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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 rcu_barrier_tasks_rude(void);
 41 void synchronize_rcu(void);
 42 
 43 #ifdef CONFIG_PREEMPT_RCU
 44 
 45 void __rcu_read_lock(void);
 46 void __rcu_read_unlock(void);
 47 
 48 /*
 49  * Defined as a macro as it is a very low level header included from
 50  * areas that don't even know about current.  This gives the rcu_read_lock()
 51  * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
 52  * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
 53  */
 54 #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
 55 
 56 #else /* #ifdef CONFIG_PREEMPT_RCU */
 57 
 58 static inline void __rcu_read_lock(void)
 59 {
 60         preempt_disable();
 61 }
 62 
 63 static inline void __rcu_read_unlock(void)
 64 {
 65         preempt_enable();
 66 }
 67 
 68 static inline int rcu_preempt_depth(void)
 69 {
 70         return 0;
 71 }
 72 
 73 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 74 
 75 /* Internal to kernel */
 76 void rcu_init(void);
 77 extern int rcu_scheduler_active __read_mostly;
 78 void rcu_sched_clock_irq(int user);
 79 void rcu_report_dead(unsigned int cpu);
 80 void rcutree_migrate_callbacks(int cpu);
 81 
 82 #ifdef CONFIG_RCU_STALL_COMMON
 83 void rcu_sysrq_start(void);
 84 void rcu_sysrq_end(void);
 85 #else /* #ifdef CONFIG_RCU_STALL_COMMON */
 86 static inline void rcu_sysrq_start(void) { }
 87 static inline void rcu_sysrq_end(void) { }
 88 #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
 89 
 90 #ifdef CONFIG_NO_HZ_FULL
 91 void rcu_user_enter(void);
 92 void rcu_user_exit(void);
 93 #else
 94 static inline void rcu_user_enter(void) { }
 95 static inline void rcu_user_exit(void) { }
 96 #endif /* CONFIG_NO_HZ_FULL */
 97 
 98 #ifdef CONFIG_RCU_NOCB_CPU
 99 void rcu_init_nohz(void);
100 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
101 static inline void rcu_init_nohz(void) { }
102 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
103 
104 /**
105  * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
106  * @a: Code that RCU needs to pay attention to.
107  *
108  * RCU read-side critical sections are forbidden in the inner idle loop,
109  * that is, between the rcu_idle_enter() and the rcu_idle_exit() -- RCU
110  * will happily ignore any such read-side critical sections.  However,
111  * things like powertop need tracepoints in the inner idle loop.
112  *
113  * This macro provides the way out:  RCU_NONIDLE(do_something_with_RCU())
114  * will tell RCU that it needs to pay attention, invoke its argument
115  * (in this example, calling the do_something_with_RCU() function),
116  * and then tell RCU to go back to ignoring this CPU.  It is permissible
117  * to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is
118  * on the order of a million or so, even on 32-bit systems).  It is
119  * not legal to block within RCU_NONIDLE(), nor is it permissible to
120  * transfer control either into or out of RCU_NONIDLE()'s statement.
121  */
122 #define RCU_NONIDLE(a) \
123         do { \
124                 rcu_irq_enter_irqson(); \
125                 do { a; } while (0); \
126                 rcu_irq_exit_irqson(); \
127         } while (0)
128 
129 /*
130  * Note a quasi-voluntary context switch for RCU-tasks's benefit.
131  * This is a macro rather than an inline function to avoid #include hell.
