include/linux/seqlock.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef __LINUX_SEQLOCK_H
   3 #define __LINUX_SEQLOCK_H
   4
   5 /*
   6  * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
   7  * lockless readers (read-only retry loops), and no writer starvation.
   8  *
   9  * See Documentation/locking/seqlock.rst
  10  *
  11  * Copyrights:
  12  * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
  13  * - Sequence counters with associated locks, (C) 2020 Linutronix GmbH
  14  */
  15
  16 #include <linux/compiler.h>
  17 #include <linux/kcsan-checks.h>
  18 #include <linux/lockdep.h>
  19 #include <linux/mutex.h>
  20 #include <linux/preempt.h>
  21 #include <linux/spinlock.h>
  22
  23 #include <asm/processor.h>
  24
  25 /*
  26  * The seqlock seqcount_t interface does not prescribe a precise sequence of
  27  * read begin/retry/end. For readers, typically there is a call to
  28  * read_seqcount_begin() and read_seqcount_retry(), however, there are more
  29  * esoteric cases which do not follow this pattern.
  30  *
  31  * As a consequence, we take the following best-effort approach for raw usage
  32  * via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
  33  * pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
  34  * atomics; if there is a matching read_seqcount_retry() call, no following
  35  * memory operations are considered atomic. Usage of the seqlock_t interface
  36  * is not affected.
  37  */
  38 #define KCSAN_SEQLOCK_REGION_MAX 1000
  39
  40 /*
  41  * Sequence counters (seqcount_t)
  42  *
  43  * This is the raw counting mechanism, without any writer protection.
  44  *
  45  * Write side critical sections must be serialized and non-preemptible.
  46  *
  47  * If readers can be invoked from hardirq or softirq contexts,
  48  * interrupts or bottom halves must also be respectively disabled before
  49  * entering the write section.
  50  *
  51  * This mechanism can't be used if the protected data contains pointers,
  52  * as the writer can invalidate a pointer that a reader is following.
  53  *
  54  * If the write serialization mechanism is one of the common kernel
  55  * locking primitives, use a sequence counter with associated lock
  56  * (seqcount_LOCKNAME_t) instead.
  57  *
  58  * If it's desired to automatically handle the sequence counter writer
  59  * serialization and non-preemptibility requirements, use a sequential
  60  * lock (seqlock_t) instead.
  61  *
  62  * See Documentation/locking/seqlock.rst
  63  */
  64 typedef struct seqcount {
  65         unsigned sequence;
  66 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  67         struct lockdep_map dep_map;
  68 #endif
  69 } seqcount_t;
  70
  71 static inline void __seqcount_init(seqcount_t *s, const char *name,
  72                                           struct lock_class_key *key)
  73 {
  74         /*
  75          * Make sure we are not reinitializing a held lock:
  76          */
  77         lockdep_init_map(&s->dep_map, name, key, 0);
  78         s->sequence = 0;
  79 }
  80
  81 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  82
  83 # define SEQCOUNT_DEP_MAP_INIT(lockname)                                \
  84                 .dep_map = { .name = #lockname }
  85
  86 /**
  87  * seqcount_init() - runtime initializer for seqcount_t
  88  * @s: Pointer to the seqcount_t instance
  89  */
  90 # define seqcount_init(s)                                               \
  91         do {                                                            \
  92                 static struct lock_class_key __key;                     \
  93                 __seqcount_init((s), #s, &__key);                       \
  94         } while (0)
  95
  96 static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
  97 {
  98         seqcount_t *l = (seqcount_t *)s;
  99         unsigned long flags;
 100
 101         local_irq_save(flags);
 102         seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
 103         seqcount_release(&l->dep_map, _RET_IP_);
 104         local_irq_restore(flags);
 105 }
 106
 107 #else
 108 # define SEQCOUNT_DEP_MAP_INIT(lockname)
 109 # define seqcount_init(s) __seqcount_init(s, NULL, NULL)
 110 # define seqcount_lockdep_reader_access(x)
 111 #endif
 112
 113 /**
 114  * SEQCNT_ZERO() - static initializer for seqcount_t
 115  * @name: Name of the seqcount_t instance
 116  */
 117 #define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
 118
 119 /*
 120  * Sequence counters with associated locks (seqcount_LOCKNAME_t)
 121  *
 122  * A sequence counter which associates the lock used for writer
 123  * serialization at initialization time. This enables lockdep to validate
 124  * that the write side critical section is properly serialized.
 125  *
 126  * For associated locks which do not implicitly disable preemption,
 127  * preemption protection is enforced in the write side function.
 128  *
 129  * Lockdep is never used in any for the raw write variants.
 130  *
 131  * See Documentation/locking/seqlock.rst
 132  */
 133
 134 /*
 135  * For PREEMPT_RT, seqcount_LOCKNAME_t write side critical sections cannot
 136  * disable preemption. It can lead to higher latencies, and the write side
 137  * sections will not be able to acquire locks which become sleeping locks
 138  * (e.g. spinlock_t).
 139  *
 140  * To remain preemptible while avoiding a possible livelock caused by the
 141  * reader preempting the writer, use a different technique: let the reader
 142  * detect if a seqcount_LOCKNAME_t writer is in progress. If that is the
 143  * case, acquire then release the associated LOCKNAME writer serialization
 144  * lock. This will allow any possibly-preempted writer to make progress
 145  * until the end of its writer serialization lock critical section.
 146  *
 147  * This lock-unlock technique must be implemented for all of PREEMPT_RT
 148  * sleeping locks.  See Documentation/locking/locktypes.rst
 149  */
 150 #if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT)
 151 #define __SEQ_LOCK(expr)        expr
 152 #else
 153 #define __SEQ_LOCK(expr)
 154 #endif
 155
 156 /*
 157  * typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated
 158  * @seqcount:   The real sequence counter
 159  * @lock:       Pointer to the associated lock
 160  *
 161  * A plain sequence counter with external writer synchronization by
 162  * LOCKNAME @lock. The lock is associated to the sequence counter in the
 163  * static initializer or init function. This enables lockdep to validate
 164  * that the write side critical section is properly serialized.
