Re: [PATCH] block: re-introduce blk_mq_complete_request_sync

From: Ming Lei <ming.lei@redhat.com>
To: Sagi Grimberg <sagi@grimberg.me>
Cc: Chao Leng <lengchao@huawei.com>, Jens Axboe <axboe@kernel.dk>,
	Yi Zhang <yi.zhang@redhat.com>,
	linux-nvme@lists.infradead.org, linux-block@vger.kernel.org,
	Keith Busch <kbusch@kernel.org>, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH] block: re-introduce blk_mq_complete_request_sync
Date: Wed, 14 Oct 2020 09:08:13 +0800	[thread overview]
Message-ID: <20201014010813.GA775684@T590> (raw)
In-Reply-To: <e19073e4-06da-ce3c-519c-ece2c4d942fa@grimberg.me>

On Tue, Oct 13, 2020 at 03:36:08PM -0700, Sagi Grimberg wrote:
> 
> > > > This may just reduce the probability. The concurrency of timeout
> > > > and teardown will cause the same request
> > > > be treated repeatly, this is not we expected.
> > > 
> > > That is right, not like SCSI, NVME doesn't apply atomic request
> > > completion, so
> > > request may be completed/freed from both timeout & nvme_cancel_request().
> > > 
> > > .teardown_lock still may cover the race with Sagi's patch because
> > > teardown
> > > actually cancels requests in sync style.
> > In extreme scenarios, the request may be already retry success(rq state
> > change to inflight).
> > Timeout processing may wrongly stop the queue and abort the request.
> > teardown_lock serialize the process of timeout and teardown, but do not
> > avoid the race.
> > It might not be safe.
> 
> Not sure I understand the scenario you are describing.
> 
> what do you mean by "In extreme scenarios, the request may be already retry
> success(rq state change to inflight)"?
> 
> What will retry the request? only when the host will reconnect
> the request will be retried.
> 
> We can call nvme_sync_queues in the last part of the teardown, but
> I still don't understand the race here.

Not like SCSI, NVME doesn't complete request atomically, so double
completion/free can be done from both timeout & nvme_cancel_request()(via teardown).

Given request is completed remotely or asynchronously in the two code paths,
the teardown_lock can't protect the case.

One solution is to apply the introduced blk_mq_complete_request_sync()
in both two code paths.

Another candidate is to use nvme_sync_queues() before teardown, such as
the following change:

diff --git a/drivers/nvme/host/tcp.c b/drivers/nvme/host/tcp.c
index d6a3e1487354..dc3561ca0074 100644
--- a/drivers/nvme/host/tcp.c
+++ b/drivers/nvme/host/tcp.c
@@ -1909,6 +1909,7 @@ static void nvme_tcp_teardown_io_queues(struct nvme_ctrl *ctrl,
 	blk_mq_quiesce_queue(ctrl->admin_q);
 	nvme_start_freeze(ctrl);
 	nvme_stop_queues(ctrl);
+	nvme_sync_queues(ctrl);
 	nvme_tcp_stop_io_queues(ctrl);
 	if (ctrl->tagset) {
 		blk_mq_tagset_busy_iter(ctrl->tagset,
@@ -2171,14 +2172,11 @@ static void nvme_tcp_complete_timed_out(struct request *rq)
 	struct nvme_tcp_request *req = blk_mq_rq_to_pdu(rq);
 	struct nvme_ctrl *ctrl = &req->queue->ctrl->ctrl;
 
-	/* fence other contexts that may complete the command */
-	mutex_lock(&to_tcp_ctrl(ctrl)->teardown_lock);
 	nvme_tcp_stop_queue(ctrl, nvme_tcp_queue_id(req->queue));
 	if (!blk_mq_request_completed(rq)) {
 		nvme_req(rq)->status = NVME_SC_HOST_ABORTED_CMD;
 		blk_mq_complete_request(rq);
 	}
-	mutex_unlock(&to_tcp_ctrl(ctrl)->teardown_lock);
 }
 
 static enum blk_eh_timer_return


Thanks,
Ming

