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, write to the Free Software
  16  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  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/cache.h>
  37 #include <linux/spinlock.h>
  38 #include <linux/threads.h>
  39 #include <linux/cpumask.h>
  40 #include <linux/seqlock.h>
  41 #include <linux/lockdep.h>
  42 #include <linux/completion.h>
  43 #include <linux/debugobjects.h>
  44 #include <linux/compiler.h>
  45
  46 #ifdef CONFIG_RCU_TORTURE_TEST
  47 extern int rcutorture_runnable; /* for sysctl */
  48 #endif /* #ifdef CONFIG_RCU_TORTURE_TEST */
  49
  50 #define UINT_CMP_GE(a, b)       (UINT_MAX / 2 >= (a) - (b))
  51 #define UINT_CMP_LT(a, b)       (UINT_MAX / 2 < (a) - (b))
  52 #define ULONG_CMP_GE(a, b)      (ULONG_MAX / 2 >= (a) - (b))
  53 #define ULONG_CMP_LT(a, b)      (ULONG_MAX / 2 < (a) - (b))
  54
  55 /**
  56  * struct rcu_head - callback structure for use with RCU
  57  * @next: next update requests in a list
  58  * @func: actual update function to call after the grace period.
  59  */
  60 struct rcu_head {
  61         struct rcu_head *next;
  62         void (*func)(struct rcu_head *head);
  63 };
  64
  65 /* Exported common interfaces */
  66 extern void call_rcu_sched(struct rcu_head *head,
  67                            void (*func)(struct rcu_head *rcu));
  68 extern void synchronize_sched(void);
  69 extern void rcu_barrier_bh(void);
  70 extern void rcu_barrier_sched(void);
  71 extern int sched_expedited_torture_stats(char *page);
  72
  73 static inline void __rcu_read_lock_bh(void)
  74 {
  75         local_bh_disable();
  76 }
  77
  78 static inline void __rcu_read_unlock_bh(void)
  79 {
  80         local_bh_enable();
  81 }
  82
  83 #ifdef CONFIG_PREEMPT_RCU
  84
  85 extern void __rcu_read_lock(void);
  86 extern void __rcu_read_unlock(void);
  87 void synchronize_rcu(void);
  88
  89 /*
  90  * Defined as a macro as it is a very low level header included from
  91  * areas that don't even know about current.  This gives the rcu_read_lock()
  92  * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
  93  * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
  94  */
  95 #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
  96
  97 #else /* #ifdef CONFIG_PREEMPT_RCU */
  98
  99 static inline void __rcu_read_lock(void)
 100 {
 101         preempt_disable();
 102 }
 103
 104 static inline void __rcu_read_unlock(void)
 105 {
 106         preempt_enable();
 107 }
 108
 109 static inline void synchronize_rcu(void)
 110 {
 111         synchronize_sched();
 112 }
 113
 114 static inline int rcu_preempt_depth(void)
 115 {
 116         return 0;
 117 }
 118
 119 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 120
 121 /* Internal to kernel */
 122 extern void rcu_sched_qs(int cpu);
 123 extern void rcu_bh_qs(int cpu);
 124 extern void rcu_check_callbacks(int cpu, int user);
 125 struct notifier_block;
 126
 127 #ifdef CONFIG_NO_HZ
 128
 129 extern void rcu_enter_nohz(void);
 130 extern void rcu_exit_nohz(void);
 131
 132 #else /* #ifdef CONFIG_NO_HZ */
 133
 134 static inline void rcu_enter_nohz(void)
 135 {
 136 }
 137
 138 static inline void rcu_exit_nohz(void)
 139 {
 140 }
 141
 142 #endif /* #else #ifdef CONFIG_NO_HZ */
 143
 144 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU)
 145 #include <linux/rcutree.h>
 146 #elif defined(CONFIG_TINY_RCU) || defined(CONFIG_TINY_PREEMPT_RCU)
 147 #include <linux/rcutiny.h>
 148 #else
 149 #error "Unknown RCU implementation specified to kernel configuration"
 150 #endif
 151
 152 /*
 153  * init_rcu_head_on_stack()/destroy_rcu_head_on_stack() are needed for dynamic
 154  * initialization and destruction of rcu_head on the stack. rcu_head structures
 155  * allocated dynamically in the heap or defined statically don't need any
 156  * initialization.
