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/ww_mutex.h>
  21 #include <linux/preempt.h>
  22 #include <linux/spinlock.h>
  23
  24 #include <asm/processor.h>
  25
  26 /*
  27  * The seqlock seqcount_t interface does not prescribe a precise sequence of
  28  * read begin/retry/end. For readers, typically there is a call to
  29  * read_seqcount_begin() and read_seqcount_retry(), however, there are more
  30  * esoteric cases which do not follow this pattern.
  31  *
  32  * As a consequence, we take the following best-effort approach for raw usage
  33  * via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
  34  * pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
  35  * atomics; if there is a matching read_seqcount_retry() call, no following
  36  * memory operations are considered atomic. Usage of the seqlock_t interface
  37  * is not affected.
  38  */
  39 #define KCSAN_SEQLOCK_REGION_MAX 1000
  40
  41 /*
  42  * Sequence counters (seqcount_t)
  43  *
  44  * This is the raw counting mechanism, without any writer protection.
  45  *
  46  * Write side critical sections must be serialized and non-preemptible.
  47  *
  48  * If readers can be invoked from hardirq or softirq contexts,
  49  * interrupts or bottom halves must also be respectively disabled before
  50  * entering the write section.
  51  *
  52  * This mechanism can't be used if the protected data contains pointers,
  53  * as the writer can invalidate a pointer that a reader is following.
  54  *
  55  * If the write serialization mechanism is one of the common kernel
  56  * locking primitives, use a sequence counter with associated lock
  57  * (seqcount_LOCKNAME_t) instead.
  58  *
  59  * If it's desired to automatically handle the sequence counter writer
  60  * serialization and non-preemptibility requirements, use a sequential
  61  * lock (seqlock_t) instead.
  62  *
  63  * See Documentation/locking/seqlock.rst
  64  */
  65 typedef struct seqcount {
  66         unsigned sequence;
  67 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  68         struct lockdep_map dep_map;
  69 #endif
  70 } seqcount_t;
  71
  72 static inline void __seqcount_init(seqcount_t *s, const char *name,
  73                                           struct lock_class_key *key)
  74 {
  75         /*
  76          * Make sure we are not reinitializing a held lock:
  77          */
  78         lockdep_init_map(&s->dep_map, name, key, 0);
  79         s->sequence = 0;
  80 }
  81
  82 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  83
  84 # define SEQCOUNT_DEP_MAP_INIT(lockname)                                \
  85                 .dep_map = { .name = #lockname }
  86
  87 /**
  88  * seqcount_init() - runtime initializer for seqcount_t
  89  * @s: Pointer to the seqcount_t instance
  90  */
  91 # define seqcount_init(s)                                               \
  92         do {                                                            \
  93                 static struct lock_class_key __key;                     \
  94                 __seqcount_init((s), #s, &__key);                       \
  95         } while (0)
  96
  97 static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
  98 {
  99         seqcount_t *l = (seqcount_t *)s;
 100         unsigned long flags;
 101
 102         local_irq_save(flags);
 103         seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
 104         seqcount_release(&l->dep_map, _RET_IP_);
 105         local_irq_restore(flags);
 106 }
 107
 108 #else
 109 # define SEQCOUNT_DEP_MAP_INIT(lockname)
 110 # define seqcount_init(s) __seqcount_init(s, NULL, NULL)
 111 # define seqcount_lockdep_reader_access(x)
 112 #endif
 113
 114 /**
 115  * SEQCNT_ZERO() - static initializer for seqcount_t
 116  * @name: Name of the seqcount_t instance
 117  */
 118 #define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
 119
 120 /*
 121  * Sequence counters with associated locks (seqcount_LOCKNAME_t)
 122  *
 123  * A sequence counter which associates the lock used for writer
 124  * serialization at initialization time. This enables lockdep to validate
 125  * that the write side critical section is properly serialized.
 126  *
 127  * For associated locks which do not implicitly disable preemption,
 128  * preemption protection is enforced in the write side function.
 129  *
 130  * Lockdep is never used in any for the raw write variants.
 131  *
 132  * See Documentation/locking/seqlock.rst
 133  */
 134
 135 /*
 136  * For PREEMPT_RT, seqcount_LOCKNAME_t write side critical sections cannot
 137  * disable preemption. It can lead to higher latencies, and the write side
 138  * sections will not be able to acquire locks which become sleeping locks
 139  * (e.g. spinlock_t).
 140  *
 141  * To remain preemptible while avoiding a possible livelock caused by the
 142  * reader preempting the writer, use a different technique: let the reader
 143  * detect if a seqcount_LOCKNAME_t writer is in progress. If that is the
 144  * case, acquire then release the associated LOCKNAME writer serialization
 145  * lock. This will allow any possibly-preempted writer to make progress
 146  * until the end of its writer serialization lock critical section.
 147  *
 148  * This lock-unlock technique must be implemented for all of PREEMPT_RT
 149  * sleeping locks.  See Documentation/locking/locktypes.rst
 150  */
 151 #if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT)
 152 #define __SEQ_LOCK(expr)        expr
 153 #else
 154 #define __SEQ_LOCK(expr)
 155 #endif
 156
 157 /*
 158  * typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated
 159  * @seqcount:   The real sequence counter
 160  * @lock:       Pointer to the associated lock
 161  *
 162  * A plain sequence counter with external writer synchronization by
 163  * LOCKNAME @lock. The lock is associated to the sequence counter in the
 164  * static initializer or init function. This enables lockdep to validate
 165  * that the write side critical section is properly serialized.
 166  *
 167  * LOCKNAME:    raw_spinlock, spinlock, rwlock, mutex, or ww_mutex.
