Re: [PATCH] block, bfq: protect async queue reset with blkcg locks

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

Nice catch. The race is real, and the fix lines up with how the rest
of the blkcg code already protects blkg_list walks — the new nesting
(blkcg_mutex -> queue_lock -> bfqd->lock) is the same order
blkg_free_workfn() and bfq_pd_offline() use, so no inversion.

Reviewed-by: Tao Cui <cuitao@kylinos.cn>

在 2026/6/21 21:59, Cen Zhang 写道:
> Writing 0 to BFQ's low_latency attribute ends weight raising for active,
> idle and async queues. The async cgroup path walks q->blkg_list, converts
> each blkg to BFQ policy data and then reads bfqg->async_bfqq and
> bfqg->async_idle_bfqq.
> 
> That walk was protected only by bfqd->lock. blkcg release work is
> serialized by q->blkcg_mutex and q->queue_lock instead, and
> blkg_free_workfn() can call BFQ's pd_free_fn before it removes
> blkg->q_node from q->blkg_list. A low_latency reset can therefore still
> find the blkg on the queue list after the BFQ policy data has been freed.
> 
> The buggy scenario involves two paths, with each column showing the order
> within that path:
> 
> BFQ low_latency reset:              blkcg blkg release work:
> 1. bfq_low_latency_store()          1. blkg_free_workfn() takes
>    calls bfq_end_wr().                 q->blkcg_mutex.
> 2. bfq_end_wr_async() walks         2. BFQ pd_free_fn drops the
>    q->blkg_list.                       final bfq_group reference.
> 3. blkg_to_bfqg() returns           3. blkg->q_node remains on
>    the stale policy data.              q->blkg_list until list_del_init().
> 4. bfq_end_wr_async_queues()
>    reads async queue fields.
> 
> Fix this by taking q->blkcg_mutex and q->queue_lock around the
> q->blkg_list walk, then taking bfqd->lock before touching BFQ async
> queues. The mutex serializes against policy-data free and queue_lock
> stabilizes the list. Move the async reset out of bfq_end_wr()'s existing
> bfqd->lock critical section so the lock order matches blkcg policy
> callbacks.