 165  *
 166  * LOCKNAME:    raw_spinlock, spinlock, rwlock or mutex
 167  */
 168
 169 /*
 170  * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
 171  * @s:          Pointer to the seqcount_LOCKNAME_t instance
 172  * @lock:       Pointer to the associated lock
 173  */
 174
 175 #define seqcount_LOCKNAME_init(s, _lock, lockname)                      \
 176         do {                                                            \
 177                 seqcount_##lockname##_t *____s = (s);                   \
 178                 seqcount_init(&____s->seqcount);                        \
 179                 __SEQ_LOCK(____s->lock = (_lock));                      \
 180         } while (0)
 181
 182 #define seqcount_raw_spinlock_init(s, lock)     seqcount_LOCKNAME_init(s, lock, raw_spinlock)
 183 #define seqcount_spinlock_init(s, lock)         seqcount_LOCKNAME_init(s, lock, spinlock)
 184 #define seqcount_rwlock_init(s, lock)           seqcount_LOCKNAME_init(s, lock, rwlock)
 185 #define seqcount_mutex_init(s, lock)            seqcount_LOCKNAME_init(s, lock, mutex)
 186
 187 /*
 188  * SEQCOUNT_LOCKNAME()  - Instantiate seqcount_LOCKNAME_t and helpers
 189  * seqprop_LOCKNAME_*() - Property accessors for seqcount_LOCKNAME_t
 190  *
 191  * @lockname:           "LOCKNAME" part of seqcount_LOCKNAME_t
 192  * @locktype:           LOCKNAME canonical C data type
 193  * @preemptible:        preemptibility of above locktype
 194  * @lockmember:         argument for lockdep_assert_held()
 195  * @lockbase:           associated lock release function (prefix only)
 196  * @lock_acquire:       associated lock acquisition function (full call)
 197  */
 198 #define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember, lockbase, lock_acquire) \
 199 typedef struct seqcount_##lockname {                                    \
 200         seqcount_t              seqcount;                               \
 201         __SEQ_LOCK(locktype     *lock);                                 \
 202 } seqcount_##lockname##_t;                                              \
 203                                                                         \
 204 static __always_inline seqcount_t *                                     \
 205 __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s)                  \
 206 {                                                                       \
 207         return &s->seqcount;                                            \
 208 }                                                                       \
 209                                                                         \
 210 static __always_inline unsigned                                         \
 211 __seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s)       \
 212 {                                                                       \
 213         unsigned seq = READ_ONCE(s->seqcount.sequence);                 \
 214                                                                         \
 215         if (!IS_ENABLED(CONFIG_PREEMPT_RT))                             \
 216                 return seq;                                             \
 217                                                                         \
 218         if (preemptible && unlikely(seq & 1)) {                         \
 219                 __SEQ_LOCK(lock_acquire);                               \
 220                 __SEQ_LOCK(lockbase##_unlock(s->lock));                 \
 221                                                                         \
 222                 /*                                                      \
 223                  * Re-read the sequence counter since the (possibly     \
 224                  * preempted) writer made progress.                     \
 225                  */                                                     \
 226                 seq = READ_ONCE(s->seqcount.sequence);                  \
 227         }                                                               \
 228                                                                         \
 229         return seq;                                                     \
 230 }                                                                       \
 231                                                                         \
 232 static __always_inline bool                                             \
 233 __seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s)    \
 234 {                                                                       \
 235         if (!IS_ENABLED(CONFIG_PREEMPT_RT))                             \
 236                 return preemptible;                                     \
 237                                                                         \
 238         /* PREEMPT_RT relies on the above LOCK+UNLOCK */                \
 239         return false;                                                   \
 240 }                                                                       \
 241                                                                         \
 242 static __always_inline void                                             \
 243 __seqprop_##lockname##_assert(const seqcount_##lockname##_t *s)         \
 244 {                                                                       \
 245         __SEQ_LOCK(lockdep_assert_held(lockmember));                    \
 246 }
 247
 248 /*
 249  * __seqprop() for seqcount_t
 250  */
 251
 252 static inline seqcount_t *__seqprop_ptr(seqcount_t *s)
 253 {
 254         return s;
 255 }
 256
 257 static inline unsigned __seqprop_sequence(const seqcount_t *s)
 258 {
 259         return READ_ONCE(s->sequence);
 260 }
 261
 262 static inline bool __seqprop_preemptible(const seqcount_t *s)
 263 {
 264         return false;
 265 }
 266
 267 static inline void __seqprop_assert(const seqcount_t *s)
 268 {
 269         lockdep_assert_preemption_disabled();
 270 }
 271
 272 #define __SEQ_RT        IS_ENABLED(CONFIG_PREEMPT_RT)
 273
 274 SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t,  false,    s->lock,        raw_spin, raw_spin_lock(s->lock))
 275 SEQCOUNT_LOCKNAME(spinlock,     spinlock_t,      __SEQ_RT, s->lock,        spin,     spin_lock(s->lock))
 276 SEQCOUNT_LOCKNAME(rwlock,       rwlock_t,        __SEQ_RT, s->lock,        read,     read_lock(s->lock))
 277 SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     s->lock,        mutex,    mutex_lock(s->lock))
 278
 279 /*
 280  * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
 281  * @name:       Name of the seqcount_LOCKNAME_t instance
 282  * @lock:       Pointer to the associated LOCKNAME
 283  */
 284
 285 #define SEQCOUNT_LOCKNAME_ZERO(seq_name, assoc_lock) {                  \
 286         .seqcount               = SEQCNT_ZERO(seq_name.seqcount),       \
 287         __SEQ_LOCK(.lock        = (assoc_lock))                         \
 288 }
 289
 290 #define SEQCNT_RAW_SPINLOCK_ZERO(name, lock)    SEQCOUNT_LOCKNAME_ZERO(name, lock)
 291 #define SEQCNT_SPINLOCK_ZERO(name, lock)        SEQCOUNT_LOCKNAME_ZERO(name, lock)
 292 #define SEQCNT_RWLOCK_ZERO(name, lock)          SEQCOUNT_LOCKNAME_ZERO(name, lock)
 293 #define SEQCNT_MUTEX_ZERO(name, lock)           SEQCOUNT_LOCKNAME_ZERO(name, lock)
