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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: linke li

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))
 				;