[dm-devel] [PATCH 8/9] dm: Fix two race conditions related to stopping and starting queues

[Mike Snitzer]

On Thu, Sep 01 2016 at 10:23am -0400,
Bart Van Assche <Bart.VanAssche at sandisk.com> wrote:

> On 08/31/16 20:14, Mike Snitzer wrote:
> > On Wed, Aug 31 2016 at  6:18pm -0400,
> > Bart Van Assche <bart.vanassche at sandisk.com> wrote:
> >
> >> Ensure that all ongoing dm_mq_queue_rq() and dm_mq_requeue_request()
> >> calls have stopped before setting the "queue stopped" flag. This
> >> allows to remove the "queue stopped" test from dm_mq_queue_rq() and
> >> dm_mq_requeue_request(). Use BLK_MQ_S_STOPPED instead of
> >> QUEUE_FLAG_STOPPED.
> >
> > At first glance, at a minimum this patch needs a better header.  It
> > seems you're doing 2 things:
> >
> > 1) using blk_mq_{freeze,unfreeze}_queue() actually makes dm_stop_queue()
> >    work for blk-mq?  Whereby fixing blk-mq race(s)?
> >
> > 2) switching away from QUEUE_FLAG_STOPPED to BLK_MQ_S_STOPPED (via
> >    blk_mq_queue_stopped)
> >    - not clear to me that dm-mq's use of QUEUE_FLAG_STOPPED wasn't fine;
> >      NVMe also uses it for blk-mq
> 
> Hello Mike,
> 
> Adding the blk_mq_{freeze,unfreeze}_queue() calls is indeed what fixes 
> the race conditions related to stopping dm queues and what makes 
> dm_stop_queue() work.

OK, thanks for confirming.

> If other blk-mq users and developers agree that QUEUE_FLAG_STOPPED 
> should be set for stopped blk-mq queues then I think the code to set 
> that flag should be moved into the blk-mq core. However, setting that 
> flag for blk-mq queues seems redundant to me. Hence my proposal to 
> introduce blk_mq_queue_stopped() instead.

This is a secondary issue that can be dealt with independent of the rest
of your DM patchset.

I've staged most of your changes (with slight tweaks), see:
https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-4.9

Only remaining issue is the queue dying race(s) in dm-multipath.