nbd: clear waiting_queue on shutdown

commit fded4e09 upstream.

Fix a serious but uncommon bug in nbd which occurs when there is heavy
I/O going to the nbd device while, at the same time, a failure (server,
network) or manual disconnect of the nbd connection occurs.

There is a small window between the time that the nbd_thread is stopped
and the socket is shutdown where requests can continue to be queued to
nbd's internal waiting_queue.  When this happens, those requests are
never completed or freed.

The fix is to clear the waiting_queue on shutdown of the nbd device, in
the same way that the nbd request queue (queue_head) is already being
cleared.
Signed-off-by: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: local nbd_device pointers are called 'lo' not 'nbd']
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/block/nbd.c

@@ -445,6 +445,14 @@ static void nbd_clear_que(struct nbd_device *lo)
 		req->errors++;
 		nbd_end_request(req);
 	}
+
+	while (!list_empty(&lo->waiting_queue)) {
+		req = list_entry(lo->waiting_queue.next, struct request,
+				 queuelist);
+		list_del_init(&req->queuelist);
+		req->errors++;
+		nbd_end_request(req);
+	}
 }
 
 
@@ -594,6 +602,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *lo,
 		lo->file = NULL;
 		nbd_clear_que(lo);
 		BUG_ON(!list_empty(&lo->queue_head));
+		BUG_ON(!list_empty(&lo->waiting_queue));
 		if (file)
 			fput(file);
 		return 0;