Re: dm-rq: don't call blk_mq_queue_stopped in dm_stop_queue()

[Ming Lei <ming.lei@redhat.com>]

On Fri, Jun 19, 2020 at 12:06:57PM -0400, Mike Snitzer wrote:
> On Fri, Jun 19 2020 at  6:11am -0400,
> Ming Lei <ming.lei@redhat.com> wrote:
> 
> > Hi Mike,
> > 
> > On Fri, Jun 19, 2020 at 05:42:50AM -0400, Mike Snitzer wrote:
> > > Hi Ming,
> > > 
> > > Thanks for the patch!  But I'm having a hard time understanding what
> > > you've written in the patch header,
> > > 
> > > On Fri, Jun 19 2020 at  4:42am -0400,
> > > Ming Lei <ming.lei@redhat.com> wrote:
> > > 
> > > > dm-rq won't stop queue, meantime blk-mq won't stop one queue too, so
> > > > remove the check.
> > > 
> > > It'd be helpful if you could unpack this with more detail before going on
> > > to explain why using blk_queue_quiesced, despite dm-rq using
> > > blk_mq_queue_stopped, would also be ineffective.
> > > 
> > > SO:
> > > 
> > > > dm-rq won't stop queue
> > > 
> > > 1) why won't dm-rq stop the queue?  Do you mean it won't reliably
> > >    _always_ stop the queue because of the blk_mq_queue_stopped() check?
> > 
> > device mapper doesn't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues.
> > 
> > > 
> > > > meantime blk-mq won't stop one queue too, so remove the check.
> > > 
> > > 2) Meaning?: blk_mq_queue_stopped() will return true even if only one hw
> > > queue is stopped, given blk-mq must stop all hw queues a positive return
> > > from this blk_mq_queue_stopped() check is incorrectly assuming it meanss
> > > all hw queues are stopped.
> > 
> > blk-mq won't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues for
> > dm-rq's queue too, so dm-rq's hw queue won't be stopped.
> > 
> > BTW blk_mq_stop_hw_queue or blk_mq_stop_hw_queues are supposed to be
> > used for throttling queue.
> 
> I'm going to look at actually stopping the queue (using one of these
> interfaces).  I didn't realize I wasn't actually stopping the queue.
> The intent was to do so.
> 
> In speaking with Jens yesterday about freeze vs stop: it is clear that
> dm-rq needs to still be able to allocate new requests, but _not_ call
> the queue_rq to issue the requests, while "stopped" (due to dm-mpath
> potentially deferring retries of failed requests because of path failure
> while quiescing the queue during DM device suspend).  But that freezing
> the queue goes too far because it won't allow such request allocation.

Freezing shouldn't be a good choice for driver usually, and quiesce is
exactly what you expect: request allocation is allowed, meantime, no
.queue_rq is possible.

> 
> > > > dm_stop_queue() actually tries to quiesce hw queues via blk_mq_quiesce_queue(),
> > > > we can't check via blk_queue_quiesced for avoiding unnecessary queue
> > > > quiesce because the flag is set before synchronize_rcu() and dm_stop_queue
> > > > may be called when synchronize_rcu from another blk_mq_quiesce_queue is
> > > > in-progress.
> > > 
> > > But I'm left with questions/confusion on this too:
> > > 
> > > 1) you mention blk_queue_quiesced instead of blk_mq_queue_stopped, so I
> > >    assume you mean that: not only is blk_mq_queue_stopped()
> > >    ineffective, blk_queue_quiesced() would be too?
> > 
> > blk_mq_queue_stopped isn't necessary because dm-rq's hw queue won't be
> > stopped by anyone, meantime replacing it with blk_queue_quiesced() is wrong.
> > 
> > > 
> > > 2) the race you detail (with competing blk_mq_quiesce_queue) relative to
> > >    synchronize_rcu() and testing "the flag" is very detailed yet vague.
> > 
> > If two code paths are calling dm_stop_queue() at the same time, one path may
> > return immediately and it is wrong, sine synchronize_rcu() from another path
> > may not be done.
> > 
> > > 
> > > Anyway, once we get this heaader cleaned up a bit more I'll be happy to
> > > get this staged as a stable@ fix for 5.8 inclusion ASAP.
> > 
> > This patch isn't a fix, and it shouldn't be related with rhel8's issue.
> 
> I realize that now.  I've changed the patch header to be a bit clearer
> and staged it for 5.9, see:
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.9&id=06e788ed59e0095b679bdce9e39c1a251032ae62

Thanks!

-- 
Ming