 168  */
 169
 170 /*
 171  * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
 172  * @s:          Pointer to the seqcount_LOCKNAME_t instance
 173  * @lock:       Pointer to the associated lock
 174  */
 175
 176 #define seqcount_LOCKNAME_init(s, _lock, lockname)                      \
 177         do {                                                            \
 178                 seqcount_##lockname##_t *____s = (s);                   \
 179                 seqcount_init(&____s->seqcount);                        \
 180                 __SEQ_LOCK(____s->lock = (_lock));                      \
 181         } while (0)
 182
 183 #define seqcount_raw_spinlock_init(s, lock)     seqcount_LOCKNAME_init(s, lock, raw_spinlock)
 184 #define seqcount_spinlock_init(s, lock)         seqcount_LOCKNAME_init(s, lock, spinlock)
 185 #define seqcount_rwlock_init(s, lock)           seqcount_LOCKNAME_init(s, lock, rwlock)
 186 #define seqcount_mutex_init(s, lock)            seqcount_LOCKNAME_init(s, lock, mutex)
 187 #define seqcount_ww_mutex_init(s, lock)         seqcount_LOCKNAME_init(s, lock, ww_mutex)
 188
 189 /*
 190  * SEQCOUNT_LOCKNAME()  - Instantiate seqcount_LOCKNAME_t and helpers
 191  * seqprop_LOCKNAME_*() - Property accessors for seqcount_LOCKNAME_t
 192  *
 193  * @lockname:           "LOCKNAME" part of seqcount_LOCKNAME_t
 194  * @locktype:           LOCKNAME canonical C data type
 195  * @preemptible:        preemptibility of above locktype
 196  * @lockmember:         argument for lockdep_assert_held()
 197  * @lockbase:           associated lock release function (prefix only)
 198  * @lock_acquire:       associated lock acquisition function (full call)
 199  */
 200 #define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember, lockbase, lock_acquire) \
 201 typedef struct seqcount_##lockname {                                    \
 202         seqcount_t              seqcount;                               \
 203         __SEQ_LOCK(locktype     *lock);                                 \
 204 } seqcount_##lockname##_t;                                              \
 205                                                                         \
 206 static __always_inline seqcount_t *                                     \
 207 __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s)                  \
 208 {                                                                       \
 209         return &s->seqcount;                                            \
 210 }                                                                       \
 211                                                                         \
 212 static __always_inline unsigned                                         \
 213 __seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s)       \
 214 {                                                                       \
 215         unsigned seq = READ_ONCE(s->seqcount.sequence);                 \
 216                                                                         \
 217         if (!IS_ENABLED(CONFIG_PREEMPT_RT))                             \
 218                 return seq;                                             \
 219                                                                         \
 220         if (preemptible && unlikely(seq & 1)) {                         \
 221                 __SEQ_LOCK(lock_acquire);                               \
 222                 __SEQ_LOCK(lockbase##_unlock(s->lock));                 \
 223                                                                         \
 224                 /*                                                      \
 225                  * Re-read the sequence counter since the (possibly     \
 226                  * preempted) writer made progress.                     \
 227                  */                                                     \
 228                 seq = READ_ONCE(s->seqcount.sequence);                  \
 229         }                                                               \
 230                                                                         \
 231         return seq;                                                     \
 232 }                                                                       \
 233                                                                         \
 234 static __always_inline bool                                             \
 235 __seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s)    \
 236 {                                                                       \
 237         if (!IS_ENABLED(CONFIG_PREEMPT_RT))                             \
 238                 return preemptible;                                     \
 239                                                                         \
 240         /* PREEMPT_RT relies on the above LOCK+UNLOCK */                \
 241         return false;                                                   \
 242 }                                                                       \
 243                                                                         \
 244 static __always_inline void                                             \
 245 __seqprop_##lockname##_assert(const seqcount_##lockname##_t *s)         \
 246 {                                                                       \
 247         __SEQ_LOCK(lockdep_assert_held(lockmember));                    \
 248 }
 249
 250 /*
 251  * __seqprop() for seqcount_t
 252  */
 253
 254 static inline seqcount_t *__seqprop_ptr(seqcount_t *s)
 255 {
 256         return s;
 257 }
 258
 259 static inline unsigned __seqprop_sequence(const seqcount_t *s)
 260 {
 261         return READ_ONCE(s->sequence);
 262 }
 263
 264 static inline bool __seqprop_preemptible(const seqcount_t *s)
 265 {
 266         return false;
 267 }
 268
 269 static inline void __seqprop_assert(const seqcount_t *s)
 270 {
 271         lockdep_assert_preemption_disabled();
 272 }
 273
 274 #define __SEQ_RT        IS_ENABLED(CONFIG_PREEMPT_RT)
 275
 276 SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t,  false,    s->lock,        raw_spin, raw_spin_lock(s->lock))
 277 SEQCOUNT_LOCKNAME(spinlock,     spinlock_t,      __SEQ_RT, s->lock,        spin,     spin_lock(s->lock))
 278 SEQCOUNT_LOCKNAME(rwlock,       rwlock_t,        __SEQ_RT, s->lock,        read,     read_lock(s->lock))
 279 SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     s->lock,        mutex,    mutex_lock(s->lock))
 280 SEQCOUNT_LOCKNAME(ww_mutex,     struct ww_mutex, true,     &s->lock->base, ww_mutex, ww_mutex_lock(s->lock, NULL))
 281
 282 /*
 283  * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
 284  * @name:       Name of the seqcount_LOCKNAME_t instance
 285  * @lock:       Pointer to the associated LOCKNAME
 286  */
 287
 288 #define SEQCOUNT_LOCKNAME_ZERO(seq_name, assoc_lock) {                  \
 289         .seqcount               = SEQCNT_ZERO(seq_name.seqcount),       \
 290         __SEQ_LOCK(.lock        = (assoc_lock))                         \
 291 }
 292
 293 #define SEQCNT_RAW_SPINLOCK_ZERO(name, lock)    SEQCOUNT_LOCKNAME_ZERO(name, lock)
 294 #define SEQCNT_SPINLOCK_ZERO(name, lock)        SEQCOUNT_LOCKNAME_ZERO(name, lock)
 295 #define SEQCNT_RWLOCK_ZERO(name, lock)          SEQCOUNT_LOCKNAME_ZERO(name, lock)
