Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme

[tj@xxxxxxxxxx]

Hello, Bart.

On Tue, Dec 12, 2017 at 09:37:11PM +0000, Bart Van Assche wrote:
> Have you considered the following instead of introducing MQ_RQ_IDLE and
> MQ_RQ_IN_FLIGHT? I think this could help to limit the number of new atomic
> operations introduced in the hot path by this patch series.

But nothing in the hot paths is atomic.

> static inline bool blk_mq_rq_in_flight(struct request *rq)
> {
> 	return list_empty(&rq->queuelist);
> }

And the fact that we encode the generation number and state into a
single variable contributes to not needing atomic operations.
Breaking up the state and generation like the above would need more
synchronization, not less.

Thanks.

-- 
tejun