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sbitmap: Replace cmpxchg with xchg 

cmpxchg() with an immediate value could be replaced with less expensive
xchg(). The same true if new value don't _depend_ on the old one.

In the second block, atomic_cmpxchg() return value isn't checked, so
after atomic_cmpxchg() ->  atomic_xchg() conversion it could be replaced
with atomic_set(). Comparison with atomic_read() in the second chunk was
left as an optimisation (if that was the initial intention).

Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Pavel Begunkov 2019-05-23 18:39:16 +03:00 committed by Jens Axboe
parent 79d08f89bb
commit 417232880c
1 changed files with 3 additions and 7 deletions
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lib/sbitmap.c
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@ -26,9 +26,7 @@ static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
/*
* First get a stable cleared mask, setting the old mask to 0.
*/
do {
mask = sb->map[index].cleared;
} while (cmpxchg(&sb->map[index].cleared, mask, 0) != mask);
mask = xchg(&sb->map[index].cleared, 0);
/*
* Now clear the masked bits in our free word
@ -516,10 +514,8 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
struct sbq_wait_state *ws = &sbq->ws[wake_index];
if (waitqueue_active(&ws->wait)) {
int o = atomic_read(&sbq->wake_index);
if (wake_index != o)
atomic_cmpxchg(&sbq->wake_index, o, wake_index);
if (wake_index != atomic_read(&sbq->wake_index))
atomic_set(&sbq->wake_index, wake_index);
return ws;
}