Re: [RFC PATCH v2 7/8] bfq: avoid blkg lookup from locked cgroup update

[Tao Cui <cui.tao@linux.dev>]

在 2026/7/24 20:30, Yu Kuai 写道:
> From: Yu Kuai <yukuai@fygo.io>
> 
> bfq_bio_bfqg() is called while bfqd->lock is held from the merge and
> request insertion paths.  It walks bio->bi_blkg and its parent chain to
> find the closest online BFQ group, and re-associates the bio by looking
> the chosen blkg up again through bio_associate_blkg_from_css().
> 
> Now that blkg creation runs under q->blkcg_mutex from the sleepable submit
> path, bio_associate_blkg_from_css() can sleep on a lookup miss, which BFQ
> must not do while holding bfqd->lock.  The blkg BFQ wants is already in
> hand from the ancestry walk, so update bio->bi_blkg by swapping references
> to that existing blkg directly instead of looking it up again by css.
> 

Just checking the new bfq helper that swaps a bio's blkg: it does the
blkg_put() without the NULL guard the old helper had.  Is there any path
where BFQ could see a bio that hasn't been associated yet (bi_blkg == NULL)?
The fallback branch calls it unconditionally, so if such a path exists it
would crash.  My reading is the bio is always associated before it reaches
BFQ, so it can't happen -- but wanted to ask in case I'm missing a path.

Thanks,
Tao

> Signed-off-by: Yu Kuai <yukuai@fygo.io>
> ---
>  block/bfq-cgroup.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index 42614aa78cd4..8a3ff9510386 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -604,6 +604,16 @@ static void bfq_link_bfqg(struct bfq_data *bfqd, struct bfq_group *bfqg)
>  	}
>  }
>  
> +static void bfq_bio_update_blkg(struct bio *bio, struct blkcg_gq *blkg)
> +{
> +	if (bio->bi_blkg == blkg)
> +		return;
> +
> +	blkg_get(blkg);
> +	blkg_put(bio->bi_blkg);
> +	bio->bi_blkg = blkg;
> +}
> +
>  struct bfq_group *bfq_bio_bfqg(struct bfq_data *bfqd, struct bio *bio)
>  {
>  	struct blkcg_gq *blkg = bio->bi_blkg;
> @@ -616,13 +626,13 @@ struct bfq_group *bfq_bio_bfqg(struct bfq_data *bfqd, struct bio *bio)
>  		}
>  		bfqg = blkg_to_bfqg(blkg);
>  		if (bfqg->pd.online) {
> -			bio_associate_blkg_from_css(bio, &blkg->blkcg->css);
> +			bfq_bio_update_blkg(bio, blkg);
>  			return bfqg;
>  		}
>  		blkg = blkg->parent;
>  	}
> -	bio_associate_blkg_from_css(bio,
> -				&bfqg_to_blkg(bfqd->root_group)->blkcg->css);
> +	blkg = bfqg_to_blkg(bfqd->root_group);
> +	bfq_bio_update_blkg(bio, blkg);
>  	return bfqd->root_group;
>  }
>