[PATCH] block: don't decrement flush request refcount if it's state is idle in flush_end_io()

flush_end_io() currently decrement request refcount unconditionally.
However, it's possible that the request is already idle and it's
refcount is zero since that flush_end_io() can be called concurrently.

For example, nbd_clear_que() can be called concurrently with normal
io completion or io timeout.

Thus check idle before decrement to avoid refcount_t underflow
warning.

Signed-off-by: Yu Kuai <redacted>
---
 block/blk-flush.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 1002f6c58181..9b65dc43702c 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c

@@ -222,7 +222,8 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
 	/* release the tag's ownership to the req cloned from */
 	spin_lock_irqsave(&fq->mq_flush_lock, flags);
 
-	if (!refcount_dec_and_test(&flush_rq->ref)) {
+	if (blk_mq_rq_state(flush_rq) == MQ_RQ_IDLE ||
+	    !refcount_dec_and_test(&flush_rq->ref)) {
 		fq->rq_status = error;
 		spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
 		return;

-- 
2.31.1