 157  */
 158 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
 159 extern void init_rcu_head_on_stack(struct rcu_head *head);
 160 extern void destroy_rcu_head_on_stack(struct rcu_head *head);
 161 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
 162 static inline void init_rcu_head_on_stack(struct rcu_head *head)
 163 {
 164 }
 165
 166 static inline void destroy_rcu_head_on_stack(struct rcu_head *head)
 167 {
 168 }
 169 #endif  /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
 170
 171 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 172
 173 extern struct lockdep_map rcu_lock_map;
 174 # define rcu_read_acquire() \
 175                 lock_acquire(&rcu_lock_map, 0, 0, 2, 1, NULL, _THIS_IP_)
 176 # define rcu_read_release()     lock_release(&rcu_lock_map, 1, _THIS_IP_)
 177
 178 extern struct lockdep_map rcu_bh_lock_map;
 179 # define rcu_read_acquire_bh() \
 180                 lock_acquire(&rcu_bh_lock_map, 0, 0, 2, 1, NULL, _THIS_IP_)
 181 # define rcu_read_release_bh()  lock_release(&rcu_bh_lock_map, 1, _THIS_IP_)
 182
 183 extern struct lockdep_map rcu_sched_lock_map;
 184 # define rcu_read_acquire_sched() \
 185                 lock_acquire(&rcu_sched_lock_map, 0, 0, 2, 1, NULL, _THIS_IP_)
 186 # define rcu_read_release_sched() \
 187                 lock_release(&rcu_sched_lock_map, 1, _THIS_IP_)
 188
 189 extern int debug_lockdep_rcu_enabled(void);
 190
 191 /**
 192  * rcu_read_lock_held() - might we be in RCU read-side critical section?
 193  *
 194  * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU
 195  * read-side critical section.  In absence of CONFIG_DEBUG_LOCK_ALLOC,
 196  * this assumes we are in an RCU read-side critical section unless it can
 197  * prove otherwise.  This is useful for debug checks in functions that
 198  * require that they be called within an RCU read-side critical section.
 199  *
 200  * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
 201  * and while lockdep is disabled.
 202  */
 203 static inline int rcu_read_lock_held(void)
 204 {
 205         if (!debug_lockdep_rcu_enabled())
 206                 return 1;
 207         return lock_is_held(&rcu_lock_map);
 208 }
 209
 210 /*
 211  * rcu_read_lock_bh_held() is defined out of line to avoid #include-file
 212  * hell.
 213  */
 214 extern int rcu_read_lock_bh_held(void);
 215
 216 /**
 217  * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
 218  *
 219  * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
 220  * RCU-sched read-side critical section.  In absence of
 221  * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
 222  * critical section unless it can prove otherwise.  Note that disabling
 223  * of preemption (including disabling irqs) counts as an RCU-sched
 224  * read-side critical section.  This is useful for debug checks in functions
 225  * that required that they be called within an RCU-sched read-side
 226  * critical section.
 227  *
 228  * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
 229  * and while lockdep is disabled.
