[PATCH] nbd: add a flush_workqueue in nbd_start_device

[Sun Ke <sunke32@huawei.com>]

When kzalloc fail, may cause trying to destroy the
workqueue from inside the workqueue.

If num_connections is m (2 < m), and NO.1 ~ NO.n
(1 < n < m) kzalloc are successful. The NO.(n + 1)
failed. Then, nbd_start_device will return ENOMEM
to nbd_start_device_ioctl, and nbd_start_device_ioctl
will return immediately without running flush_workqueue.
However, we still have n recv threads. If nbd_release
run first, recv threads may have to drop the last
config_refs and try to destroy the workqueue from
inside the workqueue.

To fix it, add a flush_workqueue in nbd_start_device.

Fixes: e9e006f5fcf2 ("nbd: fix max number of supported devs")
Signed-off-by: Sun Ke <sunke32@huawei.com>
---
 drivers/block/nbd.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index b4607dd96185..dd1f8c2c6169 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1264,7 +1264,12 @@ static int nbd_start_device(struct nbd_device *nbd)
 
 		args = kzalloc(sizeof(*args), GFP_KERNEL);
 		if (!args) {
-			sock_shutdown(nbd);
+			if (i == 0)
+				sock_shutdown(nbd);
+			else {
+				sock_shutdown(nbd);
+				flush_workqueue(nbd->recv_workq);
+			}
 			return -ENOMEM;
 		}
 		sk_set_memalloc(config->socks[i]->sock->sk);
-- 
2.17.2