include/linux/seqlock.h

   1 #ifndef __LINUX_SEQLOCK_H
   2 #define __LINUX_SEQLOCK_H
   3 /*
   4  * Reader/writer consistent mechanism without starving writers. This type of
   5  * lock for data where the reader wants a consistent set of information
   6  * and is willing to retry if the information changes. There are two types
   7  * of readers:
   8  * 1. Sequence readers which never block a writer but they may have to retry
   9  *    if a writer is in progress by detecting change in sequence number.
  10  *    Writers do not wait for a sequence reader.
  11  * 2. Locking readers which will wait if a writer or another locking reader
  12  *    is in progress. A locking reader in progress will also block a writer
  13  *    from going forward. Unlike the regular rwlock, the read lock here is
  14  *    exclusive so that only one locking reader can get it.
  15  *
  16  * This is not as cache friendly as brlock. Also, this may not work well
  17  * for data that contains pointers, because any writer could
  18  * invalidate a pointer that a reader was following.
  19  *
  20  * Expected non-blocking reader usage:
  21  *      do {
  22  *          seq = read_seqbegin(&foo);
  23  *      ...
  24  *      } while (read_seqretry(&foo, seq));
  25  *
  26  *
  27  * On non-SMP the spin locks disappear but the writer still needs
  28  * to increment the sequence variables because an interrupt routine could
  29  * change the state of the data.
  30  *
  31  * Based on x86_64 vsyscall gettimeofday
  32  * by Keith Owens and Andrea Arcangeli
  33  */
  34
  35 #include <linux/spinlock.h>
  36 #include <linux/preempt.h>
  37 #include <linux/lockdep.h>
  38 #include <asm/processor.h>
  39
  40 /*
  41  * Version using sequence counter only.
  42  * This can be used when code has its own mutex protecting the
  43  * updating starting before the write_seqcountbeqin() and ending
  44  * after the write_seqcount_end().
  45  */
  46 typedef struct seqcount {
  47         unsigned sequence;
  48 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  49         struct lockdep_map dep_map;
  50 #endif
  51 } seqcount_t;
  52
  53 static inline void __seqcount_init(seqcount_t *s, const char *name,
  54                                           struct lock_class_key *key)
  55 {
  56         /*
  57          * Make sure we are not reinitializing a held lock:
  58          */
  59         lockdep_init_map(&s->dep_map, name, key, 0);
  60         s->sequence = 0;
  61 }
  62
  63 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  64 # define SEQCOUNT_DEP_MAP_INIT(lockname) \
  65                 .dep_map = { .name = #lockname } \
  66
  67 # define seqcount_init(s)                               \
  68         do {                                            \
  69                 static struct lock_class_key __key;     \
  70                 __seqcount_init((s), #s, &__key);       \
  71         } while (0)
  72
  73 static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
  74 {
  75         seqcount_t *l = (seqcount_t *)s;
  76         unsigned long flags;
  77
  78         local_irq_save(flags);
  79         seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
  80         seqcount_release(&l->dep_map, 1, _RET_IP_);
  81         local_irq_restore(flags);
  82 }
  83
  84 #else
  85 # define SEQCOUNT_DEP_MAP_INIT(lockname)
  86 # define seqcount_init(s) __seqcount_init(s, NULL, NULL)
  87 # define seqcount_lockdep_reader_access(x)
  88 #endif
  89
  90 #define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
  91
  92
  93 /**
  94  * __read_seqcount_begin - begin a seq-read critical section (without barrier)
  95  * @s: pointer to seqcount_t
  96  * Returns: count to be passed to read_seqcount_retry
  97  *
  98  * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
  99  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 100  * provided before actually loading any of the variables that are to be
 101  * protected in this critical section.
 102  *
 103  * Use carefully, only in critical code, and comment how the barrier is
 104  * provided.
 105  */
 106 static inline unsigned __read_seqcount_begin(const seqcount_t *s)
 107 {
 108         unsigned ret;
 109
 110 repeat:
 111         ret = ACCESS_ONCE(s->sequence);
 112         if (unlikely(ret & 1)) {
 113                 cpu_relax();
 114                 goto repeat;
 115         }
 116         return ret;
 117 }
 118
 119 /**
 120  * read_seqcount_begin_no_lockdep - start seq-read critical section w/o lockdep
 121  * @s: pointer to seqcount_t
 122  * Returns: count to be passed to read_seqcount_retry
 123  *
 124  * read_seqcount_begin_no_lockdep opens a read critical section of the given
 125  * seqcount, but without any lockdep checking. Validity of the critical
 126  * section is tested by checking read_seqcount_retry function.