 230  */
 231 #ifdef CONFIG_PREEMPT
 232 static inline int rcu_read_lock_sched_held(void)
 233 {
 234         int lockdep_opinion = 0;
 235
 236         if (!debug_lockdep_rcu_enabled())
 237                 return 1;
 238         if (debug_locks)
 239                 lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
 240         return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
 241 }
 242 #else /* #ifdef CONFIG_PREEMPT */
 243 static inline int rcu_read_lock_sched_held(void)
 244 {
 245         return 1;
 246 }
 247 #endif /* #else #ifdef CONFIG_PREEMPT */
 248
 249 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 250
 251 # define rcu_read_acquire()             do { } while (0)
 252 # define rcu_read_release()             do { } while (0)
 253 # define rcu_read_acquire_bh()          do { } while (0)
 254 # define rcu_read_release_bh()          do { } while (0)
 255 # define rcu_read_acquire_sched()       do { } while (0)
 256 # define rcu_read_release_sched()       do { } while (0)
 257
 258 static inline int rcu_read_lock_held(void)
 259 {
 260         return 1;
 261 }
 262
 263 static inline int rcu_read_lock_bh_held(void)
 264 {
 265         return 1;
 266 }
 267
 268 #ifdef CONFIG_PREEMPT
 269 static inline int rcu_read_lock_sched_held(void)
 270 {
 271         return preempt_count() != 0 || irqs_disabled();
 272 }
 273 #else /* #ifdef CONFIG_PREEMPT */
 274 static inline int rcu_read_lock_sched_held(void)
 275 {
 276         return 1;
 277 }
 278 #endif /* #else #ifdef CONFIG_PREEMPT */
 279
 280 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 281
 282 #ifdef CONFIG_PROVE_RCU
 283
 284 extern int rcu_my_thread_group_empty(void);
 285
 286 /**
 287  * rcu_lockdep_assert - emit lockdep splat if specified condition not met
 288  * @c: condition to check
 289  */
 290 #define rcu_lockdep_assert(c)                                           \
 291         do {                                                            \
 292                 static bool __warned;                                   \
 293                 if (debug_lockdep_rcu_enabled() && !__warned && !(c)) { \
 294                         __warned = true;                                \
 295                         lockdep_rcu_dereference(__FILE__, __LINE__);    \
 296                 }                                                       \
 297         } while (0)
 298
 299 #else /* #ifdef CONFIG_PROVE_RCU */
 300
 301 #define rcu_lockdep_assert(c) do { } while (0)
 302
 303 #endif /* #else #ifdef CONFIG_PROVE_RCU */
 304
 305 /*
 306  * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
 307  * and rcu_assign_pointer().  Some of these could be folded into their
 308  * callers, but they are left separate in order to ease introduction of
 309  * multiple flavors of pointers to match the multiple flavors of RCU
 310  * (e.g., __rcu_bh, * __rcu_sched, and __srcu), should this make sense in
 311  * the future.
 312  */
 313
 314 #ifdef __CHECKER__
 315 #define rcu_dereference_sparse(p, space) \
 316         ((void)(((typeof(*p) space *)p) == p))
 317 #else /* #ifdef __CHECKER__ */
 318 #define rcu_dereference_sparse(p, space)
 319 #endif /* #else #ifdef __CHECKER__ */
 320
 321 #define __rcu_access_pointer(p, space) \
 322         ({ \
 323                 typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
 324                 rcu_dereference_sparse(p, space); \
 325                 ((typeof(*p) __force __kernel *)(_________p1)); \
 326         })
 327 #define __rcu_dereference_check(p, c, space) \
 328         ({ \
 329                 typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
 330                 rcu_lockdep_assert(c); \
 331                 rcu_dereference_sparse(p, space); \
 332                 smp_read_barrier_depends(); \
 333                 ((typeof(*p) __force __kernel *)(_________p1)); \
 334         })
 335 #define __rcu_dereference_protected(p, c, space) \
 336         ({ \
 337                 rcu_lockdep_assert(c); \
 338                 rcu_dereference_sparse(p, space); \
 339                 ((typeof(*p) __force __kernel *)(p)); \
 340         })
 341
 342 #define __rcu_dereference_index_check(p, c) \
 343         ({ \
 344                 typeof(p) _________p1 = ACCESS_ONCE(p); \
 345                 rcu_lockdep_assert(c); \
 346                 smp_read_barrier_depends(); \
 347                 (_________p1); \
 348         })
 349 #define __rcu_assign_pointer(p, v, space) \
 350         ({ \
 351                 if (!__builtin_constant_p(v) || \
 352                     ((v) != NULL)) \
 353                         smp_wmb(); \
 354                 (p) = (typeof(*v) __force space *)(v); \
 355         })
 356
 357
 358 /**
 359  * rcu_access_pointer() - fetch RCU pointer with no dereferencing
 360  * @p: The pointer to read
 361  *
 362  * Return the value of the specified RCU-protected pointer, but omit the
 363  * smp_read_barrier_depends() and keep the ACCESS_ONCE().  This is useful
 364  * when the value of this pointer is accessed, but the pointer is not
 365  * dereferenced, for example, when testing an RCU-protected pointer against
 366  * NULL.  Although rcu_access_pointer() may also be used in cases where
 367  * update-side locks prevent the value of the pointer from changing, you
 368  * should instead use rcu_dereference_protected() for this use case.
