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1 #ifndef __LINUX_SEQLOCK_H
2 #define __LINUX_SEQLOCK_H
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
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.
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.
20 * Expected non-blocking reader usage:
22 * seq = read_seqbegin(&foo);
24 * } while (read_seqretry(&foo, seq));
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.
31 * Based on x86_64 vsyscall gettimeofday
32 * by Keith Owens and Andrea Arcangeli
35 #include <linux/spinlock.h>
36 #include <linux/preempt.h>
37 #include <linux/lockdep.h>
38 #include <asm/processor.h>
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().
46 typedef struct seqcount
{
48 #ifdef CONFIG_DEBUG_LOCK_ALLOC
49 struct lockdep_map dep_map
;
53 static inline void __seqcount_init(seqcount_t
*s
, const char *name
,
54 struct lock_class_key
*key
)
57 * Make sure we are not reinitializing a held lock:
59 lockdep_init_map(&s
->dep_map
, name
, key
, 0);
63 #ifdef CONFIG_DEBUG_LOCK_ALLOC
64 # define SEQCOUNT_DEP_MAP_INIT(lockname) \
65 .dep_map = { .name = #lockname } \
67 # define seqcount_init(s) \
69 static struct lock_class_key __key; \
70 __seqcount_init((s), #s, &__key); \
73 static inline void seqcount_lockdep_reader_access(const seqcount_t
*s
)
75 seqcount_t
*l
= (seqcount_t
*)s
;
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
);
85 # define SEQCOUNT_DEP_MAP_INIT(lockname)
86 # define seqcount_init(s) __seqcount_init(s, NULL, NULL)
87 # define seqcount_lockdep_reader_access(x)
90 #define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
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
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.
103 * Use carefully, only in critical code, and comment how the barrier is
106 static inline unsigned __read_seqcount_begin(const seqcount_t
*s
)
111 ret
= READ_ONCE(s
->sequence
);
112 if (unlikely(ret
& 1)) {
120 * raw_read_seqcount - Read the raw seqcount
121 * @s: pointer to seqcount_t
122 * Returns: count to be passed to read_seqcount_retry
124 * raw_read_seqcount opens a read critical section of the given
125 * seqcount without any lockdep checking and without checking or
126 * masking the LSB. Calling code is responsible for handling that.
128 static inline unsigned raw_read_seqcount(const seqcount_t
*s
)
130 unsigned ret
= READ_ONCE(s
->sequence
);
136 * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
137 * @s: pointer to seqcount_t
138 * Returns: count to be passed to read_seqcount_retry
140 * raw_read_seqcount_begin opens a read critical section of the given
141 * seqcount, but without any lockdep checking. Validity of the critical
142 * section is tested by checking read_seqcount_retry function.
144 static inline unsigned raw_read_seqcount_begin(const seqcount_t
*s
)
146 unsigned ret
= __read_seqcount_begin(s
);
152 * read_seqcount_begin - begin a seq-read critical section
153 * @s: pointer to seqcount_t
154 * Returns: count to be passed to read_seqcount_retry
156 * read_seqcount_begin opens a read critical section of the given seqcount.
157 * Validity of the critical section is tested by checking read_seqcount_retry
160 static inline unsigned read_seqcount_begin(const seqcount_t
*s
)
162 seqcount_lockdep_reader_access(s
);
163 return raw_read_seqcount_begin(s
);
167 * raw_seqcount_begin - begin a seq-read critical section
168 * @s: pointer to seqcount_t
169 * Returns: count to be passed to read_seqcount_retry
171 * raw_seqcount_begin opens a read critical section of the given seqcount.
172 * Validity of the critical section is tested by checking read_seqcount_retry
175 * Unlike read_seqcount_begin(), this function will not wait for the count
176 * to stabilize. If a writer is active when we begin, we will fail the
177 * read_seqcount_retry() instead of stabilizing at the beginning of the
180 static inline unsigned raw_seqcount_begin(const seqcount_t
*s
)
182 unsigned ret
= READ_ONCE(s
->sequence
);
188 * __read_seqcount_retry - end a seq-read critical section (without barrier)
189 * @s: pointer to seqcount_t
190 * @start: count, from read_seqcount_begin
191 * Returns: 1 if retry is required, else 0
193 * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
194 * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
195 * provided before actually loading any of the variables that are to be
196 * protected in this critical section.
