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

From: Keith Busch <keith.busch@linux.intel.com>
To: Ming Lei <tom.leiming@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block <linux-block@vger.kernel.org>,
	Sagi Grimberg <sagi@grimberg.me>,
	linux-nvme <linux-nvme@lists.infradead.org>,
	Ming Lei <ming.lei@redhat.com>,
	Keith Busch <keith.busch@intel.com>,
	Jianchao Wang <jianchao.w.wang@oracle.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 1/2] nvme: pci: simplify timeout handling
Date: Thu, 10 May 2018 15:18:29 -0600	[thread overview]
Message-ID: <20180510211829.GC4787@localhost.localdomain> (raw)
In-Reply-To: <CACVXFVNvsjNS0Oi15RnYQm643w-YkFZJT7eKJ+tX_qzGcnu=4A@mail.gmail.com>

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.