release/src-rt-6.x.4708/linux/linux-2.6.36/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
  45 #ifdef CONFIG_RCU_TORTURE_TEST
  46 extern int rcutorture_runnable; /* for sysctl */
  47 #endif /* #ifdef CONFIG_RCU_TORTURE_TEST */
  48
  49 /**
  50  * struct rcu_head - callback structure for use with RCU
  51  * @next: next update requests in a list
  52  * @func: actual update function to call after the grace period.
  53  */
  54 struct rcu_head {
  55         struct rcu_head *next;
  56         void (*func)(struct rcu_head *head);
  57 };
  58
  59 /* Exported common interfaces */
  60 extern void rcu_barrier(void);
  61 extern void rcu_barrier_bh(void);
  62 extern void rcu_barrier_sched(void);
  63 extern void synchronize_sched_expedited(void);
  64 extern int sched_expedited_torture_stats(char *page);
  65
  66 /* Internal to kernel */
  67 extern void rcu_init(void);
  68
  69 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU)
  70 #include <linux/rcutree.h>
  71 #elif defined(CONFIG_TINY_RCU)
  72 #include <linux/rcutiny.h>
  73 #else
  74 #error "Unknown RCU implementation specified to kernel configuration"
  75 #endif
  76
  77 #define RCU_HEAD_INIT   { .next = NULL, .func = NULL }
  78 #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
  79 #define INIT_RCU_HEAD(ptr) do { \
  80        (ptr)->next = NULL; (ptr)->func = NULL; \
  81 } while (0)
  82
  83 /*
  84  * init_rcu_head_on_stack()/destroy_rcu_head_on_stack() are needed for dynamic
  85  * initialization and destruction of rcu_head on the stack. rcu_head structures
  86  * allocated dynamically in the heap or defined statically don't need any
  87  * initialization.
  88  */
  89 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
  90 extern void init_rcu_head_on_stack(struct rcu_head *head);
  91 extern void destroy_rcu_head_on_stack(struct rcu_head *head);
  92 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
  93 static inline void init_rcu_head_on_stack(struct rcu_head *head)
  94 {
  95 }
  96
  97 static inline void destroy_rcu_head_on_stack(struct rcu_head *head)
  98 {
  99 }
 100 #endif  /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
 101
 102 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 103
 104 extern struct lockdep_map rcu_lock_map;
 105 # define rcu_read_acquire() \
 106                 lock_acquire(&rcu_lock_map, 0, 0, 2, 1, NULL, _THIS_IP_)
 107 # define rcu_read_release()     lock_release(&rcu_lock_map, 1, _THIS_IP_)
 108
 109 extern struct lockdep_map rcu_bh_lock_map;
 110 # define rcu_read_acquire_bh() \
 111                 lock_acquire(&rcu_bh_lock_map, 0, 0, 2, 1, NULL, _THIS_IP_)
 112 # define rcu_read_release_bh()  lock_release(&rcu_bh_lock_map, 1, _THIS_IP_)
 113
 114 extern struct lockdep_map rcu_sched_lock_map;
 115 # define rcu_read_acquire_sched() \
 116                 lock_acquire(&rcu_sched_lock_map, 0, 0, 2, 1, NULL, _THIS_IP_)
 117 # define rcu_read_release_sched() \
 118                 lock_release(&rcu_sched_lock_map, 1, _THIS_IP_)
 119
 120 extern int debug_lockdep_rcu_enabled(void);
 121
 122 /**
 123  * rcu_read_lock_held - might we be in RCU read-side critical section?
 124  *
 125  * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU
 126  * read-side critical section.  In absence of CONFIG_DEBUG_LOCK_ALLOC,
 127  * this assumes we are in an RCU read-side critical section unless it can
 128  * prove otherwise.
 129  *
 130  * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
 131  * and while lockdep is disabled.
 132  */
 133 static inline int rcu_read_lock_held(void)
 134 {
 135         if (!debug_lockdep_rcu_enabled())
 136                 return 1;
 137         return lock_is_held(&rcu_lock_map);
 138 }
 139
 140 /*
 141  * rcu_read_lock_bh_held() is defined out of line to avoid #include-file
 142  * hell.
 143  */
 144 extern int rcu_read_lock_bh_held(void);
 145
 146 /**
 147  * rcu_read_lock_sched_held - might we be in RCU-sched read-side critical section?