 369  */
 370 #define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
 371
 372 /**
 373  * rcu_dereference_check() - rcu_dereference with debug checking
 374  * @p: The pointer to read, prior to dereferencing
 375  * @c: The conditions under which the dereference will take place
 376  *
 377  * Do an rcu_dereference(), but check that the conditions under which the
 378  * dereference will take place are correct.  Typically the conditions
 379  * indicate the various locking conditions that should be held at that
 380  * point.  The check should return true if the conditions are satisfied.
 381  * An implicit check for being in an RCU read-side critical section
 382  * (rcu_read_lock()) is included.
 383  *
 384  * For example:
 385  *
 386  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
 387  *
 388  * could be used to indicate to lockdep that foo->bar may only be dereferenced
 389  * if either rcu_read_lock() is held, or that the lock required to replace
 390  * the bar struct at foo->bar is held.
 391  *
 392  * Note that the list of conditions may also include indications of when a lock
 393  * need not be held, for example during initialisation or destruction of the
 394  * target struct:
 395  *
 396  *      bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
 397  *                                            atomic_read(&foo->usage) == 0);
 398  *
 399  * Inserts memory barriers on architectures that require them
 400  * (currently only the Alpha), prevents the compiler from refetching
 401  * (and from merging fetches), and, more importantly, documents exactly
 402  * which pointers are protected by RCU and checks that the pointer is
 403  * annotated as __rcu.
 404  */
 405 #define rcu_dereference_check(p, c) \
 406         __rcu_dereference_check((p), rcu_read_lock_held() || (c), __rcu)
 407
 408 /**
 409  * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
 410  * @p: The pointer to read, prior to dereferencing
 411  * @c: The conditions under which the dereference will take place
 412  *
 413  * This is the RCU-bh counterpart to rcu_dereference_check().
 414  */
 415 #define rcu_dereference_bh_check(p, c) \
 416         __rcu_dereference_check((p), rcu_read_lock_bh_held() || (c), __rcu)
 417
 418 /**
 419  * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
 420  * @p: The pointer to read, prior to dereferencing
 421  * @c: The conditions under which the dereference will take place
 422  *
 423  * This is the RCU-sched counterpart to rcu_dereference_check().
 424  */
 425 #define rcu_dereference_sched_check(p, c) \
 426         __rcu_dereference_check((p), rcu_read_lock_sched_held() || (c), \
 427                                 __rcu)
 428
 429 #define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ needed? @@@*/
 430
 431 /**
 432  * rcu_dereference_index_check() - rcu_dereference for indices with debug checking
 433  * @p: The pointer to read, prior to dereferencing
 434  * @c: The conditions under which the dereference will take place
 435  *
 436  * Similar to rcu_dereference_check(), but omits the sparse checking.
 437  * This allows rcu_dereference_index_check() to be used on integers,
 438  * which can then be used as array indices.  Attempting to use
 439  * rcu_dereference_check() on an integer will give compiler warnings
 440  * because the sparse address-space mechanism relies on dereferencing
 441  * the RCU-protected pointer.  Dereferencing integers is not something
 442  * that even gcc will put up with.
 443  *
 444  * Note that this function does not implicitly check for RCU read-side
 445  * critical sections.  If this function gains lots of uses, it might
 446  * make sense to provide versions for each flavor of RCU, but it does
 447  * not make sense as of early 2010.
 448  */
 449 #define rcu_dereference_index_check(p, c) \
 450         __rcu_dereference_index_check((p), (c))
 451
 452 /**
 453  * rcu_dereference_protected() - fetch RCU pointer when updates prevented
 454  * @p: The pointer to read, prior to dereferencing
 455  * @c: The conditions under which the dereference will take place
 456  *
 457  * Return the value of the specified RCU-protected pointer, but omit
 458  * both the smp_read_barrier_depends() and the ACCESS_ONCE().  This
 459  * is useful in cases where update-side locks prevent the value of the
 460  * pointer from changing.  Please note that this primitive does -not-
 461  * prevent the compiler from repeating this reference or combining it
 462  * with other references, so it should not be used without protection
 463  * of appropriate locks.
