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Linux/include/linux/rcupdate.h

  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/cache.h>
 38 #include <linux/spinlock.h>
 39 #include <linux/threads.h>
 40 #include <linux/cpumask.h>
 41 #include <linux/seqlock.h>
 42 #include <linux/lockdep.h>
 43 #include <linux/completion.h>
 44 #include <linux/debugobjects.h>
 45 #include <linux/bug.h>
 46 #include <linux/compiler.h>
 47 #include <asm/barrier.h>
 48 
 49 extern int rcu_expedited; /* for sysctl */
 50 #ifdef CONFIG_RCU_TORTURE_TEST
 51 extern int rcutorture_runnable; /* for sysctl */
 52 #endif /* #ifdef CONFIG_RCU_TORTURE_TEST */
 53 
 54 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU)
 55 void rcutorture_record_test_transition(void);
 56 void rcutorture_record_progress(unsigned long vernum);
 57 void do_trace_rcu_torture_read(const char *rcutorturename,
 58                                struct rcu_head *rhp,
 59                                unsigned long secs,
 60                                unsigned long c_old,
 61                                unsigned long c);
 62 #else
 63 static inline void rcutorture_record_test_transition(void)
 64 {
 65 }
 66 static inline void rcutorture_record_progress(unsigned long vernum)
 67 {
 68 }
 69 #ifdef CONFIG_RCU_TRACE
 70 void do_trace_rcu_torture_read(const char *rcutorturename,
 71                                struct rcu_head *rhp,
 72                                unsigned long secs,
 73                                unsigned long c_old,
 74                                unsigned long c);
 75 #else
 76 #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \
 77         do { } while (0)
 78 #endif
 79 #endif
 80 
 81 #define UINT_CMP_GE(a, b)       (UINT_MAX / 2 >= (a) - (b))
 82 #define UINT_CMP_LT(a, b)       (UINT_MAX / 2 < (a) - (b))
 83 #define ULONG_CMP_GE(a, b)      (ULONG_MAX / 2 >= (a) - (b))
 84 #define ULONG_CMP_LT(a, b)      (ULONG_MAX / 2 < (a) - (b))
 85 #define ulong2long(a)           (*(long *)(&(a)))
 86 
 87 /* Exported common interfaces */
 88 
 89 #ifdef CONFIG_PREEMPT_RCU
 90 
 91 /**
 92  * call_rcu() - Queue an RCU callback for invocation after a grace period.
 93  * @head: structure to be used for queueing the RCU updates.
 94  * @func: actual callback function to be invoked after the grace period
 95  *
 96  * The callback function will be invoked some time after a full grace
 97  * period elapses, in other words after all pre-existing RCU read-side
 98  * critical sections have completed.  However, the callback function
 99  * might well execute concurrently with RCU read-side critical sections
100  * that started after call_rcu() was invoked.  RCU read-side critical
101  * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
102  * and may be nested.
103  *
104  * Note that all CPUs must agree that the grace period extended beyond
105  * all pre-existing RCU read-side critical section.  On systems with more
106  * than one CPU, this means that when "func()" is invoked, each CPU is
107  * guaranteed to have executed a full memory barrier since the end of its
108  * last RCU read-side critical section whose beginning preceded the call
109  * to call_rcu().  It also means that each CPU executing an RCU read-side
110  * critical section that continues beyond the start of "func()" must have
111  * executed a memory barrier after the call_rcu() but before the beginning
112  * of that RCU read-side critical section.  Note that these guarantees
113  * include CPUs that are offline, idle, or executing in user mode, as
114  * well as CPUs that are executing in the kernel.
115  *
116  * Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
117  * resulting RCU callback function "func()", then both CPU A and CPU B are
118  * guaranteed to execute a full memory barrier during the time interval
119  * between the call to call_rcu() and the invocation of "func()" -- even
120  * if CPU A and CPU B are the same CPU (but again only if the system has
121  * more than one CPU).
122  */
123 void call_rcu(struct rcu_head *head,
124               void (*func)(struct rcu_head *head));
125 
126 #else /* #ifdef CONFIG_PREEMPT_RCU */
127 
128 /* In classic RCU, call_rcu() is just call_rcu_sched(). */
129 #define call_rcu        call_rcu_sched
130 
131 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
132 
133 /**
134  * call_rcu_bh() - Queue an RCU for invocation after a quicker grace period.
135  * @head: structure to be used for queueing the RCU updates.
136  * @func: actual callback function to be invoked after the grace period
137  *
138  * The callback function will be invoked some time after a full grace
139  * period elapses, in other words after all currently executing RCU
140  * read-side critical sections have completed. call_rcu_bh() assumes
141  * that the read-side critical sections end on completion of a softirq
142  * handler. This means that read-side critical sections in process
143  * context must not be interrupted by softirqs. This interface is to be
144  * used when most of the read-side critical sections are in softirq context.
145  * RCU read-side critical sections are delimited by :
146  *  - rcu_read_lock() and  rcu_read_unlock(), if in interrupt context.