 148  *
 149  * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
 150  * RCU-sched read-side critical section.  In absence of
 151  * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
 152  * critical section unless it can prove otherwise.  Note that disabling
 153  * of preemption (including disabling irqs) counts as an RCU-sched
 154  * read-side critical section.
 155  *
 156  * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
 157  * and while lockdep is disabled.
 158  */
 159 #ifdef CONFIG_PREEMPT
 160 static inline int rcu_read_lock_sched_held(void)
 161 {
 162         int lockdep_opinion = 0;
 163
 164         if (!debug_lockdep_rcu_enabled())
 165                 return 1;
 166         if (debug_locks)
 167                 lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
 168         return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
 169 }
 170 #else /* #ifdef CONFIG_PREEMPT */
 171 static inline int rcu_read_lock_sched_held(void)
 172 {
 173         return 1;
 174 }
 175 #endif /* #else #ifdef CONFIG_PREEMPT */
 176
 177 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 178
 179 # define rcu_read_acquire()             do { } while (0)
 180 # define rcu_read_release()             do { } while (0)
 181 # define rcu_read_acquire_bh()          do { } while (0)
 182 # define rcu_read_release_bh()          do { } while (0)
 183 # define rcu_read_acquire_sched()       do { } while (0)
 184 # define rcu_read_release_sched()       do { } while (0)
 185
 186 static inline int rcu_read_lock_held(void)
 187 {
 188         return 1;
 189 }
 190
 191 static inline int rcu_read_lock_bh_held(void)
 192 {
 193         return 1;
 194 }
 195
 196 #ifdef CONFIG_PREEMPT
 197 static inline int rcu_read_lock_sched_held(void)
 198 {
 199         return preempt_count() != 0 || irqs_disabled();
 200 }
 201 #else /* #ifdef CONFIG_PREEMPT */
 202 static inline int rcu_read_lock_sched_held(void)
 203 {
 204         return 1;
 205 }
 206 #endif /* #else #ifdef CONFIG_PREEMPT */
 207
 208 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 209
 210 #ifdef CONFIG_PROVE_RCU
 211
 212 extern int rcu_my_thread_group_empty(void);
 213
 214 #define __do_rcu_dereference_check(c)                                   \
 215         do {                                                            \
 216                 static bool __warned;                                   \
 217                 if (debug_lockdep_rcu_enabled() && !__warned && !(c)) { \
 218                         __warned = true;                                \
 219                         lockdep_rcu_dereference(__FILE__, __LINE__);    \
 220                 }                                                       \
 221         } while (0)
 222
 223 /**
 224  * rcu_dereference_check - rcu_dereference with debug checking
 225  * @p: The pointer to read, prior to dereferencing
 226  * @c: The conditions under which the dereference will take place
 227  *
 228  * Do an rcu_dereference(), but check that the conditions under which the
 229  * dereference will take place are correct.  Typically the conditions indicate
 230  * the various locking conditions that should be held at that point.  The check
 231  * should return true if the conditions are satisfied.
 232  *
 233  * For example:
 234  *
 235  *      bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||
 236  *                                            lockdep_is_held(&foo->lock));
 237  *
 238  * could be used to indicate to lockdep that foo->bar may only be dereferenced
 239  * if either the RCU read lock is held, or that the lock required to replace
 240  * the bar struct at foo->bar is held.
 241  *
 242  * Note that the list of conditions may also include indications of when a lock
 243  * need not be held, for example during initialisation or destruction of the
 244  * target struct:
 245  *
 246  *      bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||
 247  *                                            lockdep_is_held(&foo->lock) ||
 248  *                                            atomic_read(&foo->usage) == 0);
 249  */
 250 #define rcu_dereference_check(p, c) \
 251         ({ \
 252                 __do_rcu_dereference_check(c); \
 253                 rcu_dereference_raw(p); \
 254         })
 255
 256 /**
 257  * rcu_dereference_protected - fetch RCU pointer when updates prevented
 258  *
 259  * Return the value of the specified RCU-protected pointer, but omit
 260  * both the smp_read_barrier_depends() and the ACCESS_ONCE().  This
 261  * is useful in cases where update-side locks prevent the value of the
 262  * pointer from changing.  Please note that this primitive does -not-
 263  * prevent the compiler from repeating this reference or combining it
 264  * with other references, so it should not be used without protection
 265  * of appropriate locks.
