[PATCH V4] blk-mq: introduce BLK_STS_DEV_RESOURCE

[Bart.VanAssche@wdc.com (Bart Van Assche)]

On Mon, 2018-01-29@17:14 -0500, Mike Snitzer wrote:
> On Mon, Jan 29 2018 at  4:51pm -0500,
> Bart Van Assche <Bart.VanAssche@wdc.com> wrote:
> 
> > On Mon, 2018-01-29@16:44 -0500, Mike Snitzer wrote:
> > > But regardless of which wins the race, the queue will have been run.
> > > Which is all we care about right?
> > 
> > Running the queue is not sufficient. With this patch applied it can happen
> > that the block driver returns BLK_STS_DEV_RESOURCE, that the two or more
> > concurrent queue runs finish before sufficient device resources are available
> > to execute the request and that blk_mq_delay_run_hw_queue() does not get
> > called at all. If no other activity triggers a queue run, e.g. request
> > completion, this will result in a queue stall.
> 
> If BLK_STS_DEV_RESOURCE is returned then the driver doesn't need to rely
> on a future queue run.  IIUC, that is the entire premise of
> BLK_STS_DEV_RESOURCE.  If the driver had doubt about whether the
> resource were going to be available in the future it should return
> BLK_STS_RESOURCE.
> 
> That may seem like putting a lot on a driver developer (to decide
> between the 2) but I'll again defer to Jens here.  This was the approach
> he was advocating be pursued.

Hello Mike,

There was a typo in my reply: I should have referred to BLK_STS_RESOURCE
instead of BLK_STS_DEV_RESOURCE. The convention for BLK_STS_DEV_RESOURCE is
namely that if .queue_rq() returns that status code that the block driver
guarantees that the queue will be rerun.

Bart.