147  *  OR
148  *  - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context.
149  *  These may be nested.
150  *
151  * See the description of call_rcu() for more detailed information on
152  * memory ordering guarantees.
153  */
154 void call_rcu_bh(struct rcu_head *head,
155                  void (*func)(struct rcu_head *head));
156 
157 /**
158  * call_rcu_sched() - Queue an RCU for invocation after sched grace period.
159  * @head: structure to be used for queueing the RCU updates.
160  * @func: actual callback function to be invoked after the grace period
161  *
162  * The callback function will be invoked some time after a full grace
163  * period elapses, in other words after all currently executing RCU
164  * read-side critical sections have completed. call_rcu_sched() assumes
165  * that the read-side critical sections end on enabling of preemption
166  * or on voluntary preemption.
167  * RCU read-side critical sections are delimited by :
168  *  - rcu_read_lock_sched() and  rcu_read_unlock_sched(),
169  *  OR
170  *  anything that disables preemption.
171  *  These may be nested.
172  *
173  * See the description of call_rcu() for more detailed information on
174  * memory ordering guarantees.
175  */
176 void call_rcu_sched(struct rcu_head *head,
177                     void (*func)(struct rcu_head *rcu));
178 
179 void synchronize_sched(void);
180 
181 #ifdef CONFIG_PREEMPT_RCU
182 
183 void __rcu_read_lock(void);
184 void __rcu_read_unlock(void);
185 void rcu_read_unlock_special(struct task_struct *t);
186 void synchronize_rcu(void);
187 
188 /*
189  * Defined as a macro as it is a very low level header included from
190  * areas that don't even know about current.  This gives the rcu_read_lock()
191  * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
192  * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
193  */
194 #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
195 
196 #else /* #ifdef CONFIG_PREEMPT_RCU */
197 
198 static inline void __rcu_read_lock(void)
199 {
200         preempt_disable();
201 }
202 
203 static inline void __rcu_read_unlock(void)
204 {
205         preempt_enable();
206 }
207 
208 static inline void synchronize_rcu(void)
209 {
210         synchronize_sched();
211 }
212 
213 static inline int rcu_preempt_depth(void)
214 {
215         return 0;
216 }
217 
218 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
219 
220 /* Internal to kernel */
221 void rcu_init(void);
222 void rcu_sched_qs(int cpu);
223 void rcu_bh_qs(int cpu);
224 void rcu_check_callbacks(int cpu, int user);
225 struct notifier_block;
226 void rcu_idle_enter(void);
227 void rcu_idle_exit(void);
228 void rcu_irq_enter(void);
229 void rcu_irq_exit(void);
230 
231 #ifdef CONFIG_RCU_USER_QS
232 void rcu_user_enter(void);
233 void rcu_user_exit(void);
234 #else
235 static inline void rcu_user_enter(void) { }
236 static inline void rcu_user_exit(void) { }
237 static inline void rcu_user_hooks_switch(struct task_struct *prev,
238                                          struct task_struct *next) { }
239 #endif /* CONFIG_RCU_USER_QS */
240 
241 /**
242  * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
243  * @a: Code that RCU needs to pay attention to.
244  *
245  * RCU, RCU-bh, and RCU-sched read-side critical sections are forbidden
246  * in the inner idle loop, that is, between the rcu_idle_enter() and
247  * the rcu_idle_exit() -- RCU will happily ignore any such read-side
248  * critical sections.  However, things like powertop need tracepoints
249  * in the inner idle loop.
250  *
251  * This macro provides the way out:  RCU_NONIDLE(do_something_with_RCU())
252  * will tell RCU that it needs to pay attending, invoke its argument
253  * (in this example, a call to the do_something_with_RCU() function),
254  * and then tell RCU to go back to ignoring this CPU.  It is permissible
255  * to nest RCU_NONIDLE() wrappers, but the nesting level is currently
256  * quite limited.  If deeper nesting is required, it will be necessary
257  * to adjust DYNTICK_TASK_NESTING_VALUE accordingly.
258  */
259 #define RCU_NONIDLE(a) \
260         do { \
261                 rcu_irq_enter(); \
262                 do { a; } while (0); \
263                 rcu_irq_exit(); \
264         } while (0)
265 
266 #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP)
267 bool __rcu_is_watching(void);
268 #endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP) */
269 
270 /*
271  * Infrastructure to implement the synchronize_() primitives in
272  * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
273  */
274 
275 typedef void call_rcu_func_t(struct rcu_head *head,
276                              void (*func)(struct rcu_head *head));
277 void wait_rcu_gp(call_rcu_func_t crf);
278 
279 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU)
280 #include <linux/rcutree.h>
281 #elif defined(CONFIG_TINY_RCU)
282 #include <linux/rcutiny.h>
283 #else
284 #error "Unknown RCU implementation specified to kernel configuration"
285 #endif
286 
287 /*
288  * init_rcu_head_on_stack()/destroy_rcu_head_on_stack() are needed for dynamic
289  * initialization and destruction of rcu_head on the stack. rcu_head structures
290  * allocated dynamically in the heap or defined statically don't need any
291  * initialization.