132  */
133 #ifdef CONFIG_TASKS_RCU_GENERIC
134 
135 # ifdef CONFIG_TASKS_RCU
136 # define rcu_tasks_classic_qs(t, preempt)                               \
137         do {                                                            \
138                 if (!(preempt) && READ_ONCE((t)->rcu_tasks_holdout))    \
139                         WRITE_ONCE((t)->rcu_tasks_holdout, false);      \
140         } while (0)
141 void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
142 void synchronize_rcu_tasks(void);
143 # else
144 # define rcu_tasks_classic_qs(t, preempt) do { } while (0)
145 # define call_rcu_tasks call_rcu
146 # define synchronize_rcu_tasks synchronize_rcu
147 # endif
148 
149 # ifdef CONFIG_TASKS_RCU_TRACE
150 # define rcu_tasks_trace_qs(t)                                          \
151         do {                                                            \
152                 if (!likely(READ_ONCE((t)->trc_reader_checked)) &&      \
153                     !unlikely(READ_ONCE((t)->trc_reader_nesting))) {    \
154                         smp_store_release(&(t)->trc_reader_checked, true); \
155                         smp_mb(); /* Readers partitioned by store. */   \
156                 }                                                       \
157         } while (0)
158 # else
159 # define rcu_tasks_trace_qs(t) do { } while (0)
160 # endif
161 
162 #define rcu_tasks_qs(t, preempt)                                        \
163 do {                                                                    \
164         rcu_tasks_classic_qs((t), (preempt));                           \
165         rcu_tasks_trace_qs((t));                                        \
166 } while (0)
167 
168 # ifdef CONFIG_TASKS_RUDE_RCU
169 void call_rcu_tasks_rude(struct rcu_head *head, rcu_callback_t func);
170 void synchronize_rcu_tasks_rude(void);
171 # endif
172 
173 #define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t, false)
174 void exit_tasks_rcu_start(void);
175 void exit_tasks_rcu_finish(void);
176 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
177 #define rcu_tasks_qs(t, preempt) do { } while (0)
178 #define rcu_note_voluntary_context_switch(t) do { } while (0)
179 #define call_rcu_tasks call_rcu
180 #define synchronize_rcu_tasks synchronize_rcu
181 static inline void exit_tasks_rcu_start(void) { }
182 static inline void exit_tasks_rcu_finish(void) { }
183 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
184 
185 /**
186  * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
187  *
188  * This macro resembles cond_resched(), except that it is defined to
189  * report potential quiescent states to RCU-tasks even if the cond_resched()
190  * machinery were to be shut off, as some advocate for PREEMPTION kernels.
191  */
192 #define cond_resched_tasks_rcu_qs() \
193 do { \
194         rcu_tasks_qs(current, false); \
195         cond_resched(); \
196 } while (0)
197 
198 /*
199  * Infrastructure to implement the synchronize_() primitives in
200  * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
201  */
202 
203 #if defined(CONFIG_TREE_RCU)
204 #include <linux/rcutree.h>
205 #elif defined(CONFIG_TINY_RCU)
206 #include <linux/rcutiny.h>
207 #else
208 #error "Unknown RCU implementation specified to kernel configuration"
209 #endif
210 
211 /*
212  * The init_rcu_head_on_stack() and destroy_rcu_head_on_stack() calls
213  * are needed for dynamic initialization and destruction of rcu_head
214  * on the stack, and init_rcu_head()/destroy_rcu_head() are needed for
215  * dynamic initialization and destruction of statically allocated rcu_head
216  * structures.  However, rcu_head structures allocated dynamically in the
217  * heap don't need any initialization.
218  */
219 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
220 void init_rcu_head(struct rcu_head *head);
221 void destroy_rcu_head(struct rcu_head *head);
222 void init_rcu_head_on_stack(struct rcu_head *head);
223 void destroy_rcu_head_on_stack(struct rcu_head *head);
224 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
225 static inline void init_rcu_head(struct rcu_head *head) { }
226 static inline void destroy_rcu_head(struct rcu_head *head) { }
227 static inline void init_rcu_head_on_stack(struct rcu_head *head) { }
228 static inline void destroy_rcu_head_on_stack(struct rcu_head *head) { }
229 #endif  /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
230 
231 #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
232 bool rcu_lockdep_current_cpu_online(void);
233 #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
234 static inline bool rcu_lockdep_current_cpu_online(void) { return true; }
235 #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
236 
237 #ifdef CONFIG_DEBUG_LOCK_ALLOC
238 
239 static inline void rcu_lock_acquire(struct lockdep_map *map)
240 {
241         lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
242 }
243 
244 static inline void rcu_lock_release(struct lockdep_map *map)
245 {
246         lock_release(map, _THIS_IP_);
247 }
248 
249 extern struct lockdep_map rcu_lock_map;
250 extern struct lockdep_map rcu_bh_lock_map;
251 extern struct lockdep_map rcu_sched_lock_map;
252 extern struct lockdep_map rcu_callback_map;
253 int debug_lockdep_rcu_enabled(void);
254 int rcu_read_lock_held(void);
255 int rcu_read_lock_bh_held(void);
256 int rcu_read_lock_sched_held(void);
257 int rcu_read_lock_any_held(void);
258 
259 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
260 
261 # define rcu_lock_acquire(a)            do { } while (0)
262 # define rcu_lock_release(a)            do { } while (0)
263 
264 static inline int rcu_read_lock_held(void)
265 {
266         return 1;
267 }
268 
269 static inline int rcu_read_lock_bh_held(void)
270 {
271         return 1;
272 }
273 
