2024-04-26 10:40:13

by linke li

Subject: [PATCH] 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]>
---
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)

2024-04-26 13:41:09

by Jens Axboe

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

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