vhost_net: switch to use mutex_trylock() in vhost_net_busy_poll()

This change “vhost_net: switch to use mutex_trylock() in vhost_net_busy_poll()” in Linux kernel is authored by Jason Wang <jasowang [at] redhat.com> on Thu Dec 13 10:53:38 2018 +0800.

vhost_net: switch to use mutex_trylock() in vhost_net_busy_poll()

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>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit 476e8ba.

There are 8 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/vhost/net.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index ab11b2b..ad7a6f4 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -513,7 +513,13 @@ static void vhost_net_busy_poll(struct vhost_net *net,
 	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;

The commit for this change in Linux stable tree is 476e8ba (patch).

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