274 static inline int rcu_read_lock_sched_held(void)
275 {
276         return !preemptible();
277 }
278 
279 static inline int rcu_read_lock_any_held(void)
280 {
281         return !preemptible();
282 }
283 
284 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
285 
286 #ifdef CONFIG_PROVE_RCU
287 
288 /**
289  * RCU_LOCKDEP_WARN - emit lockdep splat if specified condition is met
290  * @c: condition to check
291  * @s: informative message
292  */
293 #define RCU_LOCKDEP_WARN(c, s)                                          \
294         do {                                                            \
295                 static bool __section(.data.unlikely) __warned;         \
296                 if (debug_lockdep_rcu_enabled() && !__warned && (c)) {  \
297                         __warned = true;                                \
298                         lockdep_rcu_suspicious(__FILE__, __LINE__, s);  \
299                 }                                                       \
300         } while (0)
301 
302 #if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
303 static inline void rcu_preempt_sleep_check(void)
304 {
305         RCU_LOCKDEP_WARN(lock_is_held(&rcu_lock_map),
306                          "Illegal context switch in RCU read-side critical section");
307 }
308 #else /* #ifdef CONFIG_PROVE_RCU */
309 static inline void rcu_preempt_sleep_check(void) { }
310 #endif /* #else #ifdef CONFIG_PROVE_RCU */
311 
312 #define rcu_sleep_check()                                               \
313         do {                                                            \
314                 rcu_preempt_sleep_check();                              \
315                 RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map),        \
316                                  "Illegal context switch in RCU-bh read-side critical section"); \
317                 RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map),     \
318                                  "Illegal context switch in RCU-sched read-side critical section"); \
319         } while (0)
320 
321 #else /* #ifdef CONFIG_PROVE_RCU */
322 
323 #define RCU_LOCKDEP_WARN(c, s) do { } while (0)
324 #define rcu_sleep_check() do { } while (0)
325 
326 #endif /* #else #ifdef CONFIG_PROVE_RCU */
327 
328 /*
329  * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
330  * and rcu_assign_pointer().  Some of these could be folded into their
331  * callers, but they are left separate in order to ease introduction of
332  * multiple pointers markings to match different RCU implementations
333  * (e.g., __srcu), should this make sense in the future.
334  */
335 
336 #ifdef __CHECKER__
337 #define rcu_check_sparse(p, space) \
338         ((void)(((typeof(*p) space *)p) == p))
339 #else /* #ifdef __CHECKER__ */
340 #define rcu_check_sparse(p, space)
341 #endif /* #else #ifdef __CHECKER__ */
342 
343 #define __rcu_access_pointer(p, space) \
344 ({ \
345         typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
346         rcu_check_sparse(p, space); \
347         ((typeof(*p) __force __kernel *)(_________p1)); \
348 })
349 #define __rcu_dereference_check(p, c, space) \
350 ({ \
351         /* Dependency order vs. p above. */ \
352         typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
353         RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
354         rcu_check_sparse(p, space); \
355         ((typeof(*p) __force __kernel *)(________p1)); \
356 })
357 #define __rcu_dereference_protected(p, c, space) \
358 ({ \
359         RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
360         rcu_check_sparse(p, space); \
361         ((typeof(*p) __force __kernel *)(p)); \
362 })
363 #define rcu_dereference_raw(p) \
364 ({ \
365         /* Dependency order vs. p above. */ \
366         typeof(p) ________p1 = READ_ONCE(p); \
367         ((typeof(*p) __force __kernel *)(________p1)); \
368 })
369 
370 /**
371  * RCU_INITIALIZER() - statically initialize an RCU-protected global variable
372  * @v: The value to statically initialize with.
373  */
374 #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
375 
376 /**
377  * rcu_assign_pointer() - assign to RCU-protected pointer
378  * @p: pointer to assign to
379  * @v: value to assign (publish)
380  *
381  * Assigns the specified value to the specified RCU-protected
382  * pointer, ensuring that any concurrent RCU readers will see
383  * any prior initialization.
384  *
385  * Inserts memory barriers on architectures that require them
386  * (which is most of them), and also prevents the compiler from
387  * reordering the code that initializes the structure after the pointer
388  * assignment.  More importantly, this call documents which pointers
389  * will be dereferenced by RCU read-side code.
390  *
391  * In some special cases, you may use RCU_INIT_POINTER() instead
392  * of rcu_assign_pointer().  RCU_INIT_POINTER() is a bit faster due
393  * to the fact that it does not constrain either the CPU or the compiler.
394  * That said, using RCU_INIT_POINTER() when you should have used
395  * rcu_assign_pointer() is a very bad thing that results in
396  * impossible-to-diagnose memory corruption.  So please be careful.
397  * See the RCU_INIT_POINTER() comment header for details.
398  *
399  * Note that rcu_assign_pointer() evaluates each of its arguments only
400  * once, appearances notwithstanding.  One of the "extra" evaluations
401  * is in typeof() and the other visible only to sparse (__CHECKER__),
402  * neither of which actually execute the argument.  As with most cpp
403  * macros, this execute-arguments-only-once property is important, so
404  * please be careful when making changes to rcu_assign_pointer() and the
405  * other macros that it invokes.
