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

Ming Lei
2017-03-15T12:20:00Z

On Wed, Mar 15, 2017 at 12:07:37AM +0000, Bart Van Assche wrote:
> On Thu, 2017-03-09 at 21:02 +0800, Ming Lei wrote:
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 159187a28d66..0aff380099d5 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -697,17 +697,8 @@ static void blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
> > {
> > struct blk_mq_timeout_data *data = priv;
> >
> > - if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags)) {
> > - /*
> > - * If a request wasn't started before the queue was
> > - * marked dying, kill it here or it'll go unnoticed.
> > - */
> > - if (unlikely(blk_queue_dying(rq->q))) {
> > - rq->errors = -EIO;
> > - blk_mq_end_request(rq, rq->errors);
> > - }
> > + if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags))
> > return;
> > - }
> >
> > if (time_after_eq(jiffies, rq->deadline)) {
> > if (!blk_mark_rq_complete(rq))
>
> Both the old and the new check look racy to me. The REQ_ATOM_STARTED bit can
> be changed concurrently by blk_mq_start_request(), __blk_mq_finish_request()

blk_mq_start_request() and __blk_mq_finish_request() won't be run concurrently.

>From view of __blk_mq_finish_request():

- if it is run from merge queue io path(blk_mq_merge_queue_io()),
blk_mq_start_request() can't be run at all, and the COMPLETE flag
is kept as previous value(zero)

- if it is run from normal complete path, COMPLETE flag is cleared
before the req/tag is released to tag set.

so there isn't race in blk_mq_start_request() vs. __blk_mq_finish_request()
wrt. timeout.

> 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.

> request passed to this function can be reinitialized concurrently. Sorry but

Yes, that is possible, but rq->atomic_flags is kept, and in that case
when we handle timeout, COMPLETE is cleared before releasing the rq/tag to
tag set via blk_mark_rq_complete(), so we won't complete that req twice.

Thanks,
Ming