 294 #define SEQCNT_WW_MUTEX_ZERO(name, lock)        SEQCOUNT_LOCKNAME_ZERO(name, lock)
 295
 296 #define __seqprop_case(s, lockname, prop)                               \
 297         seqcount_##lockname##_t: __seqprop_##lockname##_##prop((void *)(s))
 298
 299 #define __seqprop(s, prop) _Generic(*(s),                               \
 300         seqcount_t:             __seqprop_##prop((void *)(s)),          \
 301         __seqprop_case((s),     raw_spinlock,   prop),                  \
 302         __seqprop_case((s),     spinlock,       prop),                  \
 303         __seqprop_case((s),     rwlock,         prop),                  \
 304         __seqprop_case((s),     mutex,          prop))
 305
 306 #define seqprop_ptr(s)                  __seqprop(s, ptr)
 307 #define seqprop_sequence(s)             __seqprop(s, sequence)
 308 #define seqprop_preemptible(s)          __seqprop(s, preemptible)
 309 #define seqprop_assert(s)               __seqprop(s, assert)
 310
 311 /**
 312  * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
 313  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 314  *
 315  * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
 316  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 317  * provided before actually loading any of the variables that are to be
 318  * protected in this critical section.
 319  *
 320  * Use carefully, only in critical code, and comment how the barrier is
 321  * provided.
 322  *
 323  * Return: count to be passed to read_seqcount_retry()
 324  */
 325 #define __read_seqcount_begin(s)                                        \
 326 ({                                                                      \
 327         unsigned __seq;                                                 \
 328                                                                         \
 329         while ((__seq = seqprop_sequence(s)) & 1)                       \
 330                 cpu_relax();                                            \
 331                                                                         \
 332         kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);                    \
 333         __seq;                                                          \
 334 })
 335
 336 /**
 337  * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
 338  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 339  *
 340  * Return: count to be passed to read_seqcount_retry()
 341  */
 342 #define raw_read_seqcount_begin(s)                                      \
 343 ({                                                                      \
 344         unsigned _seq = __read_seqcount_begin(s);                       \
 345                                                                         \
 346         smp_rmb();                                                      \
 347         _seq;                                                           \
 348 })
 349
 350 /**
 351  * read_seqcount_begin() - begin a seqcount_t read critical section
 352  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 353  *
 354  * Return: count to be passed to read_seqcount_retry()
 355  */
 356 #define read_seqcount_begin(s)                                          \
 357 ({                                                                      \
 358         seqcount_lockdep_reader_access(seqprop_ptr(s));                 \
 359         raw_read_seqcount_begin(s);                                     \
 360 })
 361
 362 /**
 363  * raw_read_seqcount() - read the raw seqcount_t counter value
 364  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 365  *
 366  * raw_read_seqcount opens a read critical section of the given
 367  * seqcount_t, without any lockdep checking, and without checking or
 368  * masking the sequence counter LSB. Calling code is responsible for
 369  * handling that.
 370  *
 371  * Return: count to be passed to read_seqcount_retry()
 372  */
 373 #define raw_read_seqcount(s)                                            \
 374 ({                                                                      \
 375         unsigned __seq = seqprop_sequence(s);                           \
 376                                                                         \
 377         smp_rmb();                                                      \
 378         kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);                    \
 379         __seq;                                                          \
 380 })
 381
 382 /**
 383  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
 384  *                        lockdep and w/o counter stabilization
 385  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 386  *
 387  * raw_seqcount_begin opens a read critical section of the given
 388  * seqcount_t. Unlike read_seqcount_begin(), this function will not wait
 389  * for the count to stabilize. If a writer is active when it begins, it
 390  * will fail the read_seqcount_retry() at the end of the read critical
 391  * section instead of stabilizing at the beginning of it.
 392  *
 393  * Use this only in special kernel hot paths where the read section is
 394  * small and has a high probability of success through other external
 395  * means. It will save a single branching instruction.
 396  *
 397  * Return: count to be passed to read_seqcount_retry()
 398  */
 399 #define raw_seqcount_begin(s)                                           \
 400 ({                                                                      \
 401         /*                                                              \
 402          * If the counter is odd, let read_seqcount_retry() fail        \
 403          * by decrementing the counter.                                 \
 404          */                                                             \
 405         raw_read_seqcount(s) & ~1;                                      \
 406 })
 407
 408 /**
 409  * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
 410  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 411  * @start: count, from read_seqcount_begin()
 412  *
 413  * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
 414  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 415  * provided before actually loading any of the variables that are to be
 416  * protected in this critical section.
 417  *
 418  * Use carefully, only in critical code, and comment how the barrier is
 419  * provided.
 420  *
 421  * Return: true if a read section retry is required, else false
 422  */
 423 #define __read_seqcount_retry(s, start)                                 \
 424         do___read_seqcount_retry(seqprop_ptr(s), start)
 425
 426 static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
 427 {
 428         kcsan_atomic_next(0);
 429         return unlikely(READ_ONCE(s->sequence) != start);
 430 }
 431
 432 /**
 433  * read_seqcount_retry() - end a seqcount_t read critical section
 434  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 435  * @start: count, from read_seqcount_begin()
 436  *
 437  * read_seqcount_retry closes the read critical section of given
 438  * seqcount_t.  If the critical section was invalid, it must be ignored
 439  * (and typically retried).