406  */
407 #define rcu_assign_pointer(p, v)                                              \
408 do {                                                                          \
409         uintptr_t _r_a_p__v = (uintptr_t)(v);                                 \
410         rcu_check_sparse(p, __rcu);                                           \
411                                                                               \
412         if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL)        \
413                 WRITE_ONCE((p), (typeof(p))(_r_a_p__v));                      \
414         else                                                                  \
415                 smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
416 } while (0)
417 
418 /**
419  * rcu_replace_pointer() - replace an RCU pointer, returning its old value
420  * @rcu_ptr: RCU pointer, whose old value is returned
421  * @ptr: regular pointer
422  * @c: the lockdep conditions under which the dereference will take place
423  *
424  * Perform a replacement, where @rcu_ptr is an RCU-annotated
425  * pointer and @c is the lockdep argument that is passed to the
426  * rcu_dereference_protected() call used to read that pointer.  The old
427  * value of @rcu_ptr is returned, and @rcu_ptr is set to @ptr.
428  */
429 #define rcu_replace_pointer(rcu_ptr, ptr, c)                            \
430 ({                                                                      \
431         typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));  \
432         rcu_assign_pointer((rcu_ptr), (ptr));                           \
433         __tmp;                                                          \
434 })
435 
436 /**
437  * rcu_access_pointer() - fetch RCU pointer with no dereferencing
438  * @p: The pointer to read
439  *
440  * Return the value of the specified RCU-protected pointer, but omit the
441  * lockdep checks for being in an RCU read-side critical section.  This is
442  * useful when the value of this pointer is accessed, but the pointer is
443  * not dereferenced, for example, when testing an RCU-protected pointer
444  * against NULL.  Although rcu_access_pointer() may also be used in cases
445  * where update-side locks prevent the value of the pointer from changing,
446  * you should instead use rcu_dereference_protected() for this use case.
447  *
448  * It is also permissible to use rcu_access_pointer() when read-side
449  * access to the pointer was removed at least one grace period ago, as
450  * is the case in the context of the RCU callback that is freeing up
451  * the data, or after a synchronize_rcu() returns.  This can be useful
452  * when tearing down multi-linked structures after a grace period
453  * has elapsed.
454  */
455 #define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
456 
457 /**
458  * rcu_dereference_check() - rcu_dereference with debug checking
459  * @p: The pointer to read, prior to dereferencing
460  * @c: The conditions under which the dereference will take place
461  *
462  * Do an rcu_dereference(), but check that the conditions under which the
463  * dereference will take place are correct.  Typically the conditions
464  * indicate the various locking conditions that should be held at that
465  * point.  The check should return true if the conditions are satisfied.
466  * An implicit check for being in an RCU read-side critical section
467  * (rcu_read_lock()) is included.
468  *
469  * For example:
470  *
471  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
472  *
473  * could be used to indicate to lockdep that foo->bar may only be dereferenced
474  * if either rcu_read_lock() is held, or that the lock required to replace
475  * the bar struct at foo->bar is held.
476  *
477  * Note that the list of conditions may also include indications of when a lock
478  * need not be held, for example during initialisation or destruction of the
479  * target struct:
480  *
481  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
482  *                                            atomic_read(&foo->usage) == 0);
483  *
484  * Inserts memory barriers on architectures that require them
485  * (currently only the Alpha), prevents the compiler from refetching
486  * (and from merging fetches), and, more importantly, documents exactly
487  * which pointers are protected by RCU and checks that the pointer is
488  * annotated as __rcu.
489  */
490 #define rcu_dereference_check(p, c) \
491         __rcu_dereference_check((p), (c) || rcu_read_lock_held(), __rcu)
492 
493 /**
494  * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
495  * @p: The pointer to read, prior to dereferencing
496  * @c: The conditions under which the dereference will take place
497  *
498  * This is the RCU-bh counterpart to rcu_dereference_check().
499  */
500 #define rcu_dereference_bh_check(p, c) \
501         __rcu_dereference_check((p), (c) || rcu_read_lock_bh_held(), __rcu)
502 
503 /**
504  * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
505  * @p: The pointer to read, prior to dereferencing
506  * @c: The conditions under which the dereference will take place
507  *
508  * This is the RCU-sched counterpart to rcu_dereference_check().
509  */
510 #define rcu_dereference_sched_check(p, c) \
511         __rcu_dereference_check((p), (c) || rcu_read_lock_sched_held(), \
512                                 __rcu)
513 
514 /*
515  * The tracing infrastructure traces RCU (we want that), but unfortunately
516  * some of the RCU checks causes tracing to lock up the system.
517  *
518  * The no-tracing version of rcu_dereference_raw() must not call
519  * rcu_read_lock_held().
