Re: [RFC PATCH v3 2/3] blk-mq: Freeze and quiesce all queues for tagset in elevator_exit()

[John Garry]

On 06/03/2021 04:32, Bart Van Assche wrote:
> On 3/5/21 7:14 AM, John Garry wrote:
> > diff --git a/block/blk.h b/block/blk.h
> > index 3b53e44b967e..1a948bfd91e4 100644
> > --- a/block/blk.h
> > +++ b/block/blk.h
> > @@ -201,10 +201,29 @@ void elv_unregister_queue(struct request_queue *q);
> >   static inline void elevator_exit(struct request_queue *q,
> >   		struct elevator_queue *e)
> >   {
> > +	struct blk_mq_tag_set *set = q->tag_set;
> > +	struct request_queue *tmp;
> > +
> >   	lockdep_assert_held(&q->sysfs_lock);
> >   
> > +	mutex_lock(&set->tag_list_lock);
> > +	list_for_each_entry(tmp, &set->tag_list, tag_set_list) {
> > +		if (tmp == q)
> > +			continue;
> > +		blk_mq_freeze_queue(tmp);
> > +		blk_mq_quiesce_queue(tmp);
> > +	}
> > +
> >   	blk_mq_sched_free_requests(q);
> >   	__elevator_exit(q, e);
> > +
> > +	list_for_each_entry(tmp, &set->tag_list, tag_set_list) {
> > +		if (tmp == q)
> > +			continue;
> > +		blk_mq_unquiesce_queue(tmp);
> > +		blk_mq_unfreeze_queue(tmp);
> > +	}
> > +	mutex_unlock(&set->tag_list_lock);
> >   }

Hi Bart,

> This patch introduces nesting of tag_list_lock inside sysfs_lock. The
> latter is per request queue while the former can be shared across
> multiple request queues. Has it been analyzed whether this is safe?

Firstly - ignoring implementation details for a moment - this patch is 
to ensure that the concept is consistent with your suggestion and 
whether it is sound.

As for nested locking, I can analyze more, but I did assume that we 
don't care about locking-out sysfs intervention during this time. And it 
seems pretty difficult to avoid nesting the locks.

And further to this, I see that 
https://lore.kernel.org/linux-block/3aa5407c-0800-2482-597b-4264781a7eac@xxxxxxxxxxx/T/#mc3e3175642660578c0ae2a6c32185b1e34ec4b8a 
has a new interface for tagset quiesce, which could make this process 
more efficient.

Please let me know further thoughts.

Thanks,
John