 266  */
 267 #define rcu_dereference_protected(p, c) \
 268         ({ \
 269                 __do_rcu_dereference_check(c); \
 270                 (p); \
 271         })
 272
 273 #else /* #ifdef CONFIG_PROVE_RCU */
 274
 275 #define rcu_dereference_check(p, c)     rcu_dereference_raw(p)
 276 #define rcu_dereference_protected(p, c) (p)
 277
 278 #endif /* #else #ifdef CONFIG_PROVE_RCU */
 279
 280 /**
 281  * rcu_access_pointer - fetch RCU pointer with no dereferencing
 282  *
 283  * Return the value of the specified RCU-protected pointer, but omit the
 284  * smp_read_barrier_depends() and keep the ACCESS_ONCE().  This is useful
 285  * when the value of this pointer is accessed, but the pointer is not
 286  * dereferenced, for example, when testing an RCU-protected pointer against
 287  * NULL.  This may also be used in cases where update-side locks prevent
 288  * the value of the pointer from changing, but rcu_dereference_protected()
 289  * is a lighter-weight primitive for this use case.
 290  */
 291 #define rcu_access_pointer(p)   ACCESS_ONCE(p)
 292
 293 /**
 294  * rcu_read_lock - mark the beginning of an RCU read-side critical section.
 295  *
 296  * When synchronize_rcu() is invoked on one CPU while other CPUs
 297  * are within RCU read-side critical sections, then the
 298  * synchronize_rcu() is guaranteed to block until after all the other
 299  * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
 300  * on one CPU while other CPUs are within RCU read-side critical
 301  * sections, invocation of the corresponding RCU callback is deferred
 302  * until after the all the other CPUs exit their critical sections.
 303  *
 304  * Note, however, that RCU callbacks are permitted to run concurrently
 305  * with RCU read-side critical sections.  One way that this can happen
 306  * is via the following sequence of events: (1) CPU 0 enters an RCU
 307  * read-side critical section, (2) CPU 1 invokes call_rcu() to register
 308  * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
 309  * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
 310  * callback is invoked.  This is legal, because the RCU read-side critical
 311  * section that was running concurrently with the call_rcu() (and which
 312  * therefore might be referencing something that the corresponding RCU
 313  * callback would free up) has completed before the corresponding
 314  * RCU callback is invoked.
 315  *
 316  * RCU read-side critical sections may be nested.  Any deferred actions
 317  * will be deferred until the outermost RCU read-side critical section
 318  * completes.
 319  *
 320  * It is illegal to block while in an RCU read-side critical section.
 321  */
 322 static inline void rcu_read_lock(void)
 323 {
 324         __rcu_read_lock();
 325         __acquire(RCU);
 326         rcu_read_acquire();
 327 }
 328
 329 /*
 330  * So where is rcu_write_lock()?  It does not exist, as there is no
 331  * way for writers to lock out RCU readers.  This is a feature, not
 332  * a bug -- this property is what provides RCU's performance benefits.
 333  * Of course, writers must coordinate with each other.  The normal
 334  * spinlock primitives work well for this, but any other technique may be
 335  * used as well.  RCU does not care how the writers keep out of each
 336  * others' way, as long as they do so.
 337  */
 338
 339 /**
 340  * rcu_read_unlock - marks the end of an RCU read-side critical section.
 341  *
 342  * See rcu_read_lock() for more information.
 343  */
 344 static inline void rcu_read_unlock(void)
 345 {
 346         rcu_read_release();
 347         __release(RCU);
 348         __rcu_read_unlock();
 349 }
 350
 351 /**
 352  * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section
 353  *
 354  * This is equivalent of rcu_read_lock(), but to be used when updates
 355  * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks
 356  * consider completion of a softirq handler to be a quiescent state,
 357  * a process in RCU read-side critical section must be protected by
 358  * disabling softirqs. Read-side critical sections in interrupt context
 359  * can use just rcu_read_lock().
 360  *
 361  */
 362 static inline void rcu_read_lock_bh(void)
 363 {
 364         __rcu_read_lock_bh();
 365         __acquire(RCU_BH);
 366         rcu_read_acquire_bh();
 367 }
 368
 369 /*
 370  * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
 371  *
 372  * See rcu_read_lock_bh() for more information.
