Re: [PATCH v2 -next] blk-cgroup: synchoronize blkg creation against policy deactivation

[yukuai (C)]

On 2021/10/15 7:02, Tejun Heo wrote:
> On Wed, Oct 13, 2021 at 08:44:56PM +0800, Yu Kuai wrote:
> > @@ -1401,6 +1406,7 @@ void blkcg_deactivate_policy(struct request_queue *q,
> >   	if (queue_is_mq(q))
> >   		blk_mq_freeze_queue(q);
> >   
> > +	mutex_lock(&q->blkg_lock);
> >   	spin_lock_irq(&q->queue_lock);
>
> Given that deactivation drains q_usage_counter through
> blk_mq_freeze_queue(), can't the blkg_conf_prep() just pin the usage count?

In fact, the caller of blkcg_deactivate_policy() aready freeze the
queue, either from blk_cleanup_queue() or from elevator_switch().

Grab q_usage_counter in blk_conf_prep() can prevent concurrent with
blkcg_deactivate_policy(), I'm afraid will it be too much since it
will also be synchoronized with all the other freeze/unfreeze queues?

Thanks,
Kuai