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: linke li <lilinke99@qq.com>
Link: https://lore.kernel.org/r/tencent_0B517C25E519D3D002194E8445E86C04AD0A@qq.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 92c6b1fd898938e4613d8289cf49c09fd53bb93b..1e453f825c05d34f299abcecb12a94c2fe9d023b 100644 (file)
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -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))
                                ;