292  */
293 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
294 void init_rcu_head_on_stack(struct rcu_head *head);
295 void destroy_rcu_head_on_stack(struct rcu_head *head);
296 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
297 static inline void init_rcu_head_on_stack(struct rcu_head *head)
298 {
299 }
300 
301 static inline void destroy_rcu_head_on_stack(struct rcu_head *head)
302 {
303 }
304 #endif  /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
305 
306 #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
307 bool rcu_lockdep_current_cpu_online(void);
308 #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
309 static inline bool rcu_lockdep_current_cpu_online(void)
310 {
311         return 1;
312 }
313 #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
314 
315 #ifdef CONFIG_DEBUG_LOCK_ALLOC
316 
317 static inline void rcu_lock_acquire(struct lockdep_map *map)
318 {
319         lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
320 }
321 
322 static inline void rcu_lock_release(struct lockdep_map *map)
323 {
324         lock_release(map, 1, _THIS_IP_);
325 }
326 
327 extern struct lockdep_map rcu_lock_map;
328 extern struct lockdep_map rcu_bh_lock_map;
329 extern struct lockdep_map rcu_sched_lock_map;
330 extern struct lockdep_map rcu_callback_map;
331 extern int debug_lockdep_rcu_enabled(void);
332 
333 /**
334  * rcu_read_lock_held() - might we be in RCU read-side critical section?
335  *
336  * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU
337  * read-side critical section.  In absence of CONFIG_DEBUG_LOCK_ALLOC,
338  * this assumes we are in an RCU read-side critical section unless it can
339  * prove otherwise.  This is useful for debug checks in functions that
340  * require that they be called within an RCU read-side critical section.
341  *
342  * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
343  * and while lockdep is disabled.
344  *
345  * Note that rcu_read_lock() and the matching rcu_read_unlock() must
346  * occur in the same context, for example, it is illegal to invoke
347  * rcu_read_unlock() in process context if the matching rcu_read_lock()
348  * was invoked from within an irq handler.
349  *
350  * Note that rcu_read_lock() is disallowed if the CPU is either idle or
351  * offline from an RCU perspective, so check for those as well.
352  */
353 static inline int rcu_read_lock_held(void)
354 {
355         if (!debug_lockdep_rcu_enabled())
356                 return 1;
357         if (!rcu_is_watching())
358                 return 0;
359         if (!rcu_lockdep_current_cpu_online())
360                 return 0;
361         return lock_is_held(&rcu_lock_map);
362 }
363 
364 /*
365  * rcu_read_lock_bh_held() is defined out of line to avoid #include-file
366  * hell.
367  */
368 int rcu_read_lock_bh_held(void);
369 
370 /**
371  * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
372  *
373  * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
374  * RCU-sched read-side critical section.  In absence of
375  * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
376  * critical section unless it can prove otherwise.  Note that disabling
377  * of preemption (including disabling irqs) counts as an RCU-sched
378  * read-side critical section.  This is useful for debug checks in functions
379  * that required that they be called within an RCU-sched read-side
380  * critical section.
381  *
382  * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
383  * and while lockdep is disabled.
384  *
385  * Note that if the CPU is in the idle loop from an RCU point of
386  * view (ie: that we are in the section between rcu_idle_enter() and
387  * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU
388  * did an rcu_read_lock().  The reason for this is that RCU ignores CPUs
389  * that are in such a section, considering these as in extended quiescent
390  * state, so such a CPU is effectively never in an RCU read-side critical
391  * section regardless of what RCU primitives it invokes.  This state of
392  * affairs is required --- we need to keep an RCU-free window in idle
393  * where the CPU may possibly enter into low power mode. This way we can
394  * notice an extended quiescent state to other CPUs that started a grace
395  * period. Otherwise we would delay any grace period as long as we run in
396  * the idle task.
397  *
398  * Similarly, we avoid claiming an SRCU read lock held if the current
399  * CPU is offline.