 127  */
 128 static inline unsigned read_seqcount_begin_no_lockdep(const seqcount_t *s)
 129 {
 130         unsigned ret = __read_seqcount_begin(s);
 131         smp_rmb();
 132         return ret;
 133 }
 134
 135 /**
 136  * read_seqcount_begin - begin a seq-read critical section
 137  * @s: pointer to seqcount_t
 138  * Returns: count to be passed to read_seqcount_retry
 139  *
 140  * read_seqcount_begin opens a read critical section of the given seqcount.
 141  * Validity of the critical section is tested by checking read_seqcount_retry
 142  * function.
 143  */
 144 static inline unsigned read_seqcount_begin(const seqcount_t *s)
 145 {
 146         seqcount_lockdep_reader_access(s);
 147         return read_seqcount_begin_no_lockdep(s);
 148 }
 149
 150 /**
 151  * raw_seqcount_begin - begin a seq-read critical section
 152  * @s: pointer to seqcount_t
 153  * Returns: count to be passed to read_seqcount_retry
 154  *
 155  * raw_seqcount_begin opens a read critical section of the given seqcount.
 156  * Validity of the critical section is tested by checking read_seqcount_retry
 157  * function.
 158  *
 159  * Unlike read_seqcount_begin(), this function will not wait for the count
 160  * to stabilize. If a writer is active when we begin, we will fail the
 161  * read_seqcount_retry() instead of stabilizing at the beginning of the
 162  * critical section.
 163  */
 164 static inline unsigned raw_seqcount_begin(const seqcount_t *s)
 165 {
 166         unsigned ret = ACCESS_ONCE(s->sequence);
 167
 168         seqcount_lockdep_reader_access(s);
 169         smp_rmb();
 170         return ret & ~1;
 171 }
 172
 173 /**
 174  * __read_seqcount_retry - end a seq-read critical section (without barrier)
 175  * @s: pointer to seqcount_t
 176  * @start: count, from read_seqcount_begin
 177  * Returns: 1 if retry is required, else 0
 178  *
 179  * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
 180  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 181  * provided before actually loading any of the variables that are to be
 182  * protected in this critical section.
 183  *
 184  * Use carefully, only in critical code, and comment how the barrier is
 185  * provided.
 186  */
 187 static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
 188 {
 189         return unlikely(s->sequence != start);
 190 }
 191
 192 /**
 193  * read_seqcount_retry - end a seq-read critical section
 194  * @s: pointer to seqcount_t
 195  * @start: count, from read_seqcount_begin
 196  * Returns: 1 if retry is required, else 0
 197  *
 198  * read_seqcount_retry closes a read critical section of the given seqcount.
 199  * If the critical section was invalid, it must be ignored (and typically
 200  * retried).
 201  */
 202 static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
 203 {
 204         smp_rmb();
 205         return __read_seqcount_retry(s, start);
 206 }
 207
 208
 209 /*
 210  * Sequence counter only version assumes that callers are using their
 211  * own mutexing.
 212  */
 213 static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
 214 {
 215         s->sequence++;
 216         smp_wmb();
 217         seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
 218 }
 219
 220 static inline void write_seqcount_begin(seqcount_t *s)
 221 {
 222         write_seqcount_begin_nested(s, 0);
 223 }
 224
 225 static inline void write_seqcount_end(seqcount_t *s)
 226 {
 227         seqcount_release(&s->dep_map, 1, _RET_IP_);
 228         smp_wmb();
 229         s->sequence++;
 230 }
 231
 232 /**
 233  * write_seqcount_barrier - invalidate in-progress read-side seq operations
 234  * @s: pointer to seqcount_t
 235  *
 236  * After write_seqcount_barrier, no read-side seq operations will complete
 237  * successfully and see data older than this.
 238  */
 239 static inline void write_seqcount_barrier(seqcount_t *s)
 240 {
 241         smp_wmb();
 242         s->sequence+=2;
 243 }
 244
 245 typedef struct {
 246         struct seqcount seqcount;
 247         spinlock_t lock;
 248 } seqlock_t;
 249
 250 /*
 251  * These macros triggered gcc-3.x compile-time problems.  We think these are
 252  * OK now.  Be cautious.
 253  */
 254 #define __SEQLOCK_UNLOCKED(lockname)                    \
 255         {                                               \
 256                 .seqcount = SEQCNT_ZERO(lockname),      \
 257                 .lock = __SPIN_LOCK_UNLOCKED(lockname)  \
 258         }
 259
 260 #define seqlock_init(x)                                 \
 261         do {                                            \
 262                 seqcount_init(&(x)->seqcount);          \
 263                 spin_lock_init(&(x)->lock);             \
 264         } while (0)
 265
 266 #define DEFINE_SEQLOCK(x) \
 267                 seqlock_t x = __SEQLOCK_UNLOCKED(x)
 268
 269 /*
 270  * Read side functions for starting and finalizing a read side section.