 440  *
 441  * Return: true if a read section retry is required, else false
 442  */
 443 #define read_seqcount_retry(s, start)                                   \
 444         do_read_seqcount_retry(seqprop_ptr(s), start)
 445
 446 static inline int do_read_seqcount_retry(const seqcount_t *s, unsigned start)
 447 {
 448         smp_rmb();
 449         return do___read_seqcount_retry(s, start);
 450 }
 451
 452 /**
 453  * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
 454  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 455  *
 456  * Context: check write_seqcount_begin()
 457  */
 458 #define raw_write_seqcount_begin(s)                                     \
 459 do {                                                                    \
 460         if (seqprop_preemptible(s))                                     \
 461                 preempt_disable();                                      \
 462                                                                         \
 463         do_raw_write_seqcount_begin(seqprop_ptr(s));                    \
 464 } while (0)
 465
 466 static inline void do_raw_write_seqcount_begin(seqcount_t *s)
 467 {
 468         kcsan_nestable_atomic_begin();
 469         s->sequence++;
 470         smp_wmb();
 471 }
 472
 473 /**
 474  * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
 475  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 476  *
 477  * Context: check write_seqcount_end()
 478  */
 479 #define raw_write_seqcount_end(s)                                       \
 480 do {                                                                    \
 481         do_raw_write_seqcount_end(seqprop_ptr(s));                      \
 482                                                                         \
 483         if (seqprop_preemptible(s))                                     \
 484                 preempt_enable();                                       \
 485 } while (0)
 486
 487 static inline void do_raw_write_seqcount_end(seqcount_t *s)
 488 {
 489         smp_wmb();
 490         s->sequence++;
 491         kcsan_nestable_atomic_end();
 492 }
 493
 494 /**
 495  * write_seqcount_begin_nested() - start a seqcount_t write section with
 496  *                                 custom lockdep nesting level
 497  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 498  * @subclass: lockdep nesting level
 499  *
 500  * See Documentation/locking/lockdep-design.rst
 501  * Context: check write_seqcount_begin()
 502  */
 503 #define write_seqcount_begin_nested(s, subclass)                        \
 504 do {                                                                    \
 505         seqprop_assert(s);                                              \
 506                                                                         \
 507         if (seqprop_preemptible(s))                                     \
 508                 preempt_disable();                                      \
 509                                                                         \
 510         do_write_seqcount_begin_nested(seqprop_ptr(s), subclass);       \
 511 } while (0)
 512
 513 static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass)
 514 {
 515         do_raw_write_seqcount_begin(s);
 516         seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
 517 }
 518
 519 /**
 520  * write_seqcount_begin() - start a seqcount_t write side critical section
 521  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 522  *
 523  * Context: sequence counter write side sections must be serialized and
 524  * non-preemptible. Preemption will be automatically disabled if and
 525  * only if the seqcount write serialization lock is associated, and
 526  * preemptible.  If readers can be invoked from hardirq or softirq
 527  * context, interrupts or bottom halves must be respectively disabled.
 528  */
 529 #define write_seqcount_begin(s)                                         \
 530 do {                                                                    \
 531         seqprop_assert(s);                                              \
 532                                                                         \
 533         if (seqprop_preemptible(s))                                     \
 534                 preempt_disable();                                      \
 535                                                                         \
 536         do_write_seqcount_begin(seqprop_ptr(s));                        \
 537 } while (0)
 538
 539 static inline void do_write_seqcount_begin(seqcount_t *s)
 540 {
 541         do_write_seqcount_begin_nested(s, 0);
 542 }
 543
 544 /**
 545  * write_seqcount_end() - end a seqcount_t write side critical section
 546  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 547  *
 548  * Context: Preemption will be automatically re-enabled if and only if
 549  * the seqcount write serialization lock is associated, and preemptible.
 550  */
 551 #define write_seqcount_end(s)                                           \
 552 do {                                                                    \
 553         do_write_seqcount_end(seqprop_ptr(s));                          \
 554                                                                         \
 555         if (seqprop_preemptible(s))                                     \
 556                 preempt_enable();                                       \
 557 } while (0)
 558
 559 static inline void do_write_seqcount_end(seqcount_t *s)
 560 {
 561         seqcount_release(&s->dep_map, _RET_IP_);
 562         do_raw_write_seqcount_end(s);
 563 }
 564
 565 /**
 566  * raw_write_seqcount_barrier() - do a seqcount_t write barrier
 567  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 568  *
 569  * This can be used to provide an ordering guarantee instead of the usual
 570  * consistency guarantee. It is one wmb cheaper, because it can collapse
 571  * the two back-to-back wmb()s.
 572  *
 573  * Note that writes surrounding the barrier should be declared atomic (e.g.
 574  * via WRITE_ONCE): a) to ensure the writes become visible to other threads
 575  * atomically, avoiding compiler optimizations; b) to document which writes are
 576  * meant to propagate to the reader critical section. This is necessary because
 577  * neither writes before and after the barrier are enclosed in a seq-writer
 578  * critical section that would ensure readers are aware of ongoing writes::
 579  *
 580  *      seqcount_t seq;
 581  *      bool X = true, Y = false;
 582  *
 583  *      void read(void)
 584  *      {
 585  *              bool x, y;
 586  *
 587  *              do {
 588  *                      int s = read_seqcount_begin(&seq);
 589  *
 590  *                      x = X; y = Y;
 591  *
 592  *              } while (read_seqcount_retry(&seq, s));
 593  *
 594  *              BUG_ON(!x && !y);
 595  *      }
 596  *
 597  *      void write(void)
 598  *      {
 599  *              WRITE_ONCE(Y, true);
 600  *
 601  *              raw_write_seqcount_barrier(seq);
 602  *
 603  *              WRITE_ONCE(X, false);
 604  *      }
 605  */
 606 #define raw_write_seqcount_barrier(s)                                   \
 607         do_raw_write_seqcount_barrier(seqprop_ptr(s))
 608
 609 static inline void do_raw_write_seqcount_barrier(seqcount_t *s)
 610 {
 611         kcsan_nestable_atomic_begin();
 612         s->sequence++;
 613         smp_wmb();
 614         s->sequence++;
 615         kcsan_nestable_atomic_end();
 616 }
 617
 618 /**
 619  * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
 620  *                               side operations
 621  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 622  *
 623  * After write_seqcount_invalidate, no seqcount_t read side operations
 624  * will complete successfully and see data older than this.
