Commit c3250c8d authored Nov 22, 2020 by Pavel Begunkov Committed by Jens Axboe Dec 07, 2020

sbitmap: replace CAS with atomic and



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>

parent 661d4f55

lib/sbitmap.c

+3 −5

Original line number	Diff line number	Diff line
		@@ -14,7 +14,7 @@
		*/
		static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
		{
		unsigned long mask, val;
		unsigned long mask;

		if (!READ_ONCE(map->cleared))
		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
		*/
		do {
		val = map->word;
		} while (cmpxchg(&map->word, val, val & ~mask) != val);

		atomic_long_andnot(mask, (atomic_long_t *)&map->word);
		BUILD_BUG_ON(sizeof(atomic_long_t) != sizeof(map->word));
		return true;
		}

Admin message