400  */
401 #ifdef CONFIG_PREEMPT_COUNT
402 static inline int rcu_read_lock_sched_held(void)
403 {
404         int lockdep_opinion = 0;
405 
406         if (!debug_lockdep_rcu_enabled())
407                 return 1;
408         if (!rcu_is_watching())
409                 return 0;
410         if (!rcu_lockdep_current_cpu_online())
411                 return 0;
412         if (debug_locks)
413                 lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
414         return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
415 }
416 #else /* #ifdef CONFIG_PREEMPT_COUNT */
417 static inline int rcu_read_lock_sched_held(void)
418 {
419         return 1;
420 }
421 #endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
422 
423 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
424 
425 # define rcu_lock_acquire(a)            do { } while (0)
426 # define rcu_lock_release(a)            do { } while (0)
427 
428 static inline int rcu_read_lock_held(void)
429 {
430         return 1;
431 }
432 
433 static inline int rcu_read_lock_bh_held(void)
434 {
435         return 1;
436 }
437 
438 #ifdef CONFIG_PREEMPT_COUNT
439 static inline int rcu_read_lock_sched_held(void)
440 {
441         return preempt_count() != 0 || irqs_disabled();
442 }
443 #else /* #ifdef CONFIG_PREEMPT_COUNT */
444 static inline int rcu_read_lock_sched_held(void)
445 {
446         return 1;
447 }
448 #endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
449 
450 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
451 
452 #ifdef CONFIG_PROVE_RCU
453 
454 /**
455  * rcu_lockdep_assert - emit lockdep splat if specified condition not met
456  * @c: condition to check
457  * @s: informative message
458  */
459 #define rcu_lockdep_assert(c, s)                                        \
460         do {                                                            \
461                 static bool __section(.data.unlikely) __warned;         \
462                 if (debug_lockdep_rcu_enabled() && !__warned && !(c)) { \
463                         __warned = true;                                \
464                         lockdep_rcu_suspicious(__FILE__, __LINE__, s);  \
465                 }                                                       \
466         } while (0)
467 
468 #if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
469 static inline void rcu_preempt_sleep_check(void)
470 {
471         rcu_lockdep_assert(!lock_is_held(&rcu_lock_map),
472                            "Illegal context switch in RCU read-side critical section");
473 }
474 #else /* #ifdef CONFIG_PROVE_RCU */
475 static inline void rcu_preempt_sleep_check(void)
476 {
477 }
478 #endif /* #else #ifdef CONFIG_PROVE_RCU */
479 
480 #define rcu_sleep_check()                                               \
481         do {                                                            \
482                 rcu_preempt_sleep_check();                              \
483                 rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map),     \
484                                    "Illegal context switch in RCU-bh read-side critical section"); \
485                 rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map),  \
486                                    "Illegal context switch in RCU-sched read-side critical section"); \
487         } while (0)
488 
489 #else /* #ifdef CONFIG_PROVE_RCU */
490 
491 #define rcu_lockdep_assert(c, s) do { } while (0)
492 #define rcu_sleep_check() do { } while (0)
493 
494 #endif /* #else #ifdef CONFIG_PROVE_RCU */
495 
496 /*
497  * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
498  * and rcu_assign_pointer().  Some of these could be folded into their
499  * callers, but they are left separate in order to ease introduction of
500  * multiple flavors of pointers to match the multiple flavors of RCU
501  * (e.g., __rcu_bh, * __rcu_sched, and __srcu), should this make sense in
502  * the future.
503  */
504 
505 #ifdef __CHECKER__
506 #define rcu_dereference_sparse(p, space) \
507         ((void)(((typeof(*p) space *)p) == p))
508 #else /* #ifdef __CHECKER__ */
509 #define rcu_dereference_sparse(p, space)
510 #endif /* #else #ifdef __CHECKER__ */
511 
512 #define __rcu_access_pointer(p, space) \
513 ({ \
514         typeof(*p) *_________p1 = (typeof(*p) *__force)ACCESS_ONCE(p); \
515         rcu_dereference_sparse(p, space); \
516         ((typeof(*p) __force __kernel *)(_________p1)); \
517 })
518 #define __rcu_dereference_check(p, c, space) \
519 ({ \
520         typeof(*p) *_________p1 = (typeof(*p) *__force)ACCESS_ONCE(p); \
521         rcu_lockdep_assert(c, "suspicious rcu_dereference_check() usage"); \
522         rcu_dereference_sparse(p, space); \
523         smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
524         ((typeof(*p) __force __kernel *)(_________p1)); \
525 })
526 #define __rcu_dereference_protected(p, c, space) \
527 ({ \
528         rcu_lockdep_assert(c, "suspicious rcu_dereference_protected() usage"); \
529         rcu_dereference_sparse(p, space); \
530         ((typeof(*p) __force __kernel *)(p)); \
531 })
532 
533 #define __rcu_access_index(p, space) \
534 ({ \
535         typeof(p) _________p1 = ACCESS_ONCE(p); \
536         rcu_dereference_sparse(p, space); \
537         (_________p1); \
538 })
539 #define __rcu_dereference_index_check(p, c) \
540 ({ \
541         typeof(p) _________p1 = ACCESS_ONCE(p); \
542         rcu_lockdep_assert(c, \
543                            "suspicious rcu_dereference_index_check() usage"); \
544         smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
545         (_________p1); \
546 })
547 
548 /**
549  * RCU_INITIALIZER() - statically initialize an RCU-protected global variable
550  * @v: The value to statically initialize with.
551  */
552 #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
553 
554 /**
555  * rcu_assign_pointer() - assign to RCU-protected pointer
556  * @p: pointer to assign to
557  * @v: value to assign (publish)
558  *
559  * Assigns the specified value to the specified RCU-protected
560  * pointer, ensuring that any concurrent RCU readers will see
561  * any prior initialization.