 625  */
 626 #define write_seqcount_invalidate(s)                                    \
 627         do_write_seqcount_invalidate(seqprop_ptr(s))
 628
 629 static inline void do_write_seqcount_invalidate(seqcount_t *s)
 630 {
 631         smp_wmb();
 632         kcsan_nestable_atomic_begin();
 633         s->sequence+=2;
 634         kcsan_nestable_atomic_end();
 635 }
 636
 637 /*
 638  * Latch sequence counters (seqcount_latch_t)
 639  *
 640  * A sequence counter variant where the counter even/odd value is used to
 641  * switch between two copies of protected data. This allows the read path,
 642  * typically NMIs, to safely interrupt the write side critical section.
 643  *
 644  * As the write sections are fully preemptible, no special handling for
 645  * PREEMPT_RT is needed.
 646  */
 647 typedef struct {
 648         seqcount_t seqcount;
 649 } seqcount_latch_t;
 650
 651 /**
 652  * SEQCNT_LATCH_ZERO() - static initializer for seqcount_latch_t
 653  * @seq_name: Name of the seqcount_latch_t instance
 654  */
 655 #define SEQCNT_LATCH_ZERO(seq_name) {                                   \
 656         .seqcount               = SEQCNT_ZERO(seq_name.seqcount),       \
 657 }
 658
 659 /**
 660  * seqcount_latch_init() - runtime initializer for seqcount_latch_t
 661  * @s: Pointer to the seqcount_latch_t instance
 662  */
 663 #define seqcount_latch_init(s) seqcount_init(&(s)->seqcount)
 664
 665 /**
 666  * raw_read_seqcount_latch() - pick even/odd latch data copy
 667  * @s: Pointer to seqcount_latch_t
 668  *
 669  * See raw_write_seqcount_latch() for details and a full reader/writer
 670  * usage example.
 671  *
 672  * Return: sequence counter raw value. Use the lowest bit as an index for
 673  * picking which data copy to read. The full counter must then be checked
 674  * with read_seqcount_latch_retry().
 675  */
 676 static inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
 677 {
 678         /*
 679          * Pairs with the first smp_wmb() in raw_write_seqcount_latch().
 680          * Due to the dependent load, a full smp_rmb() is not needed.
 681          */
 682         return READ_ONCE(s->seqcount.sequence);
 683 }
 684
 685 /**
 686  * read_seqcount_latch_retry() - end a seqcount_latch_t read section
 687  * @s:          Pointer to seqcount_latch_t
 688  * @start:      count, from raw_read_seqcount_latch()
 689  *
 690  * Return: true if a read section retry is required, else false
 691  */
 692 static inline int
 693 read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
 694 {
 695         return read_seqcount_retry(&s->seqcount, start);
 696 }
 697
 698 /**
 699  * raw_write_seqcount_latch() - redirect latch readers to even/odd copy
 700  * @s: Pointer to seqcount_latch_t
 701  *
 702  * The latch technique is a multiversion concurrency control method that allows
 703  * queries during non-atomic modifications. If you can guarantee queries never
 704  * interrupt the modification -- e.g. the concurrency is strictly between CPUs
 705  * -- you most likely do not need this.
 706  *
 707  * Where the traditional RCU/lockless data structures rely on atomic
 708  * modifications to ensure queries observe either the old or the new state the
 709  * latch allows the same for non-atomic updates. The trade-off is doubling the
 710  * cost of storage; we have to maintain two copies of the entire data
 711  * structure.
 712  *
 713  * Very simply put: we first modify one copy and then the other. This ensures
 714  * there is always one copy in a stable state, ready to give us an answer.
 715  *
 716  * The basic form is a data structure like::
 717  *
 718  *      struct latch_struct {
 719  *              seqcount_latch_t        seq;
 720  *              struct data_struct      data[2];
 721  *      };
 722  *
 723  * Where a modification, which is assumed to be externally serialized, does the
 724  * following::
 725  *
 726  *      void latch_modify(struct latch_struct *latch, ...)
 727  *      {
 728  *              smp_wmb();      // Ensure that the last data[1] update is visible
 729  *              latch->seq.sequence++;
 730  *              smp_wmb();      // Ensure that the seqcount update is visible
 731  *
 732  *              modify(latch->data[0], ...);
 733  *
 734  *              smp_wmb();      // Ensure that the data[0] update is visible
 735  *              latch->seq.sequence++;
 736  *              smp_wmb();      // Ensure that the seqcount update is visible
 737  *
 738  *              modify(latch->data[1], ...);
 739  *      }
 740  *
 741  * The query will have a form like::
 742  *
 743  *      struct entry *latch_query(struct latch_struct *latch, ...)
 744  *      {
 745  *              struct entry *entry;
 746  *              unsigned seq, idx;
 747  *
 748  *              do {
 749  *                      seq = raw_read_seqcount_latch(&latch->seq);
 750  *
 751  *                      idx = seq & 0x01;
 752  *                      entry = data_query(latch->data[idx], ...);
 753  *
 754  *              // This includes needed smp_rmb()
 755  *              } while (read_seqcount_latch_retry(&latch->seq, seq));
 756  *
 757  *              return entry;
 758  *      }
 759  *
 760  * So during the modification, queries are first redirected to data[1]. Then we
 761  * modify data[0]. When that is complete, we redirect queries back to data[0]
 762  * and we can modify data[1].
