Re: [PATCH 05/14] blk-mq-sched: don't dequeue request until all in ->dispatch are flushed

Bart Van Assche
2017-08-03T01:34:37Z

On Wed, 2017-08-02 at 11:01 +0800, Ming Lei wrote:
> On Tue, Aug 01, 2017 at 04:14:07PM +0000, Bart Van Assche wrote:
> > On Tue, 2017-08-01 at 18:44 +0800, Ming Lei wrote:
> > > On Mon, Jul 31, 2017 at 11:42:21PM +0000, Bart Van Assche wrote:
> > > > Since setting, clearing and testing of BLK_MQ_S_BUSY can happen concurrently
> > > > and since clearing and testing happens without any locks held I'm afraid this
> > > > patch introduces the following race conditions:
> > > > [ ... ]
> > > > * Checking BLK_MQ_S_BUSY after requests have been removed from the dispatch list
> > > > but before that bit is cleared, resulting in test_bit(BLK_MQ_S_BUSY, &hctx->state)
> > > > reporting that the BLK_MQ_S_BUSY
> > > > has been set although there are no requests
> > > > on the dispatch list.
> > >
> > > That won't be a problem, because dispatch will be started in the
> > > context in which dispatch list is flushed, since the BUSY bit
> > > is cleared after blk_mq_dispatch_rq_list() returns. So no I/O
> > > hang.
> >
> > Hello Ming,
> >
> > Please consider changing the name of the BLK_MQ_S_BUSY constant. That bit
> > is used to serialize dispatching requests from the hctx dispatch list but
> > that's not clear from the name of that constant.
>
> Actually what we want to do is to stop taking request from sw/scheduler
> queue when ->dispatch aren't flushed completely, I think BUSY isn't
> a bad name for this case, or how about DISPATCH_BUSY? or
> FLUSHING_DISPATCH?

Hello Ming,

FLUSHING_DISPATCH sounds fine to me. In case you would prefer a shorter name,
how about BLK_MQ_S_DISPATCHING (refers to dispatching requests to the driver)?

Bart.