Re: [PATCH 1/2] nvme: pci: simplify timeout handling

[Ming Lei <ming.lei@redhat.com>]

On Thu, May 10, 2018 at 03:18:29PM -0600, Keith Busch wrote:
> On Fri, May 11, 2018 at 05:10:40AM +0800, Ming Lei wrote:
> > On Fri, May 11, 2018 at 5:05 AM, Keith Busch
> > <keith.busch@linux.intel.com> wrote:
> > > On Fri, May 11, 2018 at 04:52:11AM +0800, Ming Lei wrote:
> > >> Hi Keith,
> > >>
> > >> On Tue, May 8, 2018 at 11:30 PM, Keith Busch <keith.busch@intel.com> wrote:
> > >> > On Sat, Apr 28, 2018 at 11:50:17AM +0800, Ming Lei wrote:
> > >> >> This sync may be raced with one timed-out request, which may be handled
> > >> >> as BLK_EH_HANDLED or BLK_EH_RESET_TIMER, so the above sync queues can't
> > >> >> work reliably.
> > >> >
> > >> > Ming,
> > >> >
> > >> > As proposed, that scenario is impossible to encounter. Resetting the
> > >> > controller inline with the timeout reaps all the commands, and then
> > >> > sets the controller state to RESETTING. While blk-mq may not allow the
> > >> > driver to complete those requests, having the driver sync with the queues
> > >> > will hold the controller in the reset state until blk-mq is done with
> > >> > its timeout work; therefore, it is impossible for the NVMe driver to
> > >> > return "BLK_EH_RESET_TIMER", and all commands will be completed through
> > >> > nvme_timeout's BLK_EH_HANDLED exactly as desired.
> > >>
> > >> That isn't true for multiple namespace case,  each request queue has its
> > >> own timeout work, and all these timeout work can be triggered concurrently.
> > >
> > > The controller state is most certainly not per queue/namespace. It's
> > > global to the controller. Once the reset is triggered, nvme_timeout can
> > > only return EH_HANDLED.
> > 
> > It is related with EH_HANDLED, please see the following case:
> > 
> > 1) when req A from N1 is timed out, nvme_timeout() handles
> > it as EH_HANDLED: nvme_dev_disable() and reset is scheduled.
> > 
> > 2) when req B from N2 is timed out, nvme_timeout() handles
> > it as EH_HANDLED, then nvme_dev_disable() is called exactly
> > when reset is in-progress, so queues become quiesced, and nothing
> > can move on in the resetting triggered by N1.
> 
> Huh? The nvme_sync_queues ensures that doesn't happen. That was the
> whole point.

Could you share me the link?

Firstly, the previous nvme_sync_queues() won't work reliably, so this
patch introduces blk_unquiesce_timeout() and blk_quiesce_timeout() for
this purpose.

Secondly, I remembered that you only call nvme_sync_queues() at the
entry of nvme_reset_work(), but timeout(either admin or normal IO)
can happen again during resetting, that is another race addressed by
this patchset, but can't cover by your proposal.

Thanks,
Ming