520  */
521 #define rcu_dereference_raw_check(p) __rcu_dereference_check((p), 1, __rcu)
522 
523 /**
524  * rcu_dereference_protected() - fetch RCU pointer when updates prevented
525  * @p: The pointer to read, prior to dereferencing
526  * @c: The conditions under which the dereference will take place
527  *
528  * Return the value of the specified RCU-protected pointer, but omit
529  * the READ_ONCE().  This is useful in cases where update-side locks
530  * prevent the value of the pointer from changing.  Please note that this
531  * primitive does *not* prevent the compiler from repeating this reference
532  * or combining it with other references, so it should not be used without
533  * protection of appropriate locks.
534  *
535  * This function is only for update-side use.  Using this function
536  * when protected only by rcu_read_lock() will result in infrequent
537  * but very ugly failures.
538  */
539 #define rcu_dereference_protected(p, c) \
540         __rcu_dereference_protected((p), (c), __rcu)
541 
542 
543 /**
544  * rcu_dereference() - fetch RCU-protected pointer for dereferencing
545  * @p: The pointer to read, prior to dereferencing
546  *
547  * This is a simple wrapper around rcu_dereference_check().
548  */
549 #define rcu_dereference(p) rcu_dereference_check(p, 0)
550 
551 /**
552  * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
553  * @p: The pointer to read, prior to dereferencing
554  *
555  * Makes rcu_dereference_check() do the dirty work.
556  */
557 #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
558 
559 /**
560  * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
561  * @p: The pointer to read, prior to dereferencing
562  *
563  * Makes rcu_dereference_check() do the dirty work.
564  */
565 #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
566 
567 /**
568  * rcu_pointer_handoff() - Hand off a pointer from RCU to other mechanism
569  * @p: The pointer to hand off
570  *
571  * This is simply an identity function, but it documents where a pointer
572  * is handed off from RCU to some other synchronization mechanism, for
573  * example, reference counting or locking.  In C11, it would map to
574  * kill_dependency().  It could be used as follows::
575  *
576  *      rcu_read_lock();
577  *      p = rcu_dereference(gp);
578  *      long_lived = is_long_lived(p);
579  *      if (long_lived) {
580  *              if (!atomic_inc_not_zero(p->refcnt))
581  *                      long_lived = false;
582  *              else
583  *                      p = rcu_pointer_handoff(p);
584  *      }
585  *      rcu_read_unlock();
586  */
587 #define rcu_pointer_handoff(p) (p)
588 
589 /**
590  * rcu_read_lock() - mark the beginning of an RCU read-side critical section
591  *
592  * When synchronize_rcu() is invoked on one CPU while other CPUs
593  * are within RCU read-side critical sections, then the
594  * synchronize_rcu() is guaranteed to block until after all the other
595  * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
596  * on one CPU while other CPUs are within RCU read-side critical
597  * sections, invocation of the corresponding RCU callback is deferred
598  * until after the all the other CPUs exit their critical sections.
599  *
600  * Note, however, that RCU callbacks are permitted to run concurrently
601  * with new RCU read-side critical sections.  One way that this can happen
602  * is via the following sequence of events: (1) CPU 0 enters an RCU
603  * read-side critical section, (2) CPU 1 invokes call_rcu() to register
604  * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
605  * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
606  * callback is invoked.  This is legal, because the RCU read-side critical
607  * section that was running concurrently with the call_rcu() (and which
608  * therefore might be referencing something that the corresponding RCU
609  * callback would free up) has completed before the corresponding
610  * RCU callback is invoked.
611  *
612  * RCU read-side critical sections may be nested.  Any deferred actions
613  * will be deferred until the outermost RCU read-side critical section
614  * completes.
615  *
616  * You can avoid reading and understanding the next paragraph by
617  * following this rule: don't put anything in an rcu_read_lock() RCU
618  * read-side critical section that would block in a !PREEMPTION kernel.
619  * But if you want the full story, read on!
620  *
621  * In non-preemptible RCU implementations (pure TREE_RCU and TINY_RCU),
622  * it is illegal to block while in an RCU read-side critical section.
623  * In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPTION
624  * kernel builds, RCU read-side critical sections may be preempted,
625  * but explicit blocking is illegal.  Finally, in preemptible RCU
626  * implementations in real-time (with -rt patchset) kernel builds, RCU
627  * read-side critical sections may be preempted and they may also block, but
628  * only when acquiring spinlocks that are subject to priority inheritance.