 296 #define SEQCNT_MUTEX_ZERO(name, lock)           SEQCOUNT_LOCKNAME_ZERO(name, lock)
 297 #define SEQCNT_WW_MUTEX_ZERO(name, lock)        SEQCOUNT_LOCKNAME_ZERO(name, lock)
 298
 299 #define __seqprop_case(s, lockname, prop)                               \
 300         seqcount_##lockname##_t: __seqprop_##lockname##_##prop((void *)(s))
 301
 302 #define __seqprop(s, prop) _Generic(*(s),                               \
 303         seqcount_t:             __seqprop_##prop((void *)(s)),          \
 304         __seqprop_case((s),     raw_spinlock,   prop),                  \
 305         __seqprop_case((s),     spinlock,       prop),                  \
 306         __seqprop_case((s),     rwlock,         prop),                  \
 307         __seqprop_case((s),     mutex,          prop),                  \
 308         __seqprop_case((s),     ww_mutex,       prop))
 309
 310 #define seqprop_ptr(s)                  __seqprop(s, ptr)
 311 #define seqprop_sequence(s)             __seqprop(s, sequence)
 312 #define seqprop_preemptible(s)          __seqprop(s, preemptible)
 313 #define seqprop_assert(s)               __seqprop(s, assert)
 314
 315 /**
 316  * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
 317  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 318  *
 319  * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
 320  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 321  * provided before actually loading any of the variables that are to be
 322  * protected in this critical section.
 323  *
 324  * Use carefully, only in critical code, and comment how the barrier is
 325  * provided.
 326  *
 327  * Return: count to be passed to read_seqcount_retry()
 328  */
 329 #define __read_seqcount_begin(s)                                        \
 330 ({                                                                      \
 331         unsigned __seq;                                                 \
 332                                                                         \
 333         while ((__seq = seqprop_sequence(s)) & 1)                       \
 334                 cpu_relax();                                            \
 335                                                                         \
 336         kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);                    \
 337         __seq;                                                          \
 338 })
 339
 340 /**
 341  * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
 342  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 343  *
 344  * Return: count to be passed to read_seqcount_retry()
 345  */
 346 #define raw_read_seqcount_begin(s)                                      \
 347 ({                                                                      \
 348         unsigned _seq = __read_seqcount_begin(s);                       \
 349                                                                         \
 350         smp_rmb();                                                      \
 351         _seq;                                                           \
 352 })
 353
 354 /**
 355  * read_seqcount_begin() - begin a seqcount_t read critical section
 356  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 357  *
 358  * Return: count to be passed to read_seqcount_retry()
 359  */
 360 #define read_seqcount_begin(s)                                          \
 361 ({                                                                      \
 362         seqcount_lockdep_reader_access(seqprop_ptr(s));                 \
 363         raw_read_seqcount_begin(s);                                     \
 364 })
 365
 366 /**
 367  * raw_read_seqcount() - read the raw seqcount_t counter value
 368  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 369  *
 370  * raw_read_seqcount opens a read critical section of the given
 371  * seqcount_t, without any lockdep checking, and without checking or
 372  * masking the sequence counter LSB. Calling code is responsible for
 373  * handling that.
 374  *
 375  * Return: count to be passed to read_seqcount_retry()
 376  */
 377 #define raw_read_seqcount(s)                                            \
 378 ({                                                                      \
 379         unsigned __seq = seqprop_sequence(s);                           \
 380                                                                         \
 381         smp_rmb();                                                      \
 382         kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);                    \
 383         __seq;                                                          \
 384 })
 385
 386 /**
 387  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
 388  *                        lockdep and w/o counter stabilization
 389  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 390  *
 391  * raw_seqcount_begin opens a read critical section of the given
 392  * seqcount_t. Unlike read_seqcount_begin(), this function will not wait
 393  * for the count to stabilize. If a writer is active when it begins, it
 394  * will fail the read_seqcount_retry() at the end of the read critical
 395  * section instead of stabilizing at the beginning of it.
 396  *
 397  * Use this only in special kernel hot paths where the read section is
 398  * small and has a high probability of success through other external
 399  * means. It will save a single branching instruction.
 400  *
 401  * Return: count to be passed to read_seqcount_retry()
 402  */
 403 #define raw_seqcount_begin(s)                                           \
 404 ({                                                                      \
 405         /*                                                              \
 406          * If the counter is odd, let read_seqcount_retry() fail        \
 407          * by decrementing the counter.                                 \
 408          */                                                             \
 409         raw_read_seqcount(s) & ~1;                                      \
 410 })
 411
 412 /**
 413  * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
 414  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 415  * @start: count, from read_seqcount_begin()
 416  *
 417  * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
 418  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 419  * provided before actually loading any of the variables that are to be
 420  * protected in this critical section.
 421  *
 422  * Use carefully, only in critical code, and comment how the barrier is
 423  * provided.
 424  *
 425  * Return: true if a read section retry is required, else false
 426  */
 427 #define __read_seqcount_retry(s, start)                                 \
 428         do___read_seqcount_retry(seqprop_ptr(s), start)
 429
 430 static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
 431 {
 432         kcsan_atomic_next(0);
 433         return unlikely(READ_ONCE(s->sequence) != start);
 434 }
 435
 436 /**
 437  * read_seqcount_retry() - end a seqcount_t read critical section
 438  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 439  * @start: count, from read_seqcount_begin()
 440  *
 441  * read_seqcount_retry closes the read critical section of given
 442  * seqcount_t.  If the critical section was invalid, it must be ignored
 443  * (and typically retried).
