Re: [PATCH 2/8] block: Do not run frozen queues

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jul 02, 2025 at 06:30:34PM -0700, Bart Van Assche wrote:
> On 7/2/25 6:15 PM, Yu Kuai wrote:
> > Just a question, blk_mq_quiesce_queue also calls synchronize_rcu() to
> > wait for inflight dispatch work to be done, is it safe in following
> > patches? I think it's not, dispatch work can be running while there is
> > no request pending, menas queue can be frozen with active dispatch work.
> 
> No work is dispatched if a queue is frozen because freezing a queue only
> finishes after q_usage_counter drops to zero. queue_rq() and queue_rqs()
> are only called if one or more requests are being executed.
> q_usage_counter is increased when a request is allocated and decreased
> when a request is freed. Hence, q_usage_counter cannot be zero while
> queue_rq() or queue_rqs() is in progress. Hence, neither queue_rq() nor
> queue_rqs() are called while q_usage_counter is zero.

It isn't related with queue_rq() or queue_rqs() only.

The dispatch work is still in-progress when all requests are
completed(.q_usage_counter becomes zero), because request can complete any
time, even before queue_rq()/queue_rqs() returns.

The dispatch critical area is much _longer_ than queue_rq()/queue_rqs(),
block layer data structure may still be accessed after .q_usage_counter drops
to zero.

Please run 'git grep', and we did fix such kind of issues several times, such as:

662156641bc4 ("block: don't drain in-progress dispatch in blk_cleanup_queue()")


Thanks,
Ming