629  */
630 static __always_inline void rcu_read_lock(void)
631 {
632         __rcu_read_lock();
633         __acquire(RCU);
634         rcu_lock_acquire(&rcu_lock_map);
635         RCU_LOCKDEP_WARN(!rcu_is_watching(),
636                          "rcu_read_lock() used illegally while idle");
637 }
638 
639 /*
640  * So where is rcu_write_lock()?  It does not exist, as there is no
641  * way for writers to lock out RCU readers.  This is a feature, not
642  * a bug -- this property is what provides RCU's performance benefits.
643  * Of course, writers must coordinate with each other.  The normal
644  * spinlock primitives work well for this, but any other technique may be
645  * used as well.  RCU does not care how the writers keep out of each
646  * others' way, as long as they do so.
647  */
648 
649 /**
650  * rcu_read_unlock() - marks the end of an RCU read-side critical section.
651  *
652  * In most situations, rcu_read_unlock() is immune from deadlock.
653  * However, in kernels built with CONFIG_RCU_BOOST, rcu_read_unlock()
654  * is responsible for deboosting, which it does via rt_mutex_unlock().
655  * Unfortunately, this function acquires the scheduler's runqueue and
656  * priority-inheritance spinlocks.  This means that deadlock could result
657  * if the caller of rcu_read_unlock() already holds one of these locks or
658  * any lock that is ever acquired while holding them.
659  *
660  * That said, RCU readers are never priority boosted unless they were
661  * preempted.  Therefore, one way to avoid deadlock is to make sure
662  * that preemption never happens within any RCU read-side critical
663  * section whose outermost rcu_read_unlock() is called with one of
664  * rt_mutex_unlock()'s locks held.  Such preemption can be avoided in
665  * a number of ways, for example, by invoking preempt_disable() before
666  * critical section's outermost rcu_read_lock().
667  *
668  * Given that the set of locks acquired by rt_mutex_unlock() might change
669  * at any time, a somewhat more future-proofed approach is to make sure
670  * that that preemption never happens within any RCU read-side critical
671  * section whose outermost rcu_read_unlock() is called with irqs disabled.
672  * This approach relies on the fact that rt_mutex_unlock() currently only
673  * acquires irq-disabled locks.
674  *
675  * The second of these two approaches is best in most situations,
676  * however, the first approach can also be useful, at least to those
677  * developers willing to keep abreast of the set of locks acquired by
678  * rt_mutex_unlock().
679  *
680  * See rcu_read_lock() for more information.
681  */
682 static inline void rcu_read_unlock(void)
683 {
684         RCU_LOCKDEP_WARN(!rcu_is_watching(),
685                          "rcu_read_unlock() used illegally while idle");
686         __release(RCU);
687         __rcu_read_unlock();
688         rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
689 }
690 
691 /**
692  * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
693  *
694  * This is equivalent of rcu_read_lock(), but also disables softirqs.
695  * Note that anything else that disables softirqs can also serve as
696  * an RCU read-side critical section.
697  *
698  * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
699  * must occur in the same context, for example, it is illegal to invoke
700  * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
701  * was invoked from some other task.
702  */
703 static inline void rcu_read_lock_bh(void)
704 {
705         local_bh_disable();
706         __acquire(RCU_BH);
707         rcu_lock_acquire(&rcu_bh_lock_map);
708         RCU_LOCKDEP_WARN(!rcu_is_watching(),
709                          "rcu_read_lock_bh() used illegally while idle");
710 }
711 
712 /*
713  * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
714  *
715  * See rcu_read_lock_bh() for more information.
716  */
717 static inline void rcu_read_unlock_bh(void)
718 {
719         RCU_LOCKDEP_WARN(!rcu_is_watching(),
720                          "rcu_read_unlock_bh() used illegally while idle");
721         rcu_lock_release(&rcu_bh_lock_map);
722         __release(RCU_BH);
723         local_bh_enable();
724 }
725 
726 /**
727  * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
728  *
729  * This is equivalent of rcu_read_lock(), but disables preemption.
730  * Read-side critical sections can also be introduced by anything else
731  * that disables preemption, including local_irq_disable() and friends.
732  *
733  * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
734  * must occur in the same context, for example, it is illegal to invoke
735  * rcu_read_unlock_sched() from process context if the matching
736  * rcu_read_lock_sched() was invoked from an NMI handler.
737  */
738 static inline void rcu_read_lock_sched(void)
739 {
740         preempt_disable();
741         __acquire(RCU_SCHED);
742         rcu_lock_acquire(&rcu_sched_lock_map);
743         RCU_LOCKDEP_WARN(!rcu_is_watching(),
744                          "rcu_read_lock_sched() used illegally while idle");
745 }
746 
747 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
748 static inline notrace void rcu_read_lock_sched_notrace(void)
749 {
750         preempt_disable_notrace();
751         __acquire(RCU_SCHED);
752 }
753 
754 /*
755  * rcu_read_unlock_sched - marks the end of a RCU-classic critical section
756  *
757  * See rcu_read_lock_sched for more information.