 444  *
 445  * Return: true if a read section retry is required, else false
 446  */
 447 #define read_seqcount_retry(s, start)                                   \
 448         do_read_seqcount_retry(seqprop_ptr(s), start)
 449
 450 static inline int do_read_seqcount_retry(const seqcount_t *s, unsigned start)
 451 {
 452         smp_rmb();
 453         return do___read_seqcount_retry(s, start);
 454 }
 455
 456 /**
 457  * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
 458  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 459  *
 460  * Context: check write_seqcount_begin()
 461  */
 462 #define raw_write_seqcount_begin(s)                                     \
 463 do {                                                                    \
 464         if (seqprop_preemptible(s))                                     \
 465                 preempt_disable();                                      \
 466                                                                         \
 467         do_raw_write_seqcount_begin(seqprop_ptr(s));                    \
 468 } while (0)
 469
 470 static inline void do_raw_write_seqcount_begin(seqcount_t *s)
 471 {
 472         kcsan_nestable_atomic_begin();
 473         s->sequence++;
 474         smp_wmb();
 475 }
 476
 477 /**
 478  * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
 479  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 480  *
 481  * Context: check write_seqcount_end()
 482  */
 483 #define raw_write_seqcount_end(s)                                       \
 484 do {                                                                    \
 485         do_raw_write_seqcount_end(seqprop_ptr(s));                      \
 486                                                                         \
 487         if (seqprop_preemptible(s))                                     \
 488                 preempt_enable();                                       \
 489 } while (0)
 490
 491 static inline void do_raw_write_seqcount_end(seqcount_t *s)
 492 {
 493         smp_wmb();
 494         s->sequence++;
 495         kcsan_nestable_atomic_end();
 496 }
 497
 498 /**
 499  * write_seqcount_begin_nested() - start a seqcount_t write section with
 500  *                                 custom lockdep nesting level
 501  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 502  * @subclass: lockdep nesting level
 503  *
 504  * See Documentation/locking/lockdep-design.rst
 505  * Context: check write_seqcount_begin()
 506  */
 507 #define write_seqcount_begin_nested(s, subclass)                        \
 508 do {                                                                    \
 509         seqprop_assert(s);                                              \
 510                                                                         \
 511         if (seqprop_preemptible(s))                                     \
 512                 preempt_disable();                                      \
 513                                                                         \
 514         do_write_seqcount_begin_nested(seqprop_ptr(s), subclass);       \
 515 } while (0)
 516
 517 static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass)
 518 {
 519         do_raw_write_seqcount_begin(s);
 520         seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
 521 }
 522
 523 /**
 524  * write_seqcount_begin() - start a seqcount_t write side critical section
 525  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 526  *
 527  * Context: sequence counter write side sections must be serialized and
 528  * non-preemptible. Preemption will be automatically disabled if and
 529  * only if the seqcount write serialization lock is associated, and
 530  * preemptible.  If readers can be invoked from hardirq or softirq
 531  * context, interrupts or bottom halves must be respectively disabled.
 532  */
 533 #define write_seqcount_begin(s)                                         \
 534 do {                                                                    \
 535         seqprop_assert(s);                                              \
 536                                                                         \
 537         if (seqprop_preemptible(s))                                     \
 538                 preempt_disable();                                      \
 539                                                                         \
 540         do_write_seqcount_begin(seqprop_ptr(s));                        \
 541 } while (0)
 542
 543 static inline void do_write_seqcount_begin(seqcount_t *s)
 544 {
 545         do_write_seqcount_begin_nested(s, 0);
 546 }
 547
 548 /**
 549  * write_seqcount_end() - end a seqcount_t write side critical section
 550  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 551  *
 552  * Context: Preemption will be automatically re-enabled if and only if
 553  * the seqcount write serialization lock is associated, and preemptible.
 554  */
 555 #define write_seqcount_end(s)                                           \
 556 do {                                                                    \
 557         do_write_seqcount_end(seqprop_ptr(s));                          \
 558                                                                         \
 559         if (seqprop_preemptible(s))                                     \
 560                 preempt_enable();                                       \
 561 } while (0)
 562
 563 static inline void do_write_seqcount_end(seqcount_t *s)
 564 {
 565         seqcount_release(&s->dep_map, _RET_IP_);
 566         do_raw_write_seqcount_end(s);
 567 }
 568
 569 /**
 570  * raw_write_seqcount_barrier() - do a seqcount_t write barrier
 571  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 572  *
 573  * This can be used to provide an ordering guarantee instead of the usual
 574  * consistency guarantee. It is one wmb cheaper, because it can collapse
 575  * the two back-to-back wmb()s.
 576  *
 577  * Note that writes surrounding the barrier should be declared atomic (e.g.
 578  * via WRITE_ONCE): a) to ensure the writes become visible to other threads
 579  * atomically, avoiding compiler optimizations; b) to document which writes are
 580  * meant to propagate to the reader critical section. This is necessary because
 581  * neither writes before and after the barrier are enclosed in a seq-writer
 582  * critical section that would ensure readers are aware of ongoing writes::
 583  *
 584  *      seqcount_t seq;
 585  *      bool X = true, Y = false;
 586  *
 587  *      void read(void)
 588  *      {
 589  *              bool x, y;
 590  *
 591  *              do {
 592  *                      int s = read_seqcount_begin(&seq);
 593  *
 594  *                      x = X; y = Y;
 595  *
 596  *              } while (read_seqcount_retry(&seq, s));
 597  *
 598  *              BUG_ON(!x && !y);
 599  *      }
 600  *
 601  *      void write(void)
 602  *      {
 603  *              WRITE_ONCE(Y, true);
 604  *
 605  *              raw_write_seqcount_barrier(seq);
 606  *
 607  *              WRITE_ONCE(X, false);
 608  *      }
 609  */
 610 #define raw_write_seqcount_barrier(s)                                   \
 611         do_raw_write_seqcount_barrier(seqprop_ptr(s))
 612
 613 static inline void do_raw_write_seqcount_barrier(seqcount_t *s)
 614 {
 615         kcsan_nestable_atomic_begin();
 616         s->sequence++;
 617         smp_wmb();
 618         s->sequence++;
 619         kcsan_nestable_atomic_end();
 620 }
 621
 622 /**
 623  * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
 624  *                               side operations
 625  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
 626  *
 627  * After write_seqcount_invalidate, no seqcount_t read side operations
 628  * will complete successfully and see data older than this.