562  *
563  * Inserts memory barriers on architectures that require them
564  * (which is most of them), and also prevents the compiler from
565  * reordering the code that initializes the structure after the pointer
566  * assignment.  More importantly, this call documents which pointers
567  * will be dereferenced by RCU read-side code.
568  *
569  * In some special cases, you may use RCU_INIT_POINTER() instead
570  * of rcu_assign_pointer().  RCU_INIT_POINTER() is a bit faster due
571  * to the fact that it does not constrain either the CPU or the compiler.
572  * That said, using RCU_INIT_POINTER() when you should have used
573  * rcu_assign_pointer() is a very bad thing that results in
574  * impossible-to-diagnose memory corruption.  So please be careful.
575  * See the RCU_INIT_POINTER() comment header for details.
576  *
577  * Note that rcu_assign_pointer() evaluates each of its arguments only
578  * once, appearances notwithstanding.  One of the "extra" evaluations
579  * is in typeof() and the other visible only to sparse (__CHECKER__),
580  * neither of which actually execute the argument.  As with most cpp
581  * macros, this execute-arguments-only-once property is important, so
582  * please be careful when making changes to rcu_assign_pointer() and the
583  * other macros that it invokes.
584  */
585 #define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v))
586 
587 /**
588  * rcu_access_pointer() - fetch RCU pointer with no dereferencing
589  * @p: The pointer to read
590  *
591  * Return the value of the specified RCU-protected pointer, but omit the
592  * smp_read_barrier_depends() and keep the ACCESS_ONCE().  This is useful
593  * when the value of this pointer is accessed, but the pointer is not
594  * dereferenced, for example, when testing an RCU-protected pointer against
595  * NULL.  Although rcu_access_pointer() may also be used in cases where
596  * update-side locks prevent the value of the pointer from changing, you
597  * should instead use rcu_dereference_protected() for this use case.
598  *
599  * It is also permissible to use rcu_access_pointer() when read-side
600  * access to the pointer was removed at least one grace period ago, as
601  * is the case in the context of the RCU callback that is freeing up
602  * the data, or after a synchronize_rcu() returns.  This can be useful
603  * when tearing down multi-linked structures after a grace period
604  * has elapsed.
605  */
606 #define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
607 
608 /**
609  * rcu_dereference_check() - rcu_dereference with debug checking
610  * @p: The pointer to read, prior to dereferencing
611  * @c: The conditions under which the dereference will take place
612  *
613  * Do an rcu_dereference(), but check that the conditions under which the
614  * dereference will take place are correct.  Typically the conditions
615  * indicate the various locking conditions that should be held at that
616  * point.  The check should return true if the conditions are satisfied.
617  * An implicit check for being in an RCU read-side critical section
618  * (rcu_read_lock()) is included.
619  *
620  * For example:
621  *
622  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
623  *
624  * could be used to indicate to lockdep that foo->bar may only be dereferenced
625  * if either rcu_read_lock() is held, or that the lock required to replace
626  * the bar struct at foo->bar is held.
627  *
628  * Note that the list of conditions may also include indications of when a lock
629  * need not be held, for example during initialisation or destruction of the
630  * target struct:
631  *
632  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
633  *                                            atomic_read(&foo->usage) == 0);
634  *
635  * Inserts memory barriers on architectures that require them
636  * (currently only the Alpha), prevents the compiler from refetching
637  * (and from merging fetches), and, more importantly, documents exactly
638  * which pointers are protected by RCU and checks that the pointer is
639  * annotated as __rcu.
640  */
641 #define rcu_dereference_check(p, c) \
642         __rcu_dereference_check((p), rcu_read_lock_held() || (c), __rcu)
643 
644 /**
645  * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
646  * @p: The pointer to read, prior to dereferencing
647  * @c: The conditions under which the dereference will take place
648  *
649  * This is the RCU-bh counterpart to rcu_dereference_check().
650  */
651 #define rcu_dereference_bh_check(p, c) \
652         __rcu_dereference_check((p), rcu_read_lock_bh_held() || (c), __rcu)
653 
654 /**
655  * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
656  * @p: The pointer to read, prior to dereferencing
657  * @c: The conditions under which the dereference will take place
658  *
659  * This is the RCU-sched counterpart to rcu_dereference_check().
660  */
661 #define rcu_dereference_sched_check(p, c) \
662         __rcu_dereference_check((p), rcu_read_lock_sched_held() || (c), \
663                                 __rcu)
664 
665 #define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ needed? @@@*/
666 
667 /*
668  * The tracing infrastructure traces RCU (we want that), but unfortunately
669  * some of the RCU checks causes tracing to lock up the system.
670  *
671  * The tracing version of rcu_dereference_raw() must not call
672  * rcu_read_lock_held().
