[PATCH] NVMe: Fix possible scheduling while atomic error

[Christoph Hellwig]

On Wed, May 25, 2016 at 01:57:37PM -0400, Keith Busch wrote:
> > > On Tue, May 24, 2016 at 11:43:21AM -0700, Ming Lin wrote:
> > > > 
> > > > It's blk_mq_cancel_requeue_work() that potentially sleeps.
> > > 
> > > Right, I don't know what I was thinking... 
> > > 
> > > AFAICT, the patch is fine as-is if Christoph is okay with it.
> > 
> > I still don't why we need to delay the call to blk_mq_stop_hw_queues
> > to ->queue_rq.
> 
> Eh? It's still stopped in the same place as always, so it's not
> delayed. We just call it again from queue_rq because requeue_work can
> start h/w queues on its own. This is very similar to scsi's queue_rq
> in scsi_lib.c.

The rationale in SCSI is that we stop the queue when we are above
the hosts or targets queuing limit, and only I/O ocmpletions will
wake it up again.  Kicking of the sotpped queues in blk_mq_requeue_work
is SCSI specific behavior that leaked into the core, and it's my fault..

Well' let's get the fix in as-is for now, but I really need to sort
this out properly, so maybe add some comments.

> 
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