 629  */
 630 #define write_seqcount_invalidate(s)                                    \
 631         do_write_seqcount_invalidate(seqprop_ptr(s))
 632
 633 static inline void do_write_seqcount_invalidate(seqcount_t *s)
 634 {
 635         smp_wmb();
 636         kcsan_nestable_atomic_begin();
 637         s->sequence+=2;
 638         kcsan_nestable_atomic_end();
 639 }
 640
 641 /*
 642  * Latch sequence counters (seqcount_latch_t)
 643  *
 644  * A sequence counter variant where the counter even/odd value is used to
 645  * switch between two copies of protected data. This allows the read path,
 646  * typically NMIs, to safely interrupt the write side critical section.
 647  *
 648  * As the write sections are fully preemptible, no special handling for
 649  * PREEMPT_RT is needed.
 650  */
 651 typedef struct {
 652         seqcount_t seqcount;
 653 } seqcount_latch_t;
 654
 655 /**
 656  * SEQCNT_LATCH_ZERO() - static initializer for seqcount_latch_t
 657  * @seq_name: Name of the seqcount_latch_t instance
 658  */
 659 #define SEQCNT_LATCH_ZERO(seq_name) {                                   \
 660         .seqcount               = SEQCNT_ZERO(seq_name.seqcount),       \
 661 }
 662
 663 /**
 664  * seqcount_latch_init() - runtime initializer for seqcount_latch_t
 665  * @s: Pointer to the seqcount_latch_t instance
 666  */
 667 #define seqcount_latch_init(s) seqcount_init(&(s)->seqcount)
 668
 669 /**
 670  * raw_read_seqcount_latch() - pick even/odd latch data copy
 671  * @s: Pointer to seqcount_latch_t
 672  *
 673  * See raw_write_seqcount_latch() for details and a full reader/writer
 674  * usage example.
 675  *
 676  * Return: sequence counter raw value. Use the lowest bit as an index for
 677  * picking which data copy to read. The full counter must then be checked
 678  * with read_seqcount_latch_retry().
 679  */
 680 static inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
 681 {
 682         /*
 683          * Pairs with the first smp_wmb() in raw_write_seqcount_latch().
 684          * Due to the dependent load, a full smp_rmb() is not needed.
 685          */
 686         return READ_ONCE(s->seqcount.sequence);
 687 }
 688
 689 /**
 690  * read_seqcount_latch_retry() - end a seqcount_latch_t read section
 691  * @s:          Pointer to seqcount_latch_t
 692  * @start:      count, from raw_read_seqcount_latch()
 693  *
 694  * Return: true if a read section retry is required, else false
 695  */
 696 static inline int
 697 read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
 698 {
 699         return read_seqcount_retry(&s->seqcount, start);
 700 }
 701
 702 /**
 703  * raw_write_seqcount_latch() - redirect latch readers to even/odd copy
 704  * @s: Pointer to seqcount_latch_t
 705  *
 706  * The latch technique is a multiversion concurrency control method that allows
 707  * queries during non-atomic modifications. If you can guarantee queries never
 708  * interrupt the modification -- e.g. the concurrency is strictly between CPUs
 709  * -- you most likely do not need this.
 710  *
 711  * Where the traditional RCU/lockless data structures rely on atomic
 712  * modifications to ensure queries observe either the old or the new state the
 713  * latch allows the same for non-atomic updates. The trade-off is doubling the
 714  * cost of storage; we have to maintain two copies of the entire data
 715  * structure.
 716  *
 717  * Very simply put: we first modify one copy and then the other. This ensures
 718  * there is always one copy in a stable state, ready to give us an answer.
 719  *
 720  * The basic form is a data structure like::
 721  *
 722  *      struct latch_struct {
 723  *              seqcount_latch_t        seq;
 724  *              struct data_struct      data[2];
 725  *      };
 726  *
 727  * Where a modification, which is assumed to be externally serialized, does the
 728  * following::
 729  *
 730  *      void latch_modify(struct latch_struct *latch, ...)
 731  *      {
 732  *              smp_wmb();      // Ensure that the last data[1] update is visible
 733  *              latch->seq.sequence++;
 734  *              smp_wmb();      // Ensure that the seqcount update is visible
 735  *
 736  *              modify(latch->data[0], ...);
 737  *
 738  *              smp_wmb();      // Ensure that the data[0] update is visible
 739  *              latch->seq.sequence++;
 740  *              smp_wmb();      // Ensure that the seqcount update is visible
 741  *
 742  *              modify(latch->data[1], ...);
 743  *      }
 744  *
 745  * The query will have a form like::
 746  *
 747  *      struct entry *latch_query(struct latch_struct *latch, ...)
 748  *      {
 749  *              struct entry *entry;
 750  *              unsigned seq, idx;
 751  *
 752  *              do {
 753  *                      seq = raw_read_seqcount_latch(&latch->seq);
 754  *
 755  *                      idx = seq & 0x01;
 756  *                      entry = data_query(latch->data[idx], ...);
 757  *
 758  *              // This includes needed smp_rmb()
 759  *              } while (read_seqcount_latch_retry(&latch->seq, seq));
 760  *
 761  *              return entry;
 762  *      }
 763  *
 764  * So during the modification, queries are first redirected to data[1]. Then we
 765  * modify data[0]. When that is complete, we redirect queries back to data[0]
 766  * and we can modify data[1].
