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

Bart Van Assche
2017-03-15T21:35:04Z

On Wed, 2017-03-15 at 20:18 +0800, Ming Lei wrote:
> On Wed, Mar 15, 2017 at 12:07:37AM +0000, Bart Van Assche wrote:
> > 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.

Hello Ming,

You misinterpret what I wrote. I was referring to manipulation of
REQ_ATOM_STARTED from different contexts and not to what you explained.

Bart.