Commit 6ad0d7e0 authored by linke li's avatar linke li Committed by Jens Axboe
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sbitmap: use READ_ONCE to access map->word 



In __sbitmap_queue_get_batch(), map->word is read several times, and
update atomically using atomic_long_try_cmpxchg(). But the first two read
of map->word is not protected.

This patch moves the statement val = READ_ONCE(map->word) forward,
eliminating unprotected accesses to map->word within the function.
It is aimed at reducing the number of benign races reported by KCSAN in
order to focus future debugging effort on harmful races.

Signed-off-by: default avatarlinke li <lilinke99@qq.com>
Link: https://lore.kernel.org/r/tencent_0B517C25E519D3D002194E8445E86C04AD0A@qq.com


Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 07d1b998

lib/sbitmap.c

+4 −4
Original line number Diff line number Diff line
@@ -494,18 +494,18 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags, 
		struct sbitmap_word *map = &sb->map[index];

		unsigned long get_mask;

		unsigned int map_depth = __map_depth(sb, index);

		unsigned long val;



		sbitmap_deferred_clear(map);

		if (map->word == (1UL << (map_depth - 1)) - 1)

		val = READ_ONCE(map->word);

		if (val == (1UL << (map_depth - 1)) - 1)

			goto next;



		nr = find_first_zero_bit(&map->word, map_depth);

		nr = find_first_zero_bit(&val, map_depth);

		if (nr + nr_tags <= map_depth) {

			atomic_long_t *ptr = (atomic_long_t *) &map->word;

			unsigned long val;



			get_mask = ((1UL << nr_tags) - 1) << nr;

			val = READ_ONCE(map->word);

			while (!atomic_long_try_cmpxchg(ptr, &val,

							  get_mask | val))

				;