[PATCH] nbd: fix circular lock dependency in nbd_disconnect_and_put

[Yun Zhou <yun.zhou@windriver.com>]

Move flush_workqueue() out of the config_lock critical section in
nbd_disconnect_and_put() to break a circular lock dependency.

The lockdep splat shows:

  config_lock -> (wq_completion)nbd0-recv
    from nbd_disconnect_and_put() holding config_lock then calling
    flush_workqueue() which waits for recv_work to complete.

  (work_completion)(&args->work) -> config_lock
    from recv_work() -> nbd_config_put() -> refcount_dec_and_mutex_lock()
    which may acquire config_lock when the last reference is dropped.

Fix by splitting the config_lock region: first hold config_lock to
perform nbd_disconnect(), sock_shutdown(), and clear NBD_RT_BOUND (to
prevent nbd_genl_reconfigure from queueing new recv_work during the
window), then release config_lock before flush_workqueue(), and
re-acquire it for nbd_clear_que(). This is safe because:

- sock_shutdown() ensures recv_work will observe errors and exit
- NBD_RT_BOUND cleared prevents concurrent reconfigure from reconnecting
- flush_workqueue() guarantees all recv_work has completed before
  the second config_lock section clears the queue

Fixes: e2daec488c57 ("nbd: Fix hungtask when nbd_config_put")
Reported-by: syzbot+3add0454d5a2619b8e80@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=3add0454d5a2619b8e80
Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
---
 drivers/block/nbd.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index c5d3ae8f5fc5..87b97bd9d0d3 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -2329,14 +2329,23 @@ static void nbd_disconnect_and_put(struct nbd_device *nbd)
 	nbd_disconnect(nbd);
 	sock_shutdown(nbd);
 	wake_up(&nbd->config->conn_wait);
+	/*
+	 * Clear NBD_RT_BOUND before releasing config_lock so that
+	 * nbd_genl_reconfigure() won't queue new recv_work between
+	 * here and flush_workqueue().
+	 */
+	nbd->task_setup = NULL;
+	clear_bit(NBD_RT_BOUND, &nbd->config->runtime_flags);
+	mutex_unlock(&nbd->config_lock);
+
 	/*
 	 * Make sure recv thread has finished, we can safely call nbd_clear_que()
 	 * to cancel the inflight I/Os.
 	 */
 	flush_workqueue(nbd->recv_workq);
+
+	mutex_lock(&nbd->config_lock);
 	nbd_clear_que(nbd);
-	nbd->task_setup = NULL;
-	clear_bit(NBD_RT_BOUND, &nbd->config->runtime_flags);
 	mutex_unlock(&nbd->config_lock);
 
 	if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF,
-- 
2.43.0