[PATCH 0/4] nvme-blkmq fixes

[Keith Busch]

On Mon, 5 Jan 2015, Jens Axboe wrote:
> On 01/05/2015 08:17 AM, Keith Busch wrote:
>> On Wed, 31 Dec 2014, Jens Axboe wrote:
>> We need the driver to temporarily block tasks allocating new requests but
>> let existing requests requeue. Freeze looked good, but unfreeze expects
>> the usage count to have been 0, which it's not guaranteed with when we
>> let failed requests requeue.
>
> OK, I think that is a concern we can fix. And yes, that was the intended
> use case for it originally.
>

Okay cool, that would help a lot.

>> We also need the driver to temporarily prevent the block layer from
>> submitting requests to the driver's hw queues. 'blk_mq_stop_hw_queues'
>> looked right, but anyone can restart them at the wrong time by kicking
>> the requeue list.
>
> The driver is the only one that should kick the requeue action into
> gear, which would start those queues up again. So that should be under
> your control already.

Right, we can stop the driver from kicking if it knows a device reset
is occuring, but there can't be any requeue work prior to stopping all
h/w queues to prevent a race condition. We could have the driver's reset
handler call 'cancel_work_sync(q->requeue_work)' to address that. There's
no existing driver using q->reset_work, but it looks safe to treat
as public.