From: Jason Wang <jasowang@redhat.com>
Date: Thu, 13 Dec 2018 10:53:38 +0800
Subject: vhost_net: switch to use mutex_trylock() in vhost_net_busy_poll()
Patch-mainline: v4.20
Git-commit: 476e8ba712fa6a479192dfc4301cd3765d9ba713
References: bsc#1109837

We used to hold the mutex of paired virtqueue in
vhost_net_busy_poll(). But this will results an inconsistent lock
order which may cause deadlock if we try to bring back the protection
of device IOTLB with vq mutex that requires to hold mutex of all
virtqueues at the same time.

Fix this simply by switching to use mutex_trylock(), when fail just
skip the busy polling. This can happen when device IOTLB is under
updating which should be rare.

Fixes: commit 78139c94dc8c ("net: vhost: lock the vqs one by one")
Cc: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 drivers/vhost/net.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -511,7 +511,13 @@ static void vhost_net_busy_poll(struct v
 	struct socket *sock;
 	struct vhost_virtqueue *vq = poll_rx ? tvq : rvq;
 
-	mutex_lock_nested(&vq->mutex, poll_rx ? VHOST_NET_VQ_TX: VHOST_NET_VQ_RX);
+	/* Try to hold the vq mutex of the paired virtqueue. We can't
+	 * use mutex_lock() here since we could not guarantee a
+	 * consistenet lock ordering.
+	 */
+	if (!mutex_trylock(&vq->mutex))
+		return;
+
 	vhost_disable_notify(&net->dev, vq);
 	sock = rvq->private_data;