673  */
674 #define rcu_dereference_raw_notrace(p) __rcu_dereference_check((p), 1, __rcu)
675 
676 /**
677  * rcu_access_index() - fetch RCU index with no dereferencing
678  * @p: The index to read
679  *
680  * Return the value of the specified RCU-protected index, but omit the
681  * smp_read_barrier_depends() and keep the ACCESS_ONCE().  This is useful
682  * when the value of this index is accessed, but the index is not
683  * dereferenced, for example, when testing an RCU-protected index against
684  * -1.  Although rcu_access_index() may also be used in cases where
685  * update-side locks prevent the value of the index from changing, you
686  * should instead use rcu_dereference_index_protected() for this use case.
687  */
688 #define rcu_access_index(p) __rcu_access_index((p), __rcu)
689 
690 /**
691  * rcu_dereference_index_check() - rcu_dereference for indices with debug checking
692  * @p: The pointer to read, prior to dereferencing
693  * @c: The conditions under which the dereference will take place
694  *
695  * Similar to rcu_dereference_check(), but omits the sparse checking.
696  * This allows rcu_dereference_index_check() to be used on integers,
697  * which can then be used as array indices.  Attempting to use
698  * rcu_dereference_check() on an integer will give compiler warnings
699  * because the sparse address-space mechanism relies on dereferencing
700  * the RCU-protected pointer.  Dereferencing integers is not something
701  * that even gcc will put up with.
702  *
703  * Note that this function does not implicitly check for RCU read-side
704  * critical sections.  If this function gains lots of uses, it might
705  * make sense to provide versions for each flavor of RCU, but it does
706  * not make sense as of early 2010.
707  */
708 #define rcu_dereference_index_check(p, c) \
709         __rcu_dereference_index_check((p), (c))
710 
711 /**
712  * rcu_dereference_protected() - fetch RCU pointer when updates prevented
713  * @p: The pointer to read, prior to dereferencing
714  * @c: The conditions under which the dereference will take place
715  *
716  * Return the value of the specified RCU-protected pointer, but omit
717  * both the smp_read_barrier_depends() and the ACCESS_ONCE().  This
718  * is useful in cases where update-side locks prevent the value of the
719  * pointer from changing.  Please note that this primitive does -not-
720  * prevent the compiler from repeating this reference or combining it
721  * with other references, so it should not be used without protection
722  * of appropriate locks.
723  *
724  * This function is only for update-side use.  Using this function
725  * when protected only by rcu_read_lock() will result in infrequent
726  * but very ugly failures.
727  */
728 #define rcu_dereference_protected(p, c) \
729         __rcu_dereference_protected((p), (c), __rcu)
730 
731 
732 /**
733  * rcu_dereference() - fetch RCU-protected pointer for dereferencing
734  * @p: The pointer to read, prior to dereferencing
735  *
736  * This is a simple wrapper around rcu_dereference_check().
737  */
738 #define rcu_dereference(p) rcu_dereference_check(p, 0)
739 
740 /**
741  * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
742  * @p: The pointer to read, prior to dereferencing
743  *
744  * Makes rcu_dereference_check() do the dirty work.
745  */
746 #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
747 
748 /**
749  * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
750  * @p: The pointer to read, prior to dereferencing
751  *
752  * Makes rcu_dereference_check() do the dirty work.
753  */
754 #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
755 
756 /**
757  * rcu_read_lock() - mark the beginning of an RCU read-side critical section
758  *
759  * When synchronize_rcu() is invoked on one CPU while other CPUs
760  * are within RCU read-side critical sections, then the
761  * synchronize_rcu() is guaranteed to block until after all the other
762  * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
763  * on one CPU while other CPUs are within RCU read-side critical
764  * sections, invocation of the corresponding RCU callback is deferred
765  * until after the all the other CPUs exit their critical sections.
766  *
767  * Note, however, that RCU callbacks are permitted to run concurrently
768  * with new RCU read-side critical sections.  One way that this can happen
769  * is via the following sequence of events: (1) CPU 0 enters an RCU
770  * read-side critical section, (2) CPU 1 invokes call_rcu() to register
771  * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
772  * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
773  * callback is invoked.  This is legal, because the RCU read-side critical
774  * section that was running concurrently with the call_rcu() (and which
775  * therefore might be referencing something that the corresponding RCU
776  * callback would free up) has completed before the corresponding
777  * RCU callback is invoked.
778  *
779  * RCU read-side critical sections may be nested.  Any deferred actions
780  * will be deferred until the outermost RCU read-side critical section
781  * completes.
782  *
783  * You can avoid reading and understanding the next paragraph by
784  * following this rule: don't put anything in an rcu_read_lock() RCU
785  * read-side critical section that would block in a !PREEMPT kernel.
786  * But if you want the full story, read on!
787  *
788  * In non-preemptible RCU implementations (TREE_RCU and TINY_RCU), it
789  * is illegal to block while in an RCU read-side critical section.  In
790  * preemptible RCU implementations (TREE_PREEMPT_RCU and TINY_PREEMPT_RCU)
791  * in CONFIG_PREEMPT kernel builds, RCU read-side critical sections may
792  * be preempted, but explicit blocking is illegal.  Finally, in preemptible
793  * RCU implementations in real-time (with -rt patchset) kernel builds,
794  * RCU read-side critical sections may be preempted and they may also
795  * block, but only when acquiring spinlocks that are subject to priority
796  * inheritance.
