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