 271  */
 272 static inline unsigned read_seqbegin(const seqlock_t *sl)
 273 {
 274         return read_seqcount_begin(&sl->seqcount);
 275 }
 276
 277 static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
 278 {
 279         return read_seqcount_retry(&sl->seqcount, start);
 280 }
 281
 282 /*
 283  * Lock out other writers and update the count.
 284  * Acts like a normal spin_lock/unlock.
 285  * Don't need preempt_disable() because that is in the spin_lock already.
 286  */
 287 static inline void write_seqlock(seqlock_t *sl)
 288 {
 289         spin_lock(&sl->lock);
 290         write_seqcount_begin(&sl->seqcount);
 291 }
 292
 293 static inline void write_sequnlock(seqlock_t *sl)
 294 {
 295         write_seqcount_end(&sl->seqcount);
 296         spin_unlock(&sl->lock);
 297 }
 298
 299 static inline void write_seqlock_bh(seqlock_t *sl)
 300 {
 301         spin_lock_bh(&sl->lock);
 302         write_seqcount_begin(&sl->seqcount);
 303 }
 304
 305 static inline void write_sequnlock_bh(seqlock_t *sl)
 306 {
 307         write_seqcount_end(&sl->seqcount);
 308         spin_unlock_bh(&sl->lock);
 309 }
 310
 311 static inline void write_seqlock_irq(seqlock_t *sl)
 312 {
 313         spin_lock_irq(&sl->lock);
 314         write_seqcount_begin(&sl->seqcount);
 315 }
 316
 317 static inline void write_sequnlock_irq(seqlock_t *sl)
 318 {
 319         write_seqcount_end(&sl->seqcount);
 320         spin_unlock_irq(&sl->lock);
 321 }
 322
 323 static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
 324 {
 325         unsigned long flags;
 326
 327         spin_lock_irqsave(&sl->lock, flags);
 328         write_seqcount_begin(&sl->seqcount);
 329         return flags;
 330 }
 331
 332 #define write_seqlock_irqsave(lock, flags)                              \
 333         do { flags = __write_seqlock_irqsave(lock); } while (0)
 334
 335 static inline void
 336 write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
 337 {
 338         write_seqcount_end(&sl->seqcount);
 339         spin_unlock_irqrestore(&sl->lock, flags);
 340 }
 341
 342 /*
 343  * A locking reader exclusively locks out other writers and locking readers,
 344  * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
 345  * Don't need preempt_disable() because that is in the spin_lock already.
 346  */
 347 static inline void read_seqlock_excl(seqlock_t *sl)
 348 {
 349         spin_lock(&sl->lock);
 350 }
 351
 352 static inline void read_sequnlock_excl(seqlock_t *sl)
 353 {
 354         spin_unlock(&sl->lock);
 355 }
 356
 357 /**
 358  * read_seqbegin_or_lock - begin a sequence number check or locking block
 359  * @lock: sequence lock
 360  * @seq : sequence number to be checked
 361  *
 362  * First try it once optimistically without taking the lock. If that fails,
 363  * take the lock. The sequence number is also used as a marker for deciding
 364  * whether to be a reader (even) or writer (odd).
 365  * N.B. seq must be initialized to an even number to begin with.
 366  */
 367 static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
 368 {
 369         if (!(*seq & 1))        /* Even */
 370                 *seq = read_seqbegin(lock);
 371         else                    /* Odd */
 372                 read_seqlock_excl(lock);
 373 }
 374
 375 static inline int need_seqretry(seqlock_t *lock, int seq)
 376 {
 377         return !(seq & 1) && read_seqretry(lock, seq);
 378 }
 379
 380 static inline void done_seqretry(seqlock_t *lock, int seq)
 381 {
 382         if (seq & 1)
 383                 read_sequnlock_excl(lock);
 384 }
 385
 386 static inline void read_seqlock_excl_bh(seqlock_t *sl)
 387 {
 388         spin_lock_bh(&sl->lock);
 389 }
 390
 391 static inline void read_sequnlock_excl_bh(seqlock_t *sl)
 392 {
 393         spin_unlock_bh(&sl->lock);
 394 }
 395
 396 static inline void read_seqlock_excl_irq(seqlock_t *sl)
 397 {
 398         spin_lock_irq(&sl->lock);
 399 }
 400
 401 static inline void read_sequnlock_excl_irq(seqlock_t *sl)
 402 {
 403         spin_unlock_irq(&sl->lock);
 404 }
 405
 406 static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
 407 {
 408         unsigned long flags;
 409
 410         spin_lock_irqsave(&sl->lock, flags);
 411         return flags;
 412 }
 413
 414 #define read_seqlock_excl_irqsave(lock, flags)                          \
 415         do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
 416
 417 static inline void
 418 read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
 419 {
 420         spin_unlock_irqrestore(&sl->lock, flags);
 421 }
 422
 423 #endif /* __LINUX_SEQLOCK_H */