Re: [PATCH v13] blk-mq: Rework blk-mq timeout handling again

Keith Busch
2018-05-23T14:02:32Z

On Tue, May 22, 2018 at 09:25:15AM -0700, Bart Van Assche wrote:
> +static bool blk_mq_change_rq_state(struct request *rq,
> + enum mq_rq_state old_state,
> + enum mq_rq_state new_state)
> +{
> + union blk_generation_and_state gstate = READ_ONCE(rq->gstate);
> + union blk_generation_and_state old_val = gstate;
> + union blk_generation_and_state new_val = gstate;
> +
> + old_val.state = old_state;
> + new_val.state = new_state;
> + if (new_state == MQ_RQ_IN_FLIGHT)
> + new_val.generation++;
> + /*
> + * For transitions from state in-flight to another state cmpxchg()
> + * must be used. For other state transitions it is safe to use
> + * WRITE_ONCE().
> + */
> + if (old_state != MQ_RQ_IN_FLIGHT) {
> + WRITE_ONCE(rq->gstate.val, new_val.val);
> + return true;
> + }
> + return blk_mq_set_rq_state(rq, old_val, new_val);
> +}

<snip>

> void blk_mq_complete_request(struct request *rq)
> {
> struct request_queue *q = rq->q;
> - struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, rq->mq_ctx->cpu);
> - int srcu_idx;
>
> if (unlikely(blk_should_fake_timeout(q)))
> return;
>
> - /*
> - * If @rq->aborted_gstate equals the current instance, timeout is
> - * claiming @rq and we lost. This is synchronized through
> - * hctx_lock(). See blk_mq_timeout_work() for details.
> - *
> - * Completion path never blocks and we can directly use RCU here
> - * instead of hctx_lock() which can be either RCU or SRCU.
> - * However, that would complicate paths which want to synchronize
> - * against us. Let stay in sync with the issue path so that
> - * hctx_lock() covers both issue and completion paths.
> - */
> - hctx_lock(hctx, &srcu_idx);
> - if (blk_mq_rq_aborted_gstate(rq) != rq->gstate)
> - __blk_mq_complete_request(rq);
> - hctx_unlock(hctx, srcu_idx);
> + /* The loop is for the unlikely case of a race with the timeout code. */
> + while (true) {
> + if (blk_mq_change_rq_state(rq, MQ_RQ_IN_FLIGHT,
> + MQ_RQ_COMPLETE)) {
> + __blk_mq_complete_request(rq);
> + break;
> + }
> + if (blk_mq_change_rq_state(rq, MQ_RQ_TIMED_OUT, MQ_RQ_COMPLETE))
> + break;
> + }
> }

Looks like the cmpxchg is also needed if old_state is MQ_RQ_TIMED_OUT,
otherwise its guaranteed to return 'true' and there's no point to the
loop and 'if' check.