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SubjectRe: [PATCH v1 2/3] blk-mq: comment on races related with timeout handler
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On Fri, 2017-03-17 at 17:57 +0800, Ming Lei wrote:
> +/*
> + * When we reach here because queue is busy, REQ_ATOM_COMPLETE
> + * flag isn't set yet, so there may be race with timeout hanlder,
> + * but given rq->deadline is just set in .queue_rq() under
> + * this sitation, the race won't be possible in reality because
> + * rq->timeout should be set as big enough to cover the window
> + * between blk_mq_start_request() called from .queue_rq() and
> + * clearing REQ_ATOM_STARTED here.
> + */
> static void __blk_mq_requeue_request(struct request *rq)
> {
> struct request_queue *q = rq->q;
> @@ -700,6 +709,19 @@ static void blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
> if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags))
> return;
>
> + /*
> + * The rq being checked may have been freed and reallocated
> + * out already here, we avoid this race by checking rq->deadline
> + * and REQ_ATOM_COMPLETE flag together:
> + *
> + * - if rq->deadline is observed as new value because of
> + * reusing, the rq won't be timed out because of timing.
> + * - if rq->deadline is observed as previous value,
> + * REQ_ATOM_COMPLETE flag won't be cleared in reuse path
> + * because we put a barrier between setting rq->deadline
> + * and clearing the flag in blk_mq_start_request(), so
> + * this rq won't be timed out too.
> + */
> if (time_after_eq(jiffies, rq->deadline)) {
> if (!blk_mark_rq_complete(rq))
> blk_mq_rq_timed_out(rq, reserved);

Since this explanation applies to the same race addressed by patch 1/3,
please consider squashing this patch into patch 1/3.

Thanks,

Bart.
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