 767  *
 768  * NOTE:
 769  *
 770  *      The non-requirement for atomic modifications does _NOT_ include
 771  *      the publishing of new entries in the case where data is a dynamic
 772  *      data structure.
 773  *
 774  *      An iteration might start in data[0] and get suspended long enough
 775  *      to miss an entire modification sequence, once it resumes it might
 776  *      observe the new entry.
 777  *
 778  * NOTE2:
 779  *
 780  *      When data is a dynamic data structure; one should use regular RCU
 781  *      patterns to manage the lifetimes of the objects within.
 782  */
 783 static inline void raw_write_seqcount_latch(seqcount_latch_t *s)
 784 {
 785         smp_wmb();      /* prior stores before incrementing "sequence" */
 786         s->seqcount.sequence++;
 787         smp_wmb();      /* increment "sequence" before following stores */
 788 }
 789
 790 /*
 791  * Sequential locks (seqlock_t)
 792  *
 793  * Sequence counters with an embedded spinlock for writer serialization
 794  * and non-preemptibility.
 795  *
 796  * For more info, see:
 797  *    - Comments on top of seqcount_t
 798  *    - Documentation/locking/seqlock.rst
 799  */
 800 typedef struct {
 801         /*
 802          * Make sure that readers don't starve writers on PREEMPT_RT: use
 803          * seqcount_spinlock_t instead of seqcount_t. Check __SEQ_LOCK().
 804          */
 805         seqcount_spinlock_t seqcount;
 806         spinlock_t lock;
 807 } seqlock_t;
 808
 809 #define __SEQLOCK_UNLOCKED(lockname)                                    \
 810         {                                                               \
 811                 .seqcount = SEQCNT_SPINLOCK_ZERO(lockname, &(lockname).lock), \
 812                 .lock = __SPIN_LOCK_UNLOCKED(lockname)                  \
 813         }
 814
 815 /**
 816  * seqlock_init() - dynamic initializer for seqlock_t
 817  * @sl: Pointer to the seqlock_t instance
 818  */
 819 #define seqlock_init(sl)                                                \
 820         do {                                                            \
 821                 spin_lock_init(&(sl)->lock);                            \
 822                 seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock);   \
 823         } while (0)
 824
 825 /**
 826  * DEFINE_SEQLOCK(sl) - Define a statically allocated seqlock_t
 827  * @sl: Name of the seqlock_t instance
 828  */
 829 #define DEFINE_SEQLOCK(sl) \
 830                 seqlock_t sl = __SEQLOCK_UNLOCKED(sl)
 831
 832 /**
 833  * read_seqbegin() - start a seqlock_t read side critical section
 834  * @sl: Pointer to seqlock_t
 835  *
 836  * Return: count, to be passed to read_seqretry()
 837  */
 838 static inline unsigned read_seqbegin(const seqlock_t *sl)
 839 {
 840         unsigned ret = read_seqcount_begin(&sl->seqcount);
 841
 842         kcsan_atomic_next(0);  /* non-raw usage, assume closing read_seqretry() */
 843         kcsan_flat_atomic_begin();
 844         return ret;
 845 }
 846
 847 /**
 848  * read_seqretry() - end a seqlock_t read side section
 849  * @sl: Pointer to seqlock_t
 850  * @start: count, from read_seqbegin()
 851  *
 852  * read_seqretry closes the read side critical section of given seqlock_t.
 853  * If the critical section was invalid, it must be ignored (and typically
 854  * retried).
 855  *
 856  * Return: true if a read section retry is required, else false
 857  */
 858 static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
 859 {
 860         /*
 861          * Assume not nested: read_seqretry() may be called multiple times when
 862          * completing read critical section.
 863          */
 864         kcsan_flat_atomic_end();
 865
 866         return read_seqcount_retry(&sl->seqcount, start);
 867 }
 868
 869 /*
 870  * For all seqlock_t write side functions, use the the internal
 871  * do_write_seqcount_begin() instead of generic write_seqcount_begin().
 872  * This way, no redundant lockdep_assert_held() checks are added.
 873  */
 874
 875 /**
 876  * write_seqlock() - start a seqlock_t write side critical section
 877  * @sl: Pointer to seqlock_t
 878  *
 879  * write_seqlock opens a write side critical section for the given
 880  * seqlock_t.  It also implicitly acquires the spinlock_t embedded inside
 881  * that sequential lock. All seqlock_t write side sections are thus
 882  * automatically serialized and non-preemptible.
 883  *
 884  * Context: if the seqlock_t read section, or other write side critical
 885  * sections, can be invoked from hardirq or softirq contexts, use the
 886  * _irqsave or _bh variants of this function instead.
 887  */
 888 static inline void write_seqlock(seqlock_t *sl)
 889 {
 890         spin_lock(&sl->lock);
 891         do_write_seqcount_begin(&sl->seqcount.seqcount);
 892 }
 893
 894 /**
 895  * write_sequnlock() - end a seqlock_t write side critical section
 896  * @sl: Pointer to seqlock_t
 897  *
 898  * write_sequnlock closes the (serialized and non-preemptible) write side
 899  * critical section of given seqlock_t.
 900  */
 901 static inline void write_sequnlock(seqlock_t *sl)
 902 {
 903         do_write_seqcount_end(&sl->seqcount.seqcount);
 904         spin_unlock(&sl->lock);
 905 }
 906
 907 /**
 908  * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
 909  * @sl: Pointer to seqlock_t
 910  *
 911  * _bh variant of write_seqlock(). Use only if the read side section, or
 912  * other write side sections, can be invoked from softirq contexts.
