[PATCH] sbitmap: use READ_ONCE to access map->word

[PATCH] sbitmap: use READ_ONCE to access map->word

[linke li]

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>
---
 lib/sbitmap.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 92c6b1fd8989..1e453f825c05 100644
--- 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))
 				;
-- 
2.39.3 (Apple Git-146)

Re: [PATCH] sbitmap: use READ_ONCE to access map->word

[Jens Axboe]

On Fri, 26 Apr 2024 18:34:44 +0800, linke li wrote:
> 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.
> 
> [...]

Applied, thanks!

[1/1] sbitmap: use READ_ONCE to access map->word
      commit: 6ad0d7e0f4b68f87a98ea2b239123b7d865df86b

Best regards,
-- 
Jens Axboe