 763  *
 764  * NOTE:
 765  *
 766  *      The non-requirement for atomic modifications does _NOT_ include
 767  *      the publishing of new entries in the case where data is a dynamic
 768  *      data structure.
 769  *
 770  *      An iteration might start in data[0] and get suspended long enough
 771  *      to miss an entire modification sequence, once it resumes it might
 772  *      observe the new entry.
 773  *
 774  * NOTE2:
 775  *
 776  *      When data is a dynamic data structure; one should use regular RCU
 777  *      patterns to manage the lifetimes of the objects within.
 778  */
 779 static inline void raw_write_seqcount_latch(seqcount_latch_t *s)
 780 {
 781         smp_wmb();      /* prior stores before incrementing "sequence" */
 782         s->seqcount.sequence++;
 783         smp_wmb();      /* increment "sequence" before following stores */
 784 }
 785
 786 /*
 787  * Sequential locks (seqlock_t)
 788  *
 789  * Sequence counters with an embedded spinlock for writer serialization
 790  * and non-preemptibility.
 791  *
 792  * For more info, see:
 793  *    - Comments on top of seqcount_t
 794  *    - Documentation/locking/seqlock.rst
 795  */
 796 typedef struct {
 797         /*
 798          * Make sure that readers don't starve writers on PREEMPT_RT: use
 799          * seqcount_spinlock_t instead of seqcount_t. Check __SEQ_LOCK().
 800          */
 801         seqcount_spinlock_t seqcount;
 802         spinlock_t lock;
 803 } seqlock_t;
 804
 805 #define __SEQLOCK_UNLOCKED(lockname)                                    \
 806         {                                                               \
 807                 .seqcount = SEQCNT_SPINLOCK_ZERO(lockname, &(lockname).lock), \
 808                 .lock = __SPIN_LOCK_UNLOCKED(lockname)                  \
 809         }
 810
 811 /**
 812  * seqlock_init() - dynamic initializer for seqlock_t
 813  * @sl: Pointer to the seqlock_t instance
 814  */
 815 #define seqlock_init(sl)                                                \
 816         do {                                                            \
 817                 spin_lock_init(&(sl)->lock);                            \
 818                 seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock);   \
 819         } while (0)
 820
 821 /**
 822  * DEFINE_SEQLOCK(sl) - Define a statically allocated seqlock_t
 823  * @sl: Name of the seqlock_t instance
 824  */
 825 #define DEFINE_SEQLOCK(sl) \
 826                 seqlock_t sl = __SEQLOCK_UNLOCKED(sl)
 827
 828 /**
 829  * read_seqbegin() - start a seqlock_t read side critical section
 830  * @sl: Pointer to seqlock_t
 831  *
 832  * Return: count, to be passed to read_seqretry()
 833  */
 834 static inline unsigned read_seqbegin(const seqlock_t *sl)
 835 {
 836         unsigned ret = read_seqcount_begin(&sl->seqcount);
 837
 838         kcsan_atomic_next(0);  /* non-raw usage, assume closing read_seqretry() */
 839         kcsan_flat_atomic_begin();
 840         return ret;
 841 }
 842
 843 /**
 844  * read_seqretry() - end a seqlock_t read side section
 845  * @sl: Pointer to seqlock_t
 846  * @start: count, from read_seqbegin()
 847  *
 848  * read_seqretry closes the read side critical section of given seqlock_t.
 849  * If the critical section was invalid, it must be ignored (and typically
 850  * retried).
 851  *
 852  * Return: true if a read section retry is required, else false
 853  */
 854 static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
 855 {
 856         /*
 857          * Assume not nested: read_seqretry() may be called multiple times when
 858          * completing read critical section.
 859          */
 860         kcsan_flat_atomic_end();
 861
 862         return read_seqcount_retry(&sl->seqcount, start);
 863 }
 864
 865 /*
 866  * For all seqlock_t write side functions, use the the internal
 867  * do_write_seqcount_begin() instead of generic write_seqcount_begin().
 868  * This way, no redundant lockdep_assert_held() checks are added.
 869  */
 870
 871 /**
 872  * write_seqlock() - start a seqlock_t write side critical section
 873  * @sl: Pointer to seqlock_t
 874  *
 875  * write_seqlock opens a write side critical section for the given
 876  * seqlock_t.  It also implicitly acquires the spinlock_t embedded inside
 877  * that sequential lock. All seqlock_t write side sections are thus
 878  * automatically serialized and non-preemptible.
 879  *
 880  * Context: if the seqlock_t read section, or other write side critical
 881  * sections, can be invoked from hardirq or softirq contexts, use the
 882  * _irqsave or _bh variants of this function instead.
 883  */
 884 static inline void write_seqlock(seqlock_t *sl)
 885 {
 886         spin_lock(&sl->lock);
 887         do_write_seqcount_begin(&sl->seqcount.seqcount);
 888 }
 889
 890 /**
 891  * write_sequnlock() - end a seqlock_t write side critical section
 892  * @sl: Pointer to seqlock_t
 893  *
 894  * write_sequnlock closes the (serialized and non-preemptible) write side
 895  * critical section of given seqlock_t.
 896  */
 897 static inline void write_sequnlock(seqlock_t *sl)
 898 {
 899         do_write_seqcount_end(&sl->seqcount.seqcount);
 900         spin_unlock(&sl->lock);
 901 }
 902
 903 /**
 904  * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
 905  * @sl: Pointer to seqlock_t
 906  *
 907  * _bh variant of write_seqlock(). Use only if the read side section, or
 908  * other write side sections, can be invoked from softirq contexts.