198 * Use carefully, only in critical code, and comment how the barrier is
201 static inline int __read_seqcount_retry(const seqcount_t
*s
, unsigned start
)
203 return unlikely(s
->sequence
!= start
);
207 * read_seqcount_retry - end a seq-read critical section
208 * @s: pointer to seqcount_t
209 * @start: count, from read_seqcount_begin
210 * Returns: 1 if retry is required, else 0
212 * read_seqcount_retry closes a read critical section of the given seqcount.
213 * If the critical section was invalid, it must be ignored (and typically
216 static inline int read_seqcount_retry(const seqcount_t
*s
, unsigned start
)
219 return __read_seqcount_retry(s
, start
);
224 static inline void raw_write_seqcount_begin(seqcount_t
*s
)
230 static inline void raw_write_seqcount_end(seqcount_t
*s
)
237 * raw_write_seqcount_barrier - do a seq write barrier
238 * @s: pointer to seqcount_t
240 * This can be used to provide an ordering guarantee instead of the
241 * usual consistency guarantee. It is one wmb cheaper, because we can
242 * collapse the two back-to-back wmb()s.
245 * bool X = true, Y = false;
252 * int s = read_seqcount_begin(&seq);
256 * } while (read_seqcount_retry(&seq, s));
265 * raw_write_seqcount_barrier(seq);
270 static inline void raw_write_seqcount_barrier(seqcount_t
*s
)
278 * raw_write_seqcount_latch - redirect readers to even/odd copy
279 * @s: pointer to seqcount_t
281 static inline void raw_write_seqcount_latch(seqcount_t
*s
)
283 smp_wmb(); /* prior stores before incrementing "sequence" */
285 smp_wmb(); /* increment "sequence" before following stores */
289 * Sequence counter only version assumes that callers are using their
292 static inline void write_seqcount_begin_nested(seqcount_t
*s
, int subclass
)
294 raw_write_seqcount_begin(s
);
295 seqcount_acquire(&s
->dep_map
, subclass
, 0, _RET_IP_
);
298 static inline void write_seqcount_begin(seqcount_t
*s
)
300 write_seqcount_begin_nested(s
, 0);
303 static inline void write_seqcount_end(seqcount_t
*s
)
305 seqcount_release(&s
->dep_map
, 1, _RET_IP_
);
306 raw_write_seqcount_end(s
);
310 * write_seqcount_invalidate - invalidate in-progress read-side seq operations
311 * @s: pointer to seqcount_t
313 * After write_seqcount_invalidate, no read-side seq operations will complete
314 * successfully and see data older than this.
316 static inline void write_seqcount_invalidate(seqcount_t
*s
)
323 struct seqcount seqcount
;
328 * These macros triggered gcc-3.x compile-time problems. We think these are
329 * OK now. Be cautious.
331 #define __SEQLOCK_UNLOCKED(lockname) \
333 .seqcount = SEQCNT_ZERO(lockname), \
334 .lock = __SPIN_LOCK_UNLOCKED(lockname) \
337 #define seqlock_init(x) \
339 seqcount_init(&(x)->seqcount); \
340 spin_lock_init(&(x)->lock); \
343 #define DEFINE_SEQLOCK(x) \
344 seqlock_t x = __SEQLOCK_UNLOCKED(x)
347 * Read side functions for starting and finalizing a read side section.
349 static inline unsigned read_seqbegin(const seqlock_t
*sl
)
351 return read_seqcount_begin(&sl
->seqcount
);
354 static inline unsigned read_seqretry(const seqlock_t
*sl
, unsigned start
)
356 return read_seqcount_retry(&sl
->seqcount
, start
);
360 * Lock out other writers and update the count.
361 * Acts like a normal spin_lock/unlock.
362 * Don't need preempt_disable() because that is in the spin_lock already.
