Re: [PATCH v9 2/2] blk-mq: Rework blk-mq timeout handling again

["hch@lst.de" <hch@lst.de>]

On Wed, May 16, 2018 at 04:47:54PM +0000, Bart Van Assche wrote:
> I think your patch changes the order of changing the request state and
> calling mod_timer(). In my patch the request state and the deadline are
> updated first and mod_timer() is called afterwards. I think your patch
> changes the order of these operations into the following:
> (1) __blk_mq_start_request() sets the request deadline.
> (2) __blk_mq_start_request() calls __blk_add_timer() which in turn calls
>     mod_timer().
> (3) __blk_mq_start_request() changes the request state into MQ_RQ_IN_FLIGHT.
> 
> In the unlikely event of a significant delay between (2) and (3) it can
> happen that the timer fires and examines and ignores the request because
> its state differs from MQ_RQ_IN_FLIGHT. If the request for which this
> happened times out its timeout will only be handled the next time
> blk_mq_timeout_work() is called. Is this the behavior you intended?

We can move the timer manipulation after the change easily I think.
It would make sense to add comments explaining the ordering.