 909  */
 910 static inline void write_seqlock_bh(seqlock_t *sl)
 911 {
 912         spin_lock_bh(&sl->lock);
 913         do_write_seqcount_begin(&sl->seqcount.seqcount);
 914 }
 915
 916 /**
 917  * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
 918  * @sl: Pointer to seqlock_t
 919  *
 920  * write_sequnlock_bh closes the serialized, non-preemptible, and
 921  * softirqs-disabled, seqlock_t write side critical section opened with
 922  * write_seqlock_bh().
 923  */
 924 static inline void write_sequnlock_bh(seqlock_t *sl)
 925 {
 926         do_write_seqcount_end(&sl->seqcount.seqcount);
 927         spin_unlock_bh(&sl->lock);
 928 }
 929
 930 /**
 931  * write_seqlock_irq() - start a non-interruptible seqlock_t write section
 932  * @sl: Pointer to seqlock_t
 933  *
 934  * _irq variant of write_seqlock(). Use only if the read side section, or
 935  * other write sections, can be invoked from hardirq contexts.
 936  */
 937 static inline void write_seqlock_irq(seqlock_t *sl)
 938 {
 939         spin_lock_irq(&sl->lock);
 940         do_write_seqcount_begin(&sl->seqcount.seqcount);
 941 }
 942
 943 /**
 944  * write_sequnlock_irq() - end a non-interruptible seqlock_t write section
 945  * @sl: Pointer to seqlock_t
 946  *
 947  * write_sequnlock_irq closes the serialized and non-interruptible
 948  * seqlock_t write side section opened with write_seqlock_irq().
 949  */
 950 static inline void write_sequnlock_irq(seqlock_t *sl)
 951 {
 952         do_write_seqcount_end(&sl->seqcount.seqcount);
 953         spin_unlock_irq(&sl->lock);
 954 }
 955
 956 static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
 957 {
 958         unsigned long flags;
 959
 960         spin_lock_irqsave(&sl->lock, flags);
 961         do_write_seqcount_begin(&sl->seqcount.seqcount);
 962         return flags;
 963 }
 964
 965 /**
 966  * write_seqlock_irqsave() - start a non-interruptible seqlock_t write
 967  *                           section
 968  * @lock:  Pointer to seqlock_t
 969  * @flags: Stack-allocated storage for saving caller's local interrupt
 970  *         state, to be passed to write_sequnlock_irqrestore().
 971  *
 972  * _irqsave variant of write_seqlock(). Use it only if the read side
 973  * section, or other write sections, can be invoked from hardirq context.
 974  */
 975 #define write_seqlock_irqsave(lock, flags)                              \
 976         do { flags = __write_seqlock_irqsave(lock); } while (0)
 977
 978 /**
 979  * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write
 980  *                                section
 981  * @sl:    Pointer to seqlock_t
 982  * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
 983  *
 984  * write_sequnlock_irqrestore closes the serialized and non-interruptible
 985  * seqlock_t write section previously opened with write_seqlock_irqsave().
 986  */
 987 static inline void
 988 write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
 989 {
 990         do_write_seqcount_end(&sl->seqcount.seqcount);
 991         spin_unlock_irqrestore(&sl->lock, flags);
 992 }
 993
 994 /**
 995  * read_seqlock_excl() - begin a seqlock_t locking reader section
 996  * @sl: Pointer to seqlock_t
 997  *
 998  * read_seqlock_excl opens a seqlock_t locking reader critical section.  A
 999  * locking reader exclusively locks out *both* other writers *and* other
1000  * locking readers, but it does not update the embedded sequence number.
1001  *
1002  * Locking readers act like a normal spin_lock()/spin_unlock().
1003  *
1004  * Context: if the seqlock_t write section, *or other read sections*, can
1005  * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
1006  * variant of this function instead.
1007  *
1008  * The opened read section must be closed with read_sequnlock_excl().
1009  */
1010 static inline void read_seqlock_excl(seqlock_t *sl)
1011 {
1012         spin_lock(&sl->lock);
1013 }
1014
1015 /**
1016  * read_sequnlock_excl() - end a seqlock_t locking reader critical section
1017  * @sl: Pointer to seqlock_t
1018  */
1019 static inline void read_sequnlock_excl(seqlock_t *sl)
1020 {
1021         spin_unlock(&sl->lock);
1022 }
1023
1024 /**
1025  * read_seqlock_excl_bh() - start a seqlock_t locking reader section with
1026  *                          softirqs disabled
1027  * @sl: Pointer to seqlock_t
1028  *
1029  * _bh variant of read_seqlock_excl(). Use this variant only if the
1030  * seqlock_t write side section, *or other read sections*, can be invoked
1031  * from softirq contexts.
1032  */
1033 static inline void read_seqlock_excl_bh(seqlock_t *sl)
1034 {
1035         spin_lock_bh(&sl->lock);
1036 }
1037
1038 /**
1039  * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
1040  *                            reader section
1041  * @sl: Pointer to seqlock_t
1042  */
1043 static inline void read_sequnlock_excl_bh(seqlock_t *sl)
1044 {
1045         spin_unlock_bh(&sl->lock);
1046 }
1047
1048 /**
1049  * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
1050  *                           reader section
1051  * @sl: Pointer to seqlock_t
1052  *
1053  * _irq variant of read_seqlock_excl(). Use this only if the seqlock_t
1054  * write side section, *or other read sections*, can be invoked from a
1055  * hardirq context.