797  */
798 static inline void rcu_read_lock(void)
799 {
800         __rcu_read_lock();
801         __acquire(RCU);
802         rcu_lock_acquire(&rcu_lock_map);
803         rcu_lockdep_assert(rcu_is_watching(),
804                            "rcu_read_lock() used illegally while idle");
805 }
806 
807 /*
808  * So where is rcu_write_lock()?  It does not exist, as there is no
809  * way for writers to lock out RCU readers.  This is a feature, not
810  * a bug -- this property is what provides RCU's performance benefits.
811  * Of course, writers must coordinate with each other.  The normal
812  * spinlock primitives work well for this, but any other technique may be
813  * used as well.  RCU does not care how the writers keep out of each
814  * others' way, as long as they do so.
815  */
816 
817 /**
818  * rcu_read_unlock() - marks the end of an RCU read-side critical section.
819  *
820  * See rcu_read_lock() for more information.
821  */
822 static inline void rcu_read_unlock(void)
823 {
824         rcu_lockdep_assert(rcu_is_watching(),
825                            "rcu_read_unlock() used illegally while idle");
826         rcu_lock_release(&rcu_lock_map);
827         __release(RCU);
828         __rcu_read_unlock();
829 }
830 
831 /**
832  * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
833  *
834  * This is equivalent of rcu_read_lock(), but to be used when updates
835  * are being done using call_rcu_bh() or synchronize_rcu_bh(). Since
836  * both call_rcu_bh() and synchronize_rcu_bh() consider completion of a
837  * softirq handler to be a quiescent state, a process in RCU read-side
838  * critical section must be protected by disabling softirqs. Read-side
839  * critical sections in interrupt context can use just rcu_read_lock(),
840  * though this should at least be commented to avoid confusing people
841  * reading the code.
842  *
843  * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
844  * must occur in the same context, for example, it is illegal to invoke
845  * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
846  * was invoked from some other task.
847  */
848 static inline void rcu_read_lock_bh(void)
849 {
850         local_bh_disable();
851         __acquire(RCU_BH);
852         rcu_lock_acquire(&rcu_bh_lock_map);
853         rcu_lockdep_assert(rcu_is_watching(),
854                            "rcu_read_lock_bh() used illegally while idle");
855 }
856 
857 /*
858  * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
859  *
860  * See rcu_read_lock_bh() for more information.
861  */
862 static inline void rcu_read_unlock_bh(void)
863 {
864         rcu_lockdep_assert(rcu_is_watching(),
865                            "rcu_read_unlock_bh() used illegally while idle");
866         rcu_lock_release(&rcu_bh_lock_map);
867         __release(RCU_BH);
868         local_bh_enable();
869 }
870 
871 /**
872  * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
873  *
874  * This is equivalent of rcu_read_lock(), but to be used when updates
875  * are being done using call_rcu_sched() or synchronize_rcu_sched().
876  * Read-side critical sections can also be introduced by anything that
877  * disables preemption, including local_irq_disable() and friends.
878  *
879  * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
880  * must occur in the same context, for example, it is illegal to invoke
881  * rcu_read_unlock_sched() from process context if the matching
882  * rcu_read_lock_sched() was invoked from an NMI handler.
883  */
884 static inline void rcu_read_lock_sched(void)
885 {
886         preempt_disable();
887         __acquire(RCU_SCHED);
888         rcu_lock_acquire(&rcu_sched_lock_map);
889         rcu_lockdep_assert(rcu_is_watching(),
890                            "rcu_read_lock_sched() used illegally while idle");
891 }
892 
893 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
894 static inline notrace void rcu_read_lock_sched_notrace(void)
895 {
896         preempt_disable_notrace();
897         __acquire(RCU_SCHED);
898 }
899 
900 /*
901  * rcu_read_unlock_sched - marks the end of a RCU-classic critical section
902  *
903  * See rcu_read_lock_sched for more information.
904  */
905 static inline void rcu_read_unlock_sched(void)
906 {
907         rcu_lockdep_assert(rcu_is_watching(),
908                            "rcu_read_unlock_sched() used illegally while idle");
909         rcu_lock_release(&rcu_sched_lock_map);
910         __release(RCU_SCHED);
911         preempt_enable();
912 }
913 
914 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
915 static inline notrace void rcu_read_unlock_sched_notrace(void)
916 {
917         __release(RCU_SCHED);
918         preempt_enable_notrace();
919 }
920 
921 /**
922  * RCU_INIT_POINTER() - initialize an RCU protected pointer
923  *
924  * Initialize an RCU-protected pointer in special cases where readers
925  * do not need ordering constraints on the CPU or the compiler.  These
926  * special cases are:
927  *
928  * 1.   This use of RCU_INIT_POINTER() is NULLing out the pointer -or-
929  * 2.   The caller has taken whatever steps are required to prevent
930  *      RCU readers from concurrently accessing this pointer -or-
931  * 3.   The referenced data structure has already been exposed to
932  *      readers either at compile time or via rcu_assign_pointer() -and-
933  *      a.      You have not made -any- reader-visible changes to
934  *              this structure since then -or-
935  *      b.      It is OK for readers accessing this structure from its
936  *              new location to see the old state of the structure.  (For
937  *              example, the changes were to statistical counters or to
938  *              other state where exact synchronization is not required.)
