Re: [PATCH v6 2/8] sbitmap: fix off-by-one when wrapping hint

[Caleb Sander Mateos]

On Wed, May 7, 2025 at 2:49 PM Uday Shankar <ushankar@xxxxxxxxxxxxxxx> wrote:
>
> In update_alloc_hint_after_get, we wrap the new hint back to 0 one bit
> too early. This breaks round robin tag allocation (BLK_MQ_F_TAG_RR) -
> some tags get skipped, so we don't get round robin tags even in the
> simple case of single-threaded load on a single hctx. Fix the off-by-one
> in the wrapping condition so that round robin tag allocation works
> properly.
>
> The same pattern occurs in __sbitmap_get_word, so fix it there too.

Should this have a Fixes tag? Looks like the off-by-one wrapping has
existed since 4bb659b15699 ("blk-mq: implement new and more efficient
tagging scheme"), but it's only a correctness issue with round-robin
tag allocation, which was added in 24391c0dc57c ("blk-mq: add tag
allocation policy").

I don't have much background on blk-mq's round-robin tag allocation, but FWIW,

Reviewed-by: Caleb Sander Mateos <csander@xxxxxxxxxxxxxxx>

>
> Signed-off-by: Uday Shankar <ushankar@xxxxxxxxxxxxxxx>
> ---
>  lib/sbitmap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index d3412984170c03dc6600bbe53f130404b765ac5a..aa1cec78b9649f1f3e8ef2d617dd7ee724391a8c 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -51,7 +51,7 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
>         } else if (nr == hint || unlikely(sb->round_robin)) {
>                 /* Only update the hint if we used it. */
>                 hint = nr + 1;
> -               if (hint >= depth - 1)
> +               if (hint >= depth)
>                         hint = 0;
>                 this_cpu_write(*sb->alloc_hint, hint);
>         }
> @@ -182,7 +182,7 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
>                         break;
>
>                 hint = nr + 1;
> -               if (hint >= depth - 1)
> +               if (hint >= depth)
>                         hint = 0;
>         }
>
>
> --
> 2.34.1
>