Re: [PATCH 1/2] blk-mq: don't complete un-started request in timeout handler

Bart Van Assche
2017-03-15T15:37:02Z

On Wed, 2017-03-15 at 20:40 +0800, Ming Lei wrote:
> On Wed, Mar 15, 2017 at 08:18:53PM +0800, Ming Lei wrote:
> > On Wed, Mar 15, 2017 at 12:07:37AM +0000, Bart Van Assche wrote:
> >
> > > or __blk_mq_requeue_request(). Another issue with this function is that the
> >
> > __blk_mq_requeue_request() can be run from two pathes:
> >
> > - dispatch failure, in which case the req/tag isn't released to tag set
> >
> > - IO completion path, in which COMPLETE flag is cleared before requeue.
> >
> > so I can't see races with timeout in case of start rq vs. requeue rq.
>
> Actually rq/tag won't be released to tag set if it will be requeued, so
> the timeout race is nothing to do with requeue.

Hello Ming,

Please have another look at __blk_mq_requeue_request(). In that function
the following code occurs: if (test_and_clear_bit(REQ_ATOM_STARTED,
&rq->atomic_flags)) { ... }

I think the REQ_ATOM_STARTED check in blk_mq_check_expired() races with the
test_and_clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags) call in
__blk_mq_requeue_request().

Bart.