Re: [PATCH 2/2] nvme: avoid possible double completions for the same request

[Keith Busch <kbusch@kernel.org>]

On Mon, Jun 08, 2026 at 07:34:25PM +0800, Ye Bin wrote:
> To avoid the preceding problem, the NVME_REQ_COMPLETE flag is added by
> referring to the implementation of scsi commit f1342709d18a ("scsi: Do not
> rely on blk-mq for double completions").

That scsi commit was solving a different problem for a racing
interaction between the low level driver and the timeout handler and
error injection. It wasn't about protecting against misbehaving
hardware.
 
>  static inline struct nvme_request *nvme_req(struct request *req)
> @@ -807,6 +808,8 @@ static inline bool nvme_try_complete_req(struct request *req, __le16 status,
>  	nvme_should_fail(req);
>  	if (unlikely(blk_should_fake_timeout(req->q)))
>  		return true;
> +	if (unlikely(test_and_set_bit(NVME_REQ_COMPLETE, &rq->flags)))
> +		return true;

I think you need to invert this flag from "COMPLETE" to "INFLIGHT",
because the default allocated state is that this flag is cleared, so
this check as you have it wouldn't catch a phantom completion to a
request the host never sent.

You also have this check after the driver updated its internal
generation counter for a bogus completion, so now our actual state could
be off from what's actually being prepared. 

This previous proposal for a similar problem was probably more on the
right track:

https://lore.kernel.org/linux-nvme/20260522153034.2168862-1-coshi036@gmail.com/

However, I'm really skeptical controllers actually behave this way. You
wouldn't have a viable storage device if it was doing this.