Re: [PATCH 0/5] blk-cgroup: fix blkg list and policy data races

[Yizhou Tang]

On Wed, Jun 3, 2026 at 9:35 PM Yu Kuai <yukuai@xxxxxxx> wrote:
>
> This small series fixes races between blkg destruction, q->blkg_list
> iteration, and blkcg policy activation.
>
> The first two patches serialize q->blkg_list walks in blkg_destroy_all()
> and BFQ writeback weight-raising teardown with blkcg_mutex. The next two
> patches close policy activation races with concurrent blkg destruction,
> including skipping blkgs that are already dying. The final patch factors
> the common policy data teardown loop.
>
> This uses blkcg_mutex rather than extending queue_lock coverage because
> the races are about blkg list visibility and policy-data lifetime, not
> request-queue dispatch state. blkg_free_workfn() already uses
> blkcg_mutex to serialize policy-data freeing with policy deactivation
> and removes blkgs from q->blkg_list only after that teardown. Taking the
> same mutex around the remaining q->blkg_list walkers gives one sleepable
> serialization point for blkg lifetime, avoids adding more queue_lock
> nesting, and prepares the follow-up conversion that removes queue_lock
> from blkcg list protection entirely.

LGTM.

I had noticed some time ago that blkcg_activate_policy() did not take
q->blkcg_mutex, and this patchset nicely resolves my concern.

Reviewed-by: Tang Yizhou <yizhou.tang@xxxxxxxxxx>

Best regards,
Yi


>
> Yu Kuai (2):
>   blk-cgroup: protect q->blkg_list iteration in blkg_destroy_all() with
>     blkcg_mutex
>   bfq: protect q->blkg_list iteration in bfq_end_wr_async() with
>     blkcg_mutex
>
> Zheng Qixing (3):
>   blk-cgroup: fix race between policy activation and blkg destruction
>   blk-cgroup: skip dying blkg in blkcg_activate_policy()
>   blk-cgroup: factor policy pd teardown loop into helper
>
>  block/bfq-cgroup.c  |  3 ++-
>  block/bfq-iosched.c |  6 +++++
>  block/blk-cgroup.c  | 65 ++++++++++++++++++++++++---------------------
>  3 files changed, 43 insertions(+), 31 deletions(-)
>
> --
> 2.51.0
>