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

Ming Lei
2017-03-16T00:08:00Z

On Wed, Mar 15, 2017 at 09:35:03PM +0000, Bart Van Assche wrote:
> On Thu, 2017-03-16 at 00:22 +0800, Ming Lei wrote:
> > On Wed, Mar 15, 2017 at 03:36:31PM +0000, Bart Van Assche wrote:
> > > 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().
> >
> > OK, this race should only exist in case that the requeue happens after dispatch
> > busy, because COMPLETE flag isn't set. And if the requeue is from io completion,
> > no such race because COMPLETE flag is set.
> >
> > One solution I thought of is to call blk_mark_rq_complete() before requeuing
> > when dispatch busy happened, but that looks a bit silly. Another way is to
> > set STARTED flag just after .queue_rq returns BLK_MQ_RQ_QUEUE_OK, which looks
> > reasonable too. Any comments on the 2nd solution?
>
> Sorry but I don't think that would be sufficient. There are several other
> scenarios that have not been mentioned above, e.g. that a tag gets freed and
> reused after the blk_mq_check_expired() call started and before that function
> has had the chance to examine any members of struct request. What is needed in
> blk_mq_check_expired() is the following as a single atomic operation:

We have dealt with this by checking COMPLETE & rq->deadline together in
blk_mq_check_expired() already:

- if new rq->deadline(set in reuse path) has been observed in the later
checking rq of blk_mq_check_expired(), it won't be timeouted because of the timing.

- if new rq->deadline(set in reuse path) hasn't been observed in the
later checking rq of blk_mq_check_expired(), that means COMPLETE flag isn't set
yet in reuse path because we have a barrier to enhance the order in
blk_mq_start_request(), so it won't be timeouted too.

So let me know what is the real race between free/reusing vs. timeout.

> * Check whether REQ_ATOM_STARTED has been set.
> * Check whether REQ_ATOM_COMPLETE has not yet been set.
> * If both conditions have been met, set REQ_ATOM_COMPLETE.
>
> I don't think there is another solution than using a single state variable to
> represent the REQ_ATOM_STARTED and REQ_ATOM_COMPLETE states instead of two
> independent bits. How about the patch below?

I would review it if you can confirm me that it is a real issue, :-)

Thanks,
Ming