 913  */
 914 static inline void write_seqlock_bh(seqlock_t *sl)
 915 {
 916         spin_lock_bh(&sl->lock);
 917         do_write_seqcount_begin(&sl->seqcount.seqcount);
 918 }
 919
 920 /**
 921  * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
 922  * @sl: Pointer to seqlock_t
 923  *
 924  * write_sequnlock_bh closes the serialized, non-preemptible, and
 925  * softirqs-disabled, seqlock_t write side critical section opened with
 926  * write_seqlock_bh().
 927  */
 928 static inline void write_sequnlock_bh(seqlock_t *sl)
 929 {
 930         do_write_seqcount_end(&sl->seqcount.seqcount);
 931         spin_unlock_bh(&sl->lock);
 932 }
 933
 934 /**
 935  * write_seqlock_irq() - start a non-interruptible seqlock_t write section
 936  * @sl: Pointer to seqlock_t
 937  *
 938  * _irq variant of write_seqlock(). Use only if the read side section, or
 939  * other write sections, can be invoked from hardirq contexts.
 940  */
 941 static inline void write_seqlock_irq(seqlock_t *sl)
 942 {
 943         spin_lock_irq(&sl->lock);
 944         do_write_seqcount_begin(&sl->seqcount.seqcount);
 945 }
 946
 947 /**
 948  * write_sequnlock_irq() - end a non-interruptible seqlock_t write section
 949  * @sl: Pointer to seqlock_t
 950  *
 951  * write_sequnlock_irq closes the serialized and non-interruptible
 952  * seqlock_t write side section opened with write_seqlock_irq().
 953  */
 954 static inline void write_sequnlock_irq(seqlock_t *sl)
 955 {
 956         do_write_seqcount_end(&sl->seqcount.seqcount);
 957         spin_unlock_irq(&sl->lock);
 958 }
 959
 960 static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
 961 {
 962         unsigned long flags;
 963
 964         spin_lock_irqsave(&sl->lock, flags);
 965         do_write_seqcount_begin(&sl->seqcount.seqcount);
 966         return flags;
 967 }
 968
 969 /**
 970  * write_seqlock_irqsave() - start a non-interruptible seqlock_t write
 971  *                           section
 972  * @lock:  Pointer to seqlock_t
 973  * @flags: Stack-allocated storage for saving caller's local interrupt
 974  *         state, to be passed to write_sequnlock_irqrestore().
 975  *
 976  * _irqsave variant of write_seqlock(). Use it only if the read side
 977  * section, or other write sections, can be invoked from hardirq context.
 978  */
 979 #define write_seqlock_irqsave(lock, flags)                              \
 980         do { flags = __write_seqlock_irqsave(lock); } while (0)
 981
 982 /**
 983  * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write
 984  *                                section
 985  * @sl:    Pointer to seqlock_t
 986  * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
 987  *
 988  * write_sequnlock_irqrestore closes the serialized and non-interruptible
 989  * seqlock_t write section previously opened with write_seqlock_irqsave().
 990  */
 991 static inline void
 992 write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
 993 {
 994         do_write_seqcount_end(&sl->seqcount.seqcount);
 995         spin_unlock_irqrestore(&sl->lock, flags);
 996 }
 997
 998 /**
 999  * read_seqlock_excl() - begin a seqlock_t locking reader section
1000  * @sl: Pointer to seqlock_t
1001  *
1002  * read_seqlock_excl opens a seqlock_t locking reader critical section.  A
1003  * locking reader exclusively locks out *both* other writers *and* other
1004  * locking readers, but it does not update the embedded sequence number.
1005  *
1006  * Locking readers act like a normal spin_lock()/spin_unlock().
1007  *
1008  * Context: if the seqlock_t write section, *or other read sections*, can
1009  * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
1010  * variant of this function instead.
1011  *
1012  * The opened read section must be closed with read_sequnlock_excl().
1013  */
1014 static inline void read_seqlock_excl(seqlock_t *sl)
1015 {
1016         spin_lock(&sl->lock);
1017 }
1018
1019 /**
1020  * read_sequnlock_excl() - end a seqlock_t locking reader critical section
1021  * @sl: Pointer to seqlock_t
1022  */
1023 static inline void read_sequnlock_excl(seqlock_t *sl)
1024 {
1025         spin_unlock(&sl->lock);
1026 }
1027
1028 /**
1029  * read_seqlock_excl_bh() - start a seqlock_t locking reader section with
1030  *                          softirqs disabled
1031  * @sl: Pointer to seqlock_t
1032  *
1033  * _bh variant of read_seqlock_excl(). Use this variant only if the
1034  * seqlock_t write side section, *or other read sections*, can be invoked
1035  * from softirq contexts.
1036  */
1037 static inline void read_seqlock_excl_bh(seqlock_t *sl)
1038 {
1039         spin_lock_bh(&sl->lock);
1040 }
1041
1042 /**
1043  * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
1044  *                            reader section
1045  * @sl: Pointer to seqlock_t
1046  */
1047 static inline void read_sequnlock_excl_bh(seqlock_t *sl)
1048 {
1049         spin_unlock_bh(&sl->lock);
1050 }
1051
1052 /**
1053  * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
1054  *                           reader section
1055  * @sl: Pointer to seqlock_t
1056  *
1057  * _irq variant of read_seqlock_excl(). Use this only if the seqlock_t
1058  * write side section, *or other read sections*, can be invoked from a
1059  * hardirq context.
