Re: [PATCH 04/10] blk-mq: serialize updating nr_requests with update_nr_hwq_lock

[Ming Lei]

On Sat, Aug 16, 2025 at 08:49:41AM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2025/8/15 22:47, Ming Lei 写道:
> > On Fri, Aug 15, 2025 at 04:02:10PM +0800, Yu Kuai wrote:
> > > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> > > 
> > > request_queue->nr_requests can be changed by:
> > > 
> > > a) switching elevator by update nr_hw_queues
> > > b) switching elevator by elevator sysfs attribute
> > > c) configue queue sysfs attribute nr_requests
> >   ->elevator_lock is grabbed for updating ->nr_requests except for queue
> > initialization, so what is the real problem you are trying to solve?
> > 
> In order to fix the regression and prevent deadlock, we'll have to allocate
> memory
> 
> in the case bitmap tags grow before freezing the queue, also prevent
> nr_requests
> 
> to be updated by case a and b concurrently.

Yeah, that is potential deadlock issue in updating nr_request.

I feel the patch title is a bit confusing because q->nr_requests updating
is already serialized.


Thanks,
Ming