Re: [PATCH V2 0/4] nvme: fix two kinds of IO hang from removing NSs

[Keith Busch <kbusch@kernel.org>]

On Thu, Jun 22, 2023 at 09:51:12PM +0800, Ming Lei wrote:
> On Wed, Jun 21, 2023 at 09:48:49AM -0600, Keith Busch wrote:
> > The point was to contain requests from entering while the hctx's are
> > being reconfigured. If you're going to pair up the freezes as you've
> > suggested, we might as well just not call freeze at all.
> 
> blk_mq_update_nr_hw_queues() requires queue to be frozen.

It's too late at that point. Let's work through a real example. You'll
need a system that has more CPU's than your nvme has IO queues.

Boot without any special nvme parameters. Every possible nvme IO queue
will be assigned "default" hctx type. Now start IO to every queue, then
run:

  # echo 8 > /sys/modules/nvme/parameters/poll_queues && echo 1 > /sys/class/nvme/nvme0/reset_controller

Today, we freeze prior to tearing down the "default" IO queues, so
there's nothing entered into them while the driver reconfigures the
queues.

What you're suggesting will allow IO to queue up in a queisced "default"
queue, which will become "polled" without an interrupt hanlder on the
other side of the reset. The application doesn't know that, so the IO
you're allowing to queue up will time out.