1060  */
1061 static inline void read_seqlock_excl_irq(seqlock_t *sl)
1062 {
1063         spin_lock_irq(&sl->lock);
1064 }
1065
1066 /**
1067  * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
1068  *                             locking reader section
1069  * @sl: Pointer to seqlock_t
1070  */
1071 static inline void read_sequnlock_excl_irq(seqlock_t *sl)
1072 {
1073         spin_unlock_irq(&sl->lock);
1074 }
1075
1076 static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
1077 {
1078         unsigned long flags;
1079
1080         spin_lock_irqsave(&sl->lock, flags);
1081         return flags;
1082 }
1083
1084 /**
1085  * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
1086  *                               locking reader section
1087  * @lock:  Pointer to seqlock_t
1088  * @flags: Stack-allocated storage for saving caller's local interrupt
1089  *         state, to be passed to read_sequnlock_excl_irqrestore().
1090  *
1091  * _irqsave variant of read_seqlock_excl(). Use this only if the seqlock_t
1092  * write side section, *or other read sections*, can be invoked from a
1093  * hardirq context.
1094  */
1095 #define read_seqlock_excl_irqsave(lock, flags)                          \
1096         do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
1097
1098 /**
1099  * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
1100  *                                    locking reader section
1101  * @sl:    Pointer to seqlock_t
1102  * @flags: Caller saved interrupt state, from read_seqlock_excl_irqsave()
1103  */
1104 static inline void
1105 read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
1106 {
1107         spin_unlock_irqrestore(&sl->lock, flags);
1108 }
1109
1110 /**
1111  * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
1112  * @lock: Pointer to seqlock_t
1113  * @seq : Marker and return parameter. If the passed value is even, the
1114  * reader will become a *lockless* seqlock_t reader as in read_seqbegin().
1115  * If the passed value is odd, the reader will become a *locking* reader
1116  * as in read_seqlock_excl().  In the first call to this function, the
1117  * caller *must* initialize and pass an even value to @seq; this way, a
1118  * lockless read can be optimistically tried first.
1119  *
1120  * read_seqbegin_or_lock is an API designed to optimistically try a normal
1121  * lockless seqlock_t read section first.  If an odd counter is found, the
1122  * lockless read trial has failed, and the next read iteration transforms
1123  * itself into a full seqlock_t locking reader.
1124  *
1125  * This is typically used to avoid seqlock_t lockless readers starvation
1126  * (too much retry loops) in the case of a sharp spike in write side
1127  * activity.
1128  *
1129  * Context: if the seqlock_t write section, *or other read sections*, can
1130  * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
1131  * variant of this function instead.
1132  *
1133  * Check Documentation/locking/seqlock.rst for template example code.
1134  *
1135  * Return: the encountered sequence counter value, through the @seq
1136  * parameter, which is overloaded as a return parameter. This returned
1137  * value must be checked with need_seqretry(). If the read section need to
1138  * be retried, this returned value must also be passed as the @seq
1139  * parameter of the next read_seqbegin_or_lock() iteration.
1140  */
1141 static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
1142 {
1143         if (!(*seq & 1))        /* Even */
1144                 *seq = read_seqbegin(lock);
1145         else                    /* Odd */
1146                 read_seqlock_excl(lock);
1147 }
1148
1149 /**
1150  * need_seqretry() - validate seqlock_t "locking or lockless" read section
1151  * @lock: Pointer to seqlock_t
1152  * @seq: sequence count, from read_seqbegin_or_lock()
1153  *
1154  * Return: true if a read section retry is required, false otherwise
1155  */
1156 static inline int need_seqretry(seqlock_t *lock, int seq)
1157 {
1158         return !(seq & 1) && read_seqretry(lock, seq);
1159 }
1160
1161 /**
1162  * done_seqretry() - end seqlock_t "locking or lockless" reader section
1163  * @lock: Pointer to seqlock_t
1164  * @seq: count, from read_seqbegin_or_lock()
1165  *
1166  * done_seqretry finishes the seqlock_t read side critical section started
1167  * with read_seqbegin_or_lock() and validated by need_seqretry().
1168  */
1169 static inline void done_seqretry(seqlock_t *lock, int seq)
1170 {
1171         if (seq & 1)
1172                 read_sequnlock_excl(lock);
1173 }
1174
1175 /**
1176  * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
1177  *                                   a non-interruptible locking reader
1178  * @lock: Pointer to seqlock_t
1179  * @seq:  Marker and return parameter. Check read_seqbegin_or_lock().
1180  *
1181  * This is the _irqsave variant of read_seqbegin_or_lock(). Use it only if
1182  * the seqlock_t write section, *or other read sections*, can be invoked
1183  * from hardirq context.
1184  *
1185  * Note: Interrupts will be disabled only for "locking reader" mode.
1186  *
1187  * Return:
1188  *
1189  *   1. The saved local interrupts state in case of a locking reader, to
1190  *      be passed to done_seqretry_irqrestore().
1191  *
1192  *   2. The encountered sequence counter value, returned through @seq
1193  *      overloaded as a return parameter. Check read_seqbegin_or_lock().
1194  */
1195 static inline unsigned long
1196 read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
1197 {
1198         unsigned long flags = 0;
1199
1200         if (!(*seq & 1))        /* Even */
1201                 *seq = read_seqbegin(lock);
1202         else                    /* Odd */
1203                 read_seqlock_excl_irqsave(lock, flags);
1204
1205         return flags;
1206 }
1207
1208 /**
1209  * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
1210  *                              non-interruptible locking reader section
1211  * @lock:  Pointer to seqlock_t
1212  * @seq:   Count, from read_seqbegin_or_lock_irqsave()
1213  * @flags: Caller's saved local interrupt state in case of a locking
1214  *         reader, also from read_seqbegin_or_lock_irqsave()
1215  *
1216  * This is the _irqrestore variant of done_seqretry(). The read section
1217  * must've been opened with read_seqbegin_or_lock_irqsave(), and validated
1218  * by need_seqretry().
1219  */
1220 static inline void
1221 done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
1222 {
1223         if (seq & 1)
1224                 read_sequnlock_excl_irqrestore(lock, flags);
1225 }
1226 #endif /* __LINUX_SEQLOCK_H */