 373  */
 374 static inline void rcu_read_unlock_bh(void)
 375 {
 376         rcu_read_release_bh();
 377         __release(RCU_BH);
 378         __rcu_read_unlock_bh();
 379 }
 380
 381 /**
 382  * rcu_read_lock_sched - mark the beginning of a RCU-classic critical section
 383  *
 384  * Should be used with either
 385  * - synchronize_sched()
 386  * or
 387  * - call_rcu_sched() and rcu_barrier_sched()
 388  * on the write-side to insure proper synchronization.
 389  */
 390 static inline void rcu_read_lock_sched(void)
 391 {
 392         preempt_disable();
 393         __acquire(RCU_SCHED);
 394         rcu_read_acquire_sched();
 395 }
 396
 397 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
 398 static inline notrace void rcu_read_lock_sched_notrace(void)
 399 {
 400         preempt_disable_notrace();
 401         __acquire(RCU_SCHED);
 402 }
 403
 404 /*
 405  * rcu_read_unlock_sched - marks the end of a RCU-classic critical section
 406  *
 407  * See rcu_read_lock_sched for more information.
 408  */
 409 static inline void rcu_read_unlock_sched(void)
 410 {
 411         rcu_read_release_sched();
 412         __release(RCU_SCHED);
 413         preempt_enable();
 414 }
 415
 416 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
 417 static inline notrace void rcu_read_unlock_sched_notrace(void)
 418 {
 419         __release(RCU_SCHED);
 420         preempt_enable_notrace();
 421 }
 422
 423
 424 /**
 425  * rcu_dereference_raw - fetch an RCU-protected pointer
 426  *
 427  * The caller must be within some flavor of RCU read-side critical
 428  * section, or must be otherwise preventing the pointer from changing,
 429  * for example, by holding an appropriate lock.  This pointer may later
 430  * be safely dereferenced.  It is the caller's responsibility to have
 431  * done the right thing, as this primitive does no checking of any kind.
 432  *
 433  * Inserts memory barriers on architectures that require them
 434  * (currently only the Alpha), and, more importantly, documents
 435  * exactly which pointers are protected by RCU.
 436  */
 437 #define rcu_dereference_raw(p)  ({ \
 438                                 typeof(p) _________p1 = ACCESS_ONCE(p); \
 439                                 smp_read_barrier_depends(); \
 440                                 (_________p1); \
 441                                 })
 442
 443 /**
 444  * rcu_dereference - fetch an RCU-protected pointer, checking for RCU
 445  *
 446  * Makes rcu_dereference_check() do the dirty work.
 447  */
 448 #define rcu_dereference(p) \
 449         rcu_dereference_check(p, rcu_read_lock_held())
 450
 451 /**
 452  * rcu_dereference_bh - fetch an RCU-protected pointer, checking for RCU-bh
 453  *
 454  * Makes rcu_dereference_check() do the dirty work.
 455  */
 456 #define rcu_dereference_bh(p) \
 457                 rcu_dereference_check(p, rcu_read_lock_bh_held() || irqs_disabled())
 458
 459 /**
 460  * rcu_dereference_sched - fetch RCU-protected pointer, checking for RCU-sched
 461  *
 462  * Makes rcu_dereference_check() do the dirty work.
 463  */
 464 #define rcu_dereference_sched(p) \
 465                 rcu_dereference_check(p, rcu_read_lock_sched_held())
 466
 467 /**
 468  * rcu_assign_pointer - assign (publicize) a pointer to a newly
 469  * initialized structure that will be dereferenced by RCU read-side
 470  * critical sections.  Returns the value assigned.
 471  *
 472  * Inserts memory barriers on architectures that require them
 473  * (pretty much all of them other than x86), and also prevents
 474  * the compiler from reordering the code that initializes the
 475  * structure after the pointer assignment.  More importantly, this
 476  * call documents which pointers will be dereferenced by RCU read-side
 477  * code.
 478  */
 479
 480 #define rcu_assign_pointer(p, v) \
 481         ({ \
 482                 if (!__builtin_constant_p(v) || \
 483                     ((v) != NULL)) \
 484                         smp_wmb(); \
 485                 (p) = (v); \
 486         })
 487
 488 /* Infrastructure to implement the synchronize_() primitives. */
 489
 490 struct rcu_synchronize {
 491         struct rcu_head head;
 492         struct completion completion;
 493 };
 494
 495 extern void wakeme_after_rcu(struct rcu_head  *head);
 496
 497 /**
 498  * call_rcu - Queue an RCU callback for invocation after a grace period.
 499  * @head: structure to be used for queueing the RCU updates.
