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SubjectRe: [PATCH] block, bfq: switch 'bfqg->ref' to use atomic refcount apis
On Tue 03-01-23 16:47:55, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
>
> The updating of 'bfqg->ref' should be protected by 'bfqd->lock', however,
> during code review, we found that bfq_pd_free() update 'bfqg->ref'
> without holding the lock, which is problematic:
>
> 1) bfq_pd_free() triggered by removing cgroup is called asynchronously;
> 2) bfqq will grab bfqg reference, and exit bfqq will drop the reference,
> which can concurrent with 1).
>
> Unfortunately, 'bfqd->lock' can't be held here because 'bfqd' might already
> be freed in bfq_pd_free(). Fix the problem by using atomic refcount apis.
>
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>

The patch looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> Changes from RFC:
> - refcount_dec_and_test(bfqg->ref) -> refcount_dec_and_test(&bfqg->ref)
>
> block/bfq-cgroup.c | 8 +++-----
> block/bfq-iosched.h | 2 +-
> 2 files changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index 1b2829e99dad..7d9b15f0dbd5 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -316,14 +316,12 @@ struct bfq_group *bfqq_group(struct bfq_queue *bfqq)
>
> static void bfqg_get(struct bfq_group *bfqg)
> {
> - bfqg->ref++;
> + refcount_inc(&bfqg->ref);
> }
>
> static void bfqg_put(struct bfq_group *bfqg)
> {
> - bfqg->ref--;
> -
> - if (bfqg->ref == 0)
> + if (refcount_dec_and_test(&bfqg->ref))
> kfree(bfqg);
> }
>
> @@ -530,7 +528,7 @@ static struct blkg_policy_data *bfq_pd_alloc(gfp_t gfp, struct request_queue *q,
> }
>
> /* see comments in bfq_bic_update_cgroup for why refcounting */
> - bfqg_get(bfqg);
> + refcount_set(&bfqg->ref, 1);
> return &bfqg->pd;
> }
>
> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> index 41aa151ccc22..466e4865ace6 100644
> --- a/block/bfq-iosched.h
> +++ b/block/bfq-iosched.h
> @@ -928,7 +928,7 @@ struct bfq_group {
> char blkg_path[128];
>
> /* reference counter (see comments in bfq_bic_update_cgroup) */
> - int ref;
> + refcount_t ref;
> /* Is bfq_group still online? */
> bool online;
>
> --
> 2.31.1
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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