has been passed to call_rcu() with @func,
872  * and @false otherwise.  Emits a warning in any other case, including
873  * the case where @rhp has already been invoked after a grace period.
874  * Calls to this function must not race with callback invocation.  One way
875  * to avoid such races is to enclose the call to rcu_head_after_call_rcu()
876  * in an RCU read-side critical section that includes a read-side fetch
877  * of the pointer to the structure containing @rhp.
878  */
879 static inline bool
880 rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
881 {
882         rcu_callback_t func = READ_ONCE(rhp->func);
883 
884         if (func == f)
885                 return true;
886         WARN_ON_ONCE(func != (rcu_callback_t)~0L);
887         return false;
888 }
889 
890 #endif /* __LINUX_RCUPDATE_H */
891 

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