[PATCH 5/5] nvme/pci: Complete all stuck requests

[Sagi Grimberg]

> If the nvme driver is shutting down, it will not start the queues back
> up until asked to resume. If the block layer has entered requests and
> gets a CPU hot plug event prior to the resume event, it will wait for
> those requests to exit. Those requests will never exit since the NVMe
> driver is quieced, creating a deadlock.
>
> This patch fixes that by freezing the queue and flushing all entered
> requests to either their natural completion, or forces their demise. We
> only need to do this when requesting to shutdown the controller since
> we will not be starting the IO queues back up again.

How is this is something specific to nvme? What prevents this
for other multi-queue devices that shutdown during live IO?

Can you please describe the race in specific? Is it stuck on
nvme_ns_remove (blk_cleanup_queue)? If so, then I think we
might want to fix blk_cleanup_queue to start/drain/wait
instead?

I think it's acceptable to have drivers make their own use
of freeze_start and freeze_wait, but if this is not
nvme specific perhaps we want to move it to block instead?