sbitmap: Replace cmpxchg with xchg (41723288) · Commits · 戴 / test

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>

lib/sbitmap.c

+3 −7

Original line number	Diff line number	Diff line
		@@ -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;
		}

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