Re: [PATCH v2 0/8] blk-mq: fix & improve queue quiescing

Ming Lei
2017-05-30T00:34:25Z

On Sun, May 28, 2017 at 04:01:13PM +0000, Bart Van Assche wrote:
> On Sun, 2017-05-28 at 19:11 +0800, Ming Lei wrote:
> > On Sat, May 27, 2017 at 09:32:02PM +0000, Bart Van Assche wrote:
> > > On Sat, 2017-05-27 at 22:21 +0800, Ming Lei wrote:
> > > > There are some issues in current blk_mq_quiesce_queue():
> > > >
> > > > - in case of direct issue or BLK_MQ_S_START_ON_RUN, dispatch won't
> > > > be prevented after blk_mq_quiesce_queue() is returned.
> > > > - in theory, new RCU read-side critical sections may begin while
> > > > synchronize_rcu() was waiting, and end after returning from
> > > > synchronize_rcu(), then dispatch still may be run after
> > > > synchronize_rcu() returns
> > >
> > > I disagree with the second part of your statement. blk_mq_quiesce_queue()
> >
> > You can find the similar description from line 158 to line 180.
> > of Documentation/RCU/whatisRCU.txt.
> >
> > For example, there may be schedule preempt or irq handling between
> > checking stopped flag(false) and the RCU read-side critical sections, and
> > synchronize_*rcu() may return before running this RCU read-side critical
> > sections. That is why we have to move the check into RCU read-side
> > critical sections.
>
> Hello Ming,
>
> Have you noticed that in the current blk-mq code the test of the hctx
> stopped flag already occurs *inside* an RCU read-side critical section?
> __blk_mq_run_hw_queue() grabs an RCU read lock before
> blk_mq_sched_dispatch_requests() calls blk_mq_hctx_stopped().

OK, that is what I missed, will update the commit log in next post.

Thanks,
Ming