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sbitmap: replace CAS with atomic and

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sbitmap_deferred_clear() does CAS loop to propagate cleared bits,
replace it with equivalent atomic bitwise and. That's slightly faster
and makes wait-free instead of lock-free as before.

The atomic can be relaxed (i.e. barrier-less) because following
sbitmap_get*() deal with synchronisation, see comments in
sbitmap_queue_clear().

It's ok to cast to atomic_long_t, that's what bitops/lock.h does.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Pavel Begunkov
2020-11-22 15:35:47 +00:00
committed by Jens Axboe
parent 661d4f55a7
commit c3250c8d24
1 changed files with 3 additions and 5 deletions
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8
lib/sbitmap.c
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@@ -14,7 +14,7 @@
*/ */
static inline bool sbitmap_deferred_clear(struct sbitmap_word *map) static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
{ {
unsigned long mask, val; unsigned long mask;
if (!READ_ONCE(map->cleared)) if (!READ_ONCE(map->cleared))
return false; return false;
@@ -27,10 +27,8 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
/* /*
* Now clear the masked bits in our free word * Now clear the masked bits in our free word
*/ */
do { atomic_long_andnot(mask, (atomic_long_t *)&map->word);
val = map->word; BUILD_BUG_ON(sizeof(atomic_long_t) != sizeof(map->word));
} while (cmpxchg(&map->word, val, val & ~mask) != val);
return true; return true;
} }