 464  *
 465  * This function is only for update-side use.  Using this function
 466  * when protected only by rcu_read_lock() will result in infrequent
 467  * but very ugly failures.
 468  */
 469 #define rcu_dereference_protected(p, c) \
 470         __rcu_dereference_protected((p), (c), __rcu)
 471
 472 /**
 473  * rcu_dereference_bh_protected() - fetch RCU-bh pointer when updates prevented
 474  * @p: The pointer to read, prior to dereferencing
 475  * @c: The conditions under which the dereference will take place
 476  *
 477  * This is the RCU-bh counterpart to rcu_dereference_protected().
 478  */
 479 #define rcu_dereference_bh_protected(p, c) \
 480         __rcu_dereference_protected((p), (c), __rcu)
 481
 482 /**
 483  * rcu_dereference_sched_protected() - fetch RCU-sched pointer when updates prevented
 484  * @p: The pointer to read, prior to dereferencing
 485  * @c: The conditions under which the dereference will take place
 486  *
 487  * This is the RCU-sched counterpart to rcu_dereference_protected().
 488  */
 489 #define rcu_dereference_sched_protected(p, c) \
 490         __rcu_dereference_protected((p), (c), __rcu)
 491
 492
 493 /**
 494  * rcu_dereference() - fetch RCU-protected pointer for dereferencing
 495  * @p: The pointer to read, prior to dereferencing
 496  *
 497  * This is a simple wrapper around rcu_dereference_check().
 498  */
 499 #define rcu_dereference(p) rcu_dereference_check(p, 0)
 500
 501 /**
 502  * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
 503  * @p: The pointer to read, prior to dereferencing
 504  *
 505  * Makes rcu_dereference_check() do the dirty work.
 506  */
 507 #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
 508
 509 /**
 510  * rcu_dereference_sched() - fetch RCU-sched-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_sched(p) rcu_dereference_sched_check(p, 0)
 516
 517 /**
 518  * rcu_read_lock() - mark the beginning of an RCU read-side critical section
 519  *
 520  * When synchronize_rcu() is invoked on one CPU while other CPUs
 521  * are within RCU read-side critical sections, then the
 522  * synchronize_rcu() is guaranteed to block until after all the other
 523  * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
 524  * on one CPU while other CPUs are within RCU read-side critical
 525  * sections, invocation of the corresponding RCU callback is deferred
 526  * until after the all the other CPUs exit their critical sections.
 527  *
 528  * Note, however, that RCU callbacks are permitted to run concurrently
 529  * with new RCU read-side critical sections.  One way that this can happen
 530  * is via the following sequence of events: (1) CPU 0 enters an RCU
 531  * read-side critical section, (2) CPU 1 invokes call_rcu() to register
 532  * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
 533  * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
 534  * callback is invoked.  This is legal, because the RCU read-side critical
 535  * section that was running concurrently with the call_rcu() (and which
 536  * therefore might be referencing something that the corresponding RCU
 537  * callback would free up) has completed before the corresponding
 538  * RCU callback is invoked.
 539  *
 540  * RCU read-side critical sections may be nested.  Any deferred actions
 541  * will be deferred until the outermost RCU read-side critical section
 542  * completes.
 543  *
 544  * You can avoid reading and understanding the next paragraph by
 545  * following this rule: don't put anything in an rcu_read_lock() RCU
 546  * read-side critical section that would block in a !PREEMPT kernel.
 547  * But if you want the full story, read on!
 548  *
 549  * In non-preemptible RCU implementations (TREE_RCU and TINY_RCU), it
 550  * is illegal to block while in an RCU read-side critical section.  In
 551  * preemptible RCU implementations (TREE_PREEMPT_RCU and TINY_PREEMPT_RCU)
 552  * in CONFIG_PREEMPT kernel builds, RCU read-side critical sections may
 553  * be preempted, but explicit blocking is illegal.  Finally, in preemptible
 554  * RCU implementations in real-time (CONFIG_PREEMPT_RT) kernel builds,
 555  * RCU read-side critical sections may be preempted and they may also
 556  * block, but only when acquiring spinlocks that are subject to priority
 557  * inheritance.