758  */
759 static inline void rcu_read_unlock_sched(void)
760 {
761         RCU_LOCKDEP_WARN(!rcu_is_watching(),
762                          "rcu_read_unlock_sched() used illegally while idle");
763         rcu_lock_release(&rcu_sched_lock_map);
764         __release(RCU_SCHED);
765         preempt_enable();
766 }
767 
768 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
769 static inline notrace void rcu_read_unlock_sched_notrace(void)
770 {
771         __release(RCU_SCHED);
772         preempt_enable_notrace();
773 }
774 
775 /**
776  * RCU_INIT_POINTER() - initialize an RCU protected pointer
777  * @p: The pointer to be initialized.
778  * @v: The value to initialized the pointer to.
779  *
780  * Initialize an RCU-protected pointer in special cases where readers
781  * do not need ordering constraints on the CPU or the compiler.  These
782  * special cases are:
783  *
784  * 1.   This use of RCU_INIT_POINTER() is NULLing out the pointer *or*
785  * 2.   The caller has taken whatever steps are required to prevent
786  *      RCU readers from concurrently accessing this pointer *or*
787  * 3.   The referenced data structure has already been exposed to
788  *      readers either at compile time or via rcu_assign_pointer() *and*
789  *
790  *      a.      You have not made *any* reader-visible changes to
791  *              this structure since then *or*
792  *      b.      It is OK for readers accessing this structure from its
793  *              new location to see the old state of the structure.  (For
794  *              example, the changes were to statistical counters or to
795  *              other state where exact synchronization is not required.)
796  *
797  * Failure to follow these rules governing use of RCU_INIT_POINTER() will
798  * result in impossible-to-diagnose memory corruption.  As in the structures
799  * will look OK in crash dumps, but any concurrent RCU readers might
800  * see pre-initialized values of the referenced data structure.  So
801  * please be very careful how you use RCU_INIT_POINTER()!!!
802  *
803  * If you are creating an RCU-protected linked structure that is accessed
804  * by a single external-to-structure RCU-protected pointer, then you may
805  * use RCU_INIT_POINTER() to initialize the internal RCU-protected
806  * pointers, but you must use rcu_assign_pointer() to initialize the
807  * external-to-structure pointer *after* you have completely initialized
808  * the reader-accessible portions of the linked structure.
809  *
810  * Note that unlike rcu_assign_pointer(), RCU_INIT_POINTER() provides no
811  * ordering guarantees for either the CPU or the compiler.
812  */
813 #define RCU_INIT_POINTER(p, v) \
814         do { \
815                 rcu_check_sparse(p, __rcu); \
816                 WRITE_ONCE(p, RCU_INITIALIZER(v)); \
817         } while (0)
818 
819 /**
820  * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer
821  * @p: The pointer to be initialized.
822  * @v: The value to initialized the pointer to.
823  *
824  * GCC-style initialization for an RCU-protected pointer in a structure field.
825  */
826 #define RCU_POINTER_INITIALIZER(p, v) \
827                 .p = RCU_INITIALIZER(v)
828 
829 /*
830  * Does the specified offset indicate that the corresponding rcu_head
831  * structure can be handled by kvfree_rcu()?
832  */
833 #define __is_kvfree_rcu_offset(offset) ((offset) < 4096)
834 
835 /*
836  * Helper macro for kfree_rcu() to prevent argument-expansion eyestrain.
837  */
838 #define __kvfree_rcu(head, offset) \
839         do { \
840                 BUILD_BUG_ON(!__is_kvfree_rcu_offset(offset)); \
841                 kvfree_call_rcu(head, (rcu_callback_t)(unsigned long)(offset)); \
842         } while (0)
843 
844 /**
845  * kfree_rcu() - kfree an object after a grace period.
846  * @ptr:        pointer to kfree
847  * @rhf:        the name of the struct rcu_head within the type of @ptr.
848  *
849  * Many rcu callbacks functions just call kfree() on the base structure.
850  * These functions are trivial, but their size adds up, and furthermore
851  * when they are used in a kernel module, that module must invoke the
852  * high-latency rcu_barrier() function at module-unload time.
853  *
854  * The kfree_rcu() function handles this issue.  Rather than encoding a
855  * function address in the embedded rcu_head structure, kfree_rcu() instead
856  * encodes the offset of the rcu_head structure within the base structure.
857  * Because the functions are not allowed in the low-order 4096 bytes of
858  * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
859  * If the offset is larger than 4095 bytes, a compile-time error will
860  * be generated in __kvfree_rcu(). If this error is triggered, you can
861  * either fall back to use of call_rcu() or rearrange the structure to
862  * position the rcu_head structure into the first 4096 bytes.
