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