 558  */
 559 static inline void rcu_read_lock(void)
 560 {
 561         __rcu_read_lock();
 562         __acquire(RCU);
 563         rcu_read_acquire();
 564 }
 565
 566 /*
 567  * So where is rcu_write_lock()?  It does not exist, as there is no
 568  * way for writers to lock out RCU readers.  This is a feature, not
 569  * a bug -- this property is what provides RCU's performance benefits.
 570  * Of course, writers must coordinate with each other.  The normal
 571  * spinlock primitives work well for this, but any other technique may be
 572  * used as well.  RCU does not care how the writers keep out of each
 573  * others' way, as long as they do so.
 574  */
 575
 576 /**
 577  * rcu_read_unlock() - marks the end of an RCU read-side critical section.
 578  *
 579  * See rcu_read_lock() for more information.
 580  */
 581 static inline void rcu_read_unlock(void)
 582 {
 583         rcu_read_release();
 584         __release(RCU);
 585         __rcu_read_unlock();
 586 }
 587
 588 /**
 589  * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
 590  *
 591  * This is equivalent of rcu_read_lock(), but to be used when updates
 592  * are being done using call_rcu_bh() or synchronize_rcu_bh(). Since
 593  * both call_rcu_bh() and synchronize_rcu_bh() consider completion of a
 594  * softirq handler to be a quiescent state, a process in RCU read-side
 595  * critical section must be protected by disabling softirqs. Read-side
 596  * critical sections in interrupt context can use just rcu_read_lock(),
 597  * though this should at least be commented to avoid confusing people
 598  * reading the code.
 599  */
 600 static inline void rcu_read_lock_bh(void)
 601 {
 602         __rcu_read_lock_bh();
 603         __acquire(RCU_BH);
 604         rcu_read_acquire_bh();
 605 }
 606
 607 /*
 608  * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
 609  *
 610  * See rcu_read_lock_bh() for more information.
 611  */
 612 static inline void rcu_read_unlock_bh(void)
 613 {
 614         rcu_read_release_bh();
 615         __release(RCU_BH);
 616         __rcu_read_unlock_bh();
 617 }
 618
 619 /**
 620  * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
 621  *
 622  * This is equivalent of rcu_read_lock(), but to be used when updates
 623  * are being done using call_rcu_sched() or synchronize_rcu_sched().
 624  * Read-side critical sections can also be introduced by anything that
 625  * disables preemption, including local_irq_disable() and friends.
 626  */
 627 static inline void rcu_read_lock_sched(void)
 628 {
 629         preempt_disable();
 630         __acquire(RCU_SCHED);
 631         rcu_read_acquire_sched();
 632 }
 633
 634 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
 635 static inline notrace void rcu_read_lock_sched_notrace(void)
 636 {
 637         preempt_disable_notrace();
 638         __acquire(RCU_SCHED);
 639 }
 640
 641 /*
 642  * rcu_read_unlock_sched - marks the end of a RCU-classic critical section
 643  *
 644  * See rcu_read_lock_sched for more information.
 645  */
 646 static inline void rcu_read_unlock_sched(void)
 647 {
 648         rcu_read_release_sched();
 649         __release(RCU_SCHED);
 650         preempt_enable();
 651 }
 652
 653 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
 654 static inline notrace void rcu_read_unlock_sched_notrace(void)
 655 {
 656         __release(RCU_SCHED);
 657         preempt_enable_notrace();
 658 }
 659
 660 /**
 661  * rcu_assign_pointer() - assign to RCU-protected pointer
 662  * @p: pointer to assign to
 663  * @v: value to assign (publish)
 664  *
 665  * Assigns the specified value to the specified RCU-protected
 666  * pointer, ensuring that any concurrent RCU readers will see
 667  * any prior initialization.  Returns the value assigned.
 668  *
 669  * Inserts memory barriers on architectures that require them
 670  * (pretty much all of them other than x86), and also prevents
 671  * the compiler from reordering the code that initializes the
 672  * structure after the pointer assignment.  More importantly, this
 673  * call documents which pointers will be dereferenced by RCU read-side
 674  * code.