364 static inline void write_seqlock(seqlock_t
*sl
)
366 spin_lock(&sl
->lock
);
367 write_seqcount_begin(&sl
->seqcount
);
370 static inline void write_sequnlock(seqlock_t
*sl
)
372 write_seqcount_end(&sl
->seqcount
);
373 spin_unlock(&sl
->lock
);
376 static inline void write_seqlock_bh(seqlock_t
*sl
)
378 spin_lock_bh(&sl
->lock
);
379 write_seqcount_begin(&sl
->seqcount
);
382 static inline void write_sequnlock_bh(seqlock_t
*sl
)
384 write_seqcount_end(&sl
->seqcount
);
385 spin_unlock_bh(&sl
->lock
);
388 static inline void write_seqlock_irq(seqlock_t
*sl
)
390 spin_lock_irq(&sl
->lock
);
391 write_seqcount_begin(&sl
->seqcount
);
394 static inline void write_sequnlock_irq(seqlock_t
*sl
)
396 write_seqcount_end(&sl
->seqcount
);
397 spin_unlock_irq(&sl
->lock
);
400 static inline unsigned long __write_seqlock_irqsave(seqlock_t
*sl
)
404 spin_lock_irqsave(&sl
->lock
, flags
);
405 write_seqcount_begin(&sl
->seqcount
);
409 #define write_seqlock_irqsave(lock, flags) \
410 do { flags = __write_seqlock_irqsave(lock); } while (0)
413 write_sequnlock_irqrestore(seqlock_t
*sl
, unsigned long flags
)
415 write_seqcount_end(&sl
->seqcount
);
416 spin_unlock_irqrestore(&sl
->lock
, flags
);
420 * A locking reader exclusively locks out other writers and locking readers,
421 * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
422 * Don't need preempt_disable() because that is in the spin_lock already.
424 static inline void read_seqlock_excl(seqlock_t
*sl
)
426 spin_lock(&sl
->lock
);
429 static inline void read_sequnlock_excl(seqlock_t
*sl
)
431 spin_unlock(&sl
->lock
);
435 * read_seqbegin_or_lock - begin a sequence number check or locking block
436 * @lock: sequence lock
437 * @seq : sequence number to be checked
439 * First try it once optimistically without taking the lock. If that fails,
440 * take the lock. The sequence number is also used as a marker for deciding
441 * whether to be a reader (even) or writer (odd).
442 * N.B. seq must be initialized to an even number to begin with.
444 static inline void read_seqbegin_or_lock(seqlock_t
*lock
, int *seq
)
446 if (!(*seq
& 1)) /* Even */
447 *seq
= read_seqbegin(lock
);
449 read_seqlock_excl(lock
);
452 static inline int need_seqretry(seqlock_t
*lock
, int seq
)
454 return !(seq
& 1) && read_seqretry(lock
, seq
);
457 static inline void done_seqretry(seqlock_t
*lock
, int seq
)
460 read_sequnlock_excl(lock
);
463 static inline void read_seqlock_excl_bh(seqlock_t
*sl
)
465 spin_lock_bh(&sl
->lock
);
468 static inline void read_sequnlock_excl_bh(seqlock_t
*sl
)
470 spin_unlock_bh(&sl
->lock
);
473 static inline void read_seqlock_excl_irq(seqlock_t
*sl
)
475 spin_lock_irq(&sl
->lock
);
478 static inline void read_sequnlock_excl_irq(seqlock_t
*sl
)
480 spin_unlock_irq(&sl
->lock
);
483 static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t
*sl
)
487 spin_lock_irqsave(&sl
->lock
, flags
);
491 #define read_seqlock_excl_irqsave(lock, flags) \
492 do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
495 read_sequnlock_excl_irqrestore(seqlock_t
*sl
, unsigned long flags
)
497 spin_unlock_irqrestore(&sl
->lock
, flags
);
500 static inline unsigned long
501 read_seqbegin_or_lock_irqsave(seqlock_t
*lock
, int *seq
)
503 unsigned long flags
= 0;
505 if (!(*seq
& 1)) /* Even */
506 *seq
= read_seqbegin(lock
);
508 read_seqlock_excl_irqsave(lock
, flags
);
514 done_seqretry_irqrestore(seqlock_t
*lock
, int seq
, unsigned long flags
)
517 read_sequnlock_excl_irqrestore(lock
, flags
);
519 #endif /* __LINUX_SEQLOCK_H */