 500  * @func: actual update function to be invoked after the grace period
 501  *
 502  * The update function will be invoked some time after a full grace
 503  * period elapses, in other words after all currently executing RCU
 504  * read-side critical sections have completed.  RCU read-side critical
 505  * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 506  * and may be nested.
 507  */
 508 extern void call_rcu(struct rcu_head *head,
 509                               void (*func)(struct rcu_head *head));
 510
 511 /**
 512  * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
 513  * @head: structure to be used for queueing the RCU updates.
 514  * @func: actual update function to be invoked after the grace period
 515  *
 516  * The update function will be invoked some time after a full grace
 517  * period elapses, in other words after all currently executing RCU
 518  * read-side critical sections have completed. call_rcu_bh() assumes
 519  * that the read-side critical sections end on completion of a softirq
 520  * handler. This means that read-side critical sections in process
 521  * context must not be interrupted by softirqs. This interface is to be
 522  * used when most of the read-side critical sections are in softirq context.
 523  * RCU read-side critical sections are delimited by :
 524  *  - rcu_read_lock() and  rcu_read_unlock(), if in interrupt context.
 525  *  OR
 526  *  - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context.
 527  *  These may be nested.
 528  */
 529 extern void call_rcu_bh(struct rcu_head *head,
 530                         void (*func)(struct rcu_head *head));
 531
 532 /*
 533  * debug_rcu_head_queue()/debug_rcu_head_unqueue() are used internally
 534  * by call_rcu() and rcu callback execution, and are therefore not part of the
 535  * RCU API. Leaving in rcupdate.h because they are used by all RCU flavors.
 536  */
 537
 538 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
 539 # define STATE_RCU_HEAD_READY   0
 540 # define STATE_RCU_HEAD_QUEUED  1
 541
 542 extern struct debug_obj_descr rcuhead_debug_descr;
 543
 544 static inline void debug_rcu_head_queue(struct rcu_head *head)
 545 {
 546         debug_object_activate(head, &rcuhead_debug_descr);
 547         debug_object_active_state(head, &rcuhead_debug_descr,
 548                                   STATE_RCU_HEAD_READY,
 549                                   STATE_RCU_HEAD_QUEUED);
 550 }
 551
 552 static inline void debug_rcu_head_unqueue(struct rcu_head *head)
 553 {
 554         debug_object_active_state(head, &rcuhead_debug_descr,
 555                                   STATE_RCU_HEAD_QUEUED,
 556                                   STATE_RCU_HEAD_READY);
 557         debug_object_deactivate(head, &rcuhead_debug_descr);
 558 }
 559 #else   /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
 560 static inline void debug_rcu_head_queue(struct rcu_head *head)
 561 {
 562 }
 563
 564 static inline void debug_rcu_head_unqueue(struct rcu_head *head)
 565 {
 566 }
 567 #endif  /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
 568
 569 #ifndef CONFIG_PROVE_RCU
 570 #define __do_rcu_dereference_check(c) do { } while (0)
 571 #endif /* #ifdef CONFIG_PROVE_RCU */
 572
 573 #define __rcu_dereference_index_check(p, c) \
 574         ({ \
 575                 typeof(p) _________p1 = ACCESS_ONCE(p); \
 576                 __do_rcu_dereference_check(c); \
 577                 smp_read_barrier_depends(); \
 578                 (_________p1); \
 579         })
 580
 581 /**
 582  * rcu_dereference_index_check() - rcu_dereference for indices with debug checking
 583  * @p: The pointer to read, prior to dereferencing
 584  * @c: The conditions under which the dereference will take place
 585  *
 586  * Similar to rcu_dereference_check(), but omits the sparse checking.
 587  * This allows rcu_dereference_index_check() to be used on integers,
 588  * which can then be used as array indices.  Attempting to use
 589  * rcu_dereference_check() on an integer will give compiler warnings
 590  * because the sparse address-space mechanism relies on dereferencing
 591  * the RCU-protected pointer.  Dereferencing integers is not something
 592  * that even gcc will put up with.
 593  *
 594  * Note that this function does not implicitly check for RCU read-side
 595  * critical sections.  If this function gains lots of uses, it might
 596  * make sense to provide versions for each flavor of RCU, but it does
 597  * not make sense as of early 2010.
 598  */
 599 #define rcu_dereference_index_check(p, c) \
 600         __rcu_dereference_index_check((p), (c))
 601
 602 #endif /* __LINUX_RCUPDATE_H */