 675  */
 676 #define rcu_assign_pointer(p, v) \
 677         __rcu_assign_pointer((p), (v), __rcu)
 678
 679 /**
 680  * RCU_INIT_POINTER() - initialize an RCU protected pointer
 681  *
 682  * Initialize an RCU-protected pointer in such a way to avoid RCU-lockdep
 683  * splats.
 684  */
 685 #define RCU_INIT_POINTER(p, v) \
 686                 p = (typeof(*v) __force __rcu *)(v)
 687
 688 /* Infrastructure to implement the synchronize_() primitives. */
 689
 690 struct rcu_synchronize {
 691         struct rcu_head head;
 692         struct completion completion;
 693 };
 694
 695 extern void wakeme_after_rcu(struct rcu_head  *head);
 696
 697 #ifdef CONFIG_PREEMPT_RCU
 698
 699 /**
 700  * call_rcu() - Queue an RCU callback for invocation after a grace period.
 701  * @head: structure to be used for queueing the RCU updates.
 702  * @func: actual callback function to be invoked after the grace period
 703  *
 704  * The callback function will be invoked some time after a full grace
 705  * period elapses, in other words after all pre-existing RCU read-side
 706  * critical sections have completed.  However, the callback function
 707  * might well execute concurrently with RCU read-side critical sections
 708  * that started after call_rcu() was invoked.  RCU read-side critical
 709  * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 710  * and may be nested.
 711  */
 712 extern void call_rcu(struct rcu_head *head,
 713                               void (*func)(struct rcu_head *head));
 714
 715 #else /* #ifdef CONFIG_PREEMPT_RCU */
 716
 717 /* In classic RCU, call_rcu() is just call_rcu_sched(). */
 718 #define call_rcu        call_rcu_sched
 719
 720 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 721
 722 /**
 723  * call_rcu_bh() - Queue an RCU for invocation after a quicker grace period.
 724  * @head: structure to be used for queueing the RCU updates.
 725  * @func: actual callback function to be invoked after the grace period
 726  *
 727  * The callback function will be invoked some time after a full grace
 728  * period elapses, in other words after all currently executing RCU
 729  * read-side critical sections have completed. call_rcu_bh() assumes
 730  * that the read-side critical sections end on completion of a softirq
 731  * handler. This means that read-side critical sections in process
 732  * context must not be interrupted by softirqs. This interface is to be
 733  * used when most of the read-side critical sections are in softirq context.
 734  * RCU read-side critical sections are delimited by :
 735  *  - rcu_read_lock() and  rcu_read_unlock(), if in interrupt context.
 736  *  OR
 737  *  - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context.
 738  *  These may be nested.
 739  */
 740 extern void call_rcu_bh(struct rcu_head *head,
 741                         void (*func)(struct rcu_head *head));
 742
 743 /*
 744  * debug_rcu_head_queue()/debug_rcu_head_unqueue() are used internally
 745  * by call_rcu() and rcu callback execution, and are therefore not part of the
 746  * RCU API. Leaving in rcupdate.h because they are used by all RCU flavors.
 747  */
 748
 749 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
 750 # define STATE_RCU_HEAD_READY   0
 751 # define STATE_RCU_HEAD_QUEUED  1
 752
 753 extern struct debug_obj_descr rcuhead_debug_descr;
 754
 755 static inline void debug_rcu_head_queue(struct rcu_head *head)
 756 {
 757         debug_object_activate(head, &rcuhead_debug_descr);
 758         debug_object_active_state(head, &rcuhead_debug_descr,
 759                                   STATE_RCU_HEAD_READY,
 760                                   STATE_RCU_HEAD_QUEUED);
 761 }
 762
 763 static inline void debug_rcu_head_unqueue(struct rcu_head *head)
 764 {
 765         debug_object_active_state(head, &rcuhead_debug_descr,
 766                                   STATE_RCU_HEAD_QUEUED,
 767                                   STATE_RCU_HEAD_READY);
 768         debug_object_deactivate(head, &rcuhead_debug_descr);
 769 }
 770 #else   /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
 771 static inline void debug_rcu_head_queue(struct rcu_head *head)
 772 {
 773 }
 774
 775 static inline void debug_rcu_head_unqueue(struct rcu_head *head)
 776 {
 777 }
 778 #endif  /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
 779
 780 #endif /* __LINUX_RCUPDATE_H */