939  *
940  * Failure to follow these rules governing use of RCU_INIT_POINTER() will
941  * result in impossible-to-diagnose memory corruption.  As in the structures
942  * will look OK in crash dumps, but any concurrent RCU readers might
943  * see pre-initialized values of the referenced data structure.  So
944  * please be very careful how you use RCU_INIT_POINTER()!!!
945  *
946  * If you are creating an RCU-protected linked structure that is accessed
947  * by a single external-to-structure RCU-protected pointer, then you may
948  * use RCU_INIT_POINTER() to initialize the internal RCU-protected
949  * pointers, but you must use rcu_assign_pointer() to initialize the
950  * external-to-structure pointer -after- you have completely initialized
951  * the reader-accessible portions of the linked structure.
952  */
953 #define RCU_INIT_POINTER(p, v) \
954         do { \
955                 p = RCU_INITIALIZER(v); \
956         } while (0)
957 
958 /**
959  * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer
960  *
961  * GCC-style initialization for an RCU-protected pointer in a structure field.
962  */
963 #define RCU_POINTER_INITIALIZER(p, v) \
964                 .p = RCU_INITIALIZER(v)
965 
966 /*
967  * Does the specified offset indicate that the corresponding rcu_head
968  * structure can be handled by kfree_rcu()?
969  */
970 #define __is_kfree_rcu_offset(offset) ((offset) < 4096)
971 
972 /*
973  * Helper macro for kfree_rcu() to prevent argument-expansion eyestrain.
974  */
975 #define __kfree_rcu(head, offset) \
976         do { \
977                 BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \
978                 kfree_call_rcu(head, (void (*)(struct rcu_head *))(unsigned long)(offset)); \
979         } while (0)
980 
981 /**
982  * kfree_rcu() - kfree an object after a grace period.
983  * @ptr:        pointer to kfree
984  * @rcu_head:   the name of the struct rcu_head within the type of @ptr.
985  *
986  * Many rcu callbacks functions just call kfree() on the base structure.
987  * These functions are trivial, but their size adds up, and furthermore
988  * when they are used in a kernel module, that module must invoke the
989  * high-latency rcu_barrier() function at module-unload time.
990  *
991  * The kfree_rcu() function handles this issue.  Rather than encoding a
992  * function address in the embedded rcu_head structure, kfree_rcu() instead
993  * encodes the offset of the rcu_head structure within the base structure.
994  * Because the functions are not allowed in the low-order 4096 bytes of
995  * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
996  * If the offset is larger than 4095 bytes, a compile-time error will
997  * be generated in __kfree_rcu().  If this error is triggered, you can
998  * either fall back to use of call_rcu() or rearrange the structure to
999  * position the rcu_head structure into the first 4096 bytes.
1000  *
1001  * Note that the allowable offset might decrease in the future, for example,
1002  * to allow something like kmem_cache_free_rcu().
1003  *
1004  * The BUILD_BUG_ON check must not involve any function calls, hence the
1005  * checks are done in macros here.
1006  */
1007 #define kfree_rcu(ptr, rcu_head)                                        \
1008         __kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
1009 
1010 #if defined(CONFIG_TINY_RCU) || defined(CONFIG_RCU_NOCB_CPU_ALL)
1011 static inline int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
1012 {
1013         *delta_jiffies = ULONG_MAX;
1014         return 0;
1015 }
1016 #endif /* #if defined(CONFIG_TINY_RCU) || defined(CONFIG_RCU_NOCB_CPU_ALL) */
1017 
1018 #if defined(CONFIG_RCU_NOCB_CPU_ALL)
1019 static inline bool rcu_is_nocb_cpu(int cpu) { return true; }
1020 #elif defined(CONFIG_RCU_NOCB_CPU)
1021 bool rcu_is_nocb_cpu(int cpu);
1022 #else
1023 static inline bool rcu_is_nocb_cpu(int cpu) { return false; }
1024 #endif
1025 
1026 
1027 /* Only for use by adaptive-ticks code. */
1028 #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
1029 bool rcu_sys_is_idle(void);
1030 void rcu_sysidle_force_exit(void);
1031 #else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
1032 
1033 static inline bool rcu_sys_is_idle(void)
1034 {
1035         return false;
1036 }
1037 
1038 static inline void rcu_sysidle_force_exit(void)
1039 {
1040 }
1041 
1042 #endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
1043 
1044 
1045 #endif /* __LINUX_RCUPDATE_H */
1046 

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