linux-kernel - [PATCH AUTOSEL 5.2 142/171] block: init flush rq ref count to 1

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Date:   Thu, 18 Jul 2019 23:56:13 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Josef Bacik <josef@...icpanda.com>, Jens Axboe <axboe@...nel.dk>,
        Sasha Levin <sashal@...nel.org>, linux-block@...r.kernel.org
Subject: [PATCH AUTOSEL 5.2 142/171] block: init flush rq ref count to 1

From: Josef Bacik <josef@...icpanda.com>

[ Upstream commit b554db147feea39617b533ab6bca247c91c6198a ]

We discovered a problem in newer kernels where a disconnect of a NBD
device while the flush request was pending would result in a hang.  This
is because the blk mq timeout handler does

        if (!refcount_inc_not_zero(&rq->ref))
                return true;

to determine if it's ok to run the timeout handler for the request.
Flush_rq's don't have a ref count set, so we'd skip running the timeout
handler for this request and it would just sit there in limbo forever.

Fix this by always setting the refcount of any request going through
blk_init_rq() to 1.  I tested this with a nbd-server that dropped flush
requests to verify that it hung, and then tested with this patch to
verify I got the timeout as expected and the error handling kicked in.
Thanks,

Signed-off-by: Josef Bacik <josef@...icpanda.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 block/blk-core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/block/blk-core.c b/block/blk-core.c
index 8340f69670d8..5183fca0818a 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -117,6 +117,7 @@ void blk_rq_init(struct request_queue *q, struct request *rq)
 	rq->internal_tag = -1;
 	rq->start_time_ns = ktime_get_ns();
 	rq->part = NULL;
+	refcount_set(&rq->ref, 1);
 }
 EXPORT_SYMBOL(blk_rq_init);

-- 
2.20.1