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

[Ming Lei <tom.leiming@gmail.com>]

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.

Thanks,
Ming Lei