On Tue, 2018-05-29 at 15:52 +0200, Christoph Hellwig wrote:
> From: Keith Busch <keith.busch@xxxxxxxxx>
>
> This patch simplifies the timeout handling by relying on the request
> reference counting to ensure the iterator is operating on an inflight
> and truly timed out request. Since the reference counting prevents the
> tag from being reallocated, the block layer no longer needs to prevent
> drivers from completing their requests while the timeout handler is
> operating on it: a driver completing a request is allowed to proceed to
> the next state without additional syncronization with the block layer.
>
> This also removes any need for generation sequence numbers since the
> request lifetime is prevented from being reallocated as a new sequence
> while timeout handling is operating on it.
>
> To enables this a refcount is added to struct request so that request
> users can be sure they're operating on the same request without it
> changing while they're processing it. The request's tag won't be
> released for reuse until both the timeout handler and the completion
> are done with it.
>
> Signed-off-by: Keith Busch <keith.busch@xxxxxxxxx>
> [hch: slight cleanups, added back submission side hctx lock, use cmpxchg
> for completions]
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Hello Christoph,
Can you explain why you prefer this approach? I think this patch introduces
more new atomic operations in the hot path than the approach I had proposed
("blk-mq: Rework blk-mq timeout handling again"). In other words, I think the
patch that I had proposed will yield better performance.
Thanks,
Bart.
