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

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

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
> 
> Hello Ming,

Hello Bart,

> 
> 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.

> stops all hardware queues before calling synchronize_*rcu().
> blk_mq_sched_dispatch_requests() is called with an RCU lock held and checks
> whether a hctx has been stopped before calling .queue_rq(). That is
> sufficient to prevent blk_mq_try_issue_directly() to trigger a .queue_rq()
> call.

Our case should be very similar with example "6.  ANALOGY WITH READER-WRITER
LOCKING" in Documentation/RCU/whatisRCU.txt.

Thanks,
Ming