863  *
864  * Note that the allowable offset might decrease in the future, for example,
865  * to allow something like kmem_cache_free_rcu().
866  *
867  * The BUILD_BUG_ON check must not involve any function calls, hence the
868  * checks are done in macros here.
869  */
870 #define kfree_rcu(ptr, rhf)                                             \
871 do {                                                                    \
872         typeof (ptr) ___p = (ptr);                                      \
873                                                                         \
874         if (___p)                                                       \
875                 __kvfree_rcu(&((___p)->rhf), offsetof(typeof(*(ptr)), rhf)); \
876 } while (0)
877 
878 /**
879  * kvfree_rcu() - kvfree an object after a grace period.
880  *
881  * This macro consists of one or two arguments and it is
882  * based on whether an object is head-less or not. If it
883  * has a head then a semantic stays the same as it used
884  * to be before:
885  *
886  *     kvfree_rcu(ptr, rhf);
887  *
888  * where @ptr is a pointer to kvfree(), @rhf is the name
889  * of the rcu_head structure within the type of @ptr.
890  *
891  * When it comes to head-less variant, only one argument
892  * is passed and that is just a pointer which has to be
893  * freed after a grace period. Therefore the semantic is
894  *
895  *     kvfree_rcu(ptr);
896  *
897  * where @ptr is a pointer to kvfree().
898  *
899  * Please note, head-less way of freeing is permitted to
900  * use from a context that has to follow might_sleep()
901  * annotation. Otherwise, please switch and embed the
902  * rcu_head structure within the type of @ptr.
903  */
904 #define kvfree_rcu(...) KVFREE_GET_MACRO(__VA_ARGS__,           \
905         kvfree_rcu_arg_2, kvfree_rcu_arg_1)(__VA_ARGS__)
906 
907 #define KVFREE_GET_MACRO(_1, _2, NAME, ...) NAME
908 #define kvfree_rcu_arg_2(ptr, rhf) kfree_rcu(ptr, rhf)
909 #define kvfree_rcu_arg_1(ptr)                                   \
910 do {                                                            \
911         typeof(ptr) ___p = (ptr);                               \
912                                                                 \
913         if (___p)                                               \
914                 kvfree_call_rcu(NULL, (rcu_callback_t) (___p)); \
915 } while (0)
916 
917 /*
918  * Place this after a lock-acquisition primitive to guarantee that
919  * an UNLOCK+LOCK pair acts as a full barrier.  This guarantee applies
920  * if the UNLOCK and LOCK are executed by the same CPU or if the
921  * UNLOCK and LOCK operate on the same lock variable.
922  */
923 #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE
924 #define smp_mb__after_unlock_lock()     smp_mb()  /* Full ordering for lock. */
925 #else /* #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
926 #define smp_mb__after_unlock_lock()     do { } while (0)
927 #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
928 
929 
930 /* Has the specified rcu_head structure been handed to call_rcu()? */
931 
932 /**
933  * rcu_head_init - Initialize rcu_head for rcu_head_after_call_rcu()
934  * @rhp: The rcu_head structure to initialize.
935  *
936  * If you intend to invoke rcu_head_after_call_rcu() to test whether a
937  * given rcu_head structure has already been passed to call_rcu(), then
938  * you must also invoke this rcu_head_init() function on it just after
939  * allocating that structure.  Calls to this function must not race with
940  * calls to call_rcu(), rcu_head_after_call_rcu(), or callback invocation.
941  */
942 static inline void rcu_head_init(struct rcu_head *rhp)
943 {
944         rhp->func = (rcu_callback_t)~0L;
945 }
946 
947 /**
948  * rcu_head_after_call_rcu - Has this rcu_head been passed to call_rcu()?
949  * @rhp: The rcu_head structure to test.
950  * @f: The function passed to call_rcu() along with @rhp.
951  *
952  * Returns @true if the @rhp has been passed to call_rcu() with @func,
953  * and @false otherwise.  Emits a warning in any other case, including
954  * the case where @rhp has already been invoked after a grace period.
955  * Calls to this function must not race with callback invocation.  One way
956  * to avoid such races is to enclose the call to rcu_head_after_call_rcu()
957  * in an RCU read-side critical section that includes a read-side fetch
958  * of the pointer to the structure containing @rhp.
959  */
960 static inline bool
961 rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
962 {
963         rcu_callback_t func = READ_ONCE(rhp->func);
964 
965         if (func == f)
966                 return true;
967         WARN_ON_ONCE(func != (rcu_callback_t)~0L);
968         return false;
969 }
970 
971 /* kernel/ksysfs.c definitions */
972 extern int rcu_expedited;
973 extern int rcu_normal;
974 
975 #endif /* __LINUX_RCUPDATE_H */
976 

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