1056  */
1057 static inline void read_seqlock_excl_irq(seqlock_t *sl)
1058 {
1059         spin_lock_irq(&sl->lock);
1060 }
1061
1062 /**
1063  * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
1064  *                             locking reader section
1065  * @sl: Pointer to seqlock_t
1066  */
1067 static inline void read_sequnlock_excl_irq(seqlock_t *sl)
1068 {
1069         spin_unlock_irq(&sl->lock);
1070 }
1071
1072 static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
1073 {
1074         unsigned long flags;
1075
1076         spin_lock_irqsave(&sl->lock, flags);
1077         return flags;
1078 }
1079
1080 /**
1081  * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
1082  *                               locking reader section
1083  * @lock:  Pointer to seqlock_t
1084  * @flags: Stack-allocated storage for saving caller's local interrupt
1085  *         state, to be passed to read_sequnlock_excl_irqrestore().
1086  *
1087  * _irqsave variant of read_seqlock_excl(). Use this only if the seqlock_t
1088  * write side section, *or other read sections*, can be invoked from a
1089  * hardirq context.
1090  */
1091 #define read_seqlock_excl_irqsave(lock, flags)                          \
1092         do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
1093
1094 /**
1095  * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
1096  *                                    locking reader section
1097  * @sl:    Pointer to seqlock_t
1098  * @flags: Caller saved interrupt state, from read_seqlock_excl_irqsave()
1099  */
1100 static inline void
1101 read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
1102 {
1103         spin_unlock_irqrestore(&sl->lock, flags);
1104 }
1105
1106 /**
1107  * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
1108  * @lock: Pointer to seqlock_t
1109  * @seq : Marker and return parameter. If the passed value is even, the
1110  * reader will become a *lockless* seqlock_t reader as in read_seqbegin().
1111  * If the passed value is odd, the reader will become a *locking* reader
1112  * as in read_seqlock_excl().  In the first call to this function, the
1113  * caller *must* initialize and pass an even value to @seq; this way, a
1114  * lockless read can be optimistically tried first.
1115  *
1116  * read_seqbegin_or_lock is an API designed to optimistically try a normal
1117  * lockless seqlock_t read section first.  If an odd counter is found, the
1118  * lockless read trial has failed, and the next read iteration transforms
1119  * itself into a full seqlock_t locking reader.
1120  *
1121  * This is typically used to avoid seqlock_t lockless readers starvation
1122  * (too much retry loops) in the case of a sharp spike in write side
1123  * activity.
1124  *
1125  * Context: if the seqlock_t write section, *or other read sections*, can
1126  * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
1127  * variant of this function instead.
1128  *
1129  * Check Documentation/locking/seqlock.rst for template example code.
1130  *
1131  * Return: the encountered sequence counter value, through the @seq
1132  * parameter, which is overloaded as a return parameter. This returned
1133  * value must be checked with need_seqretry(). If the read section need to
1134  * be retried, this returned value must also be passed as the @seq
1135  * parameter of the next read_seqbegin_or_lock() iteration.
1136  */
1137 static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
1138 {
1139         if (!(*seq & 1))        /* Even */
1140                 *seq = read_seqbegin(lock);
1141         else                    /* Odd */
1142                 read_seqlock_excl(lock);
1143 }
1144
1145 /**
1146  * need_seqretry() - validate seqlock_t "locking or lockless" read section
1147  * @lock: Pointer to seqlock_t
1148  * @seq: sequence count, from read_seqbegin_or_lock()
1149  *
1150  * Return: true if a read section retry is required, false otherwise
1151  */
1152 static inline int need_seqretry(seqlock_t *lock, int seq)
1153 {
1154         return !(seq & 1) && read_seqretry(lock, seq);
1155 }
1156
1157 /**
1158  * done_seqretry() - end seqlock_t "locking or lockless" reader section
1159  * @lock: Pointer to seqlock_t
1160  * @seq: count, from read_seqbegin_or_lock()
1161  *
1162  * done_seqretry finishes the seqlock_t read side critical section started
1163  * with read_seqbegin_or_lock() and validated by need_seqretry().
1164  */
1165 static inline void done_seqretry(seqlock_t *lock, int seq)
1166 {
1167         if (seq & 1)
1168                 read_sequnlock_excl(lock);
1169 }
1170
1171 /**
1172  * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
1173  *                                   a non-interruptible locking reader
1174  * @lock: Pointer to seqlock_t
1175  * @seq:  Marker and return parameter. Check read_seqbegin_or_lock().
1176  *
1177  * This is the _irqsave variant of read_seqbegin_or_lock(). Use it only if
1178  * the seqlock_t write section, *or other read sections*, can be invoked
1179  * from hardirq context.
1180  *
1181  * Note: Interrupts will be disabled only for "locking reader" mode.
1182  *
1183  * Return:
1184  *
1185  *   1. The saved local interrupts state in case of a locking reader, to
1186  *      be passed to done_seqretry_irqrestore().
1187  *
1188  *   2. The encountered sequence counter value, returned through @seq
1189  *      overloaded as a return parameter. Check read_seqbegin_or_lock().
1190  */
1191 static inline unsigned long
1192 read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
1193 {
1194         unsigned long flags = 0;
1195
1196         if (!(*seq & 1))        /* Even */
1197                 *seq = read_seqbegin(lock);
1198         else                    /* Odd */
1199                 read_seqlock_excl_irqsave(lock, flags);
1200
1201         return flags;
1202 }
1203
1204 /**
1205  * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
1206  *                              non-interruptible locking reader section
1207  * @lock:  Pointer to seqlock_t
1208  * @seq:   Count, from read_seqbegin_or_lock_irqsave()
1209  * @flags: Caller's saved local interrupt state in case of a locking
1210  *         reader, also from read_seqbegin_or_lock_irqsave()
1211  *
1212  * This is the _irqrestore variant of done_seqretry(). The read section
1213  * must've been opened with read_seqbegin_or_lock_irqsave(), and validated
1214  * by need_seqretry().
1215  */
1216 static inline void
1217 done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
1218 {
1219         if (seq & 1)
1220                 read_sequnlock_excl_irqrestore(lock, flags);
1221 }
1222 #endif /* __LINUX_SEQLOCK_H */