On 30/11/2018 13:32, Michael S. Tsirkin wrote:

On Fri, Nov 30, 2018 at 11:37:02AM +0000, Jean-Philippe Brucker wrote:

quoted

Commit 78139c94dc8c ("net: vhost: lock the vqs one by one") moved the vq
lock to improve scalability, but introduced a possible deadlock in
vhost-iotlb. vhost_iotlb_notify_vq() now takes vq->mutex while holding
the device's IOTLB spinlock.

Indeed spin_lock is just outside this snippet. Yack.

quoted

And on the vhost_iotlb_miss() path, the
spinlock is taken while holding vq->mutex.

As long as we hold dev->mutex to prevent an ioctl from modifying
vq->poll concurrently, we can safely call vhost_poll_queue() without
holding vq->mutex. Since vhost_process_iotlb_msg() holds dev->mutex when
calling vhost_iotlb_notify_vq(), avoid the deadlock by not taking
vq->mutex.

Fixes: 78139c94dc8c ("net: vhost: lock the vqs one by one")
Signed-off-by: Jean-Philippe Brucker <redacted>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

but see below for a minor comment.

I guess we now need this on stable?

I don't think so, the bug is introduced in 4.20

quoted

---
 drivers/vhost/vhost.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 3a5f81a66d34..1cbb17f898f7 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c

@@ -944,10 +944,10 @@ static void vhost_iotlb_notify_vq(struct vhost_dev *d,
 		if (msg->iova <= vq_msg->iova &&
 		    msg->iova + msg->size - 1 >= vq_msg->iova &&
 		    vq_msg->type == VHOST_IOTLB_MISS) {
-			mutex_lock(&node->vq->mutex);
+			/* Safe to call outside vq->mutex as long as dev->mutex
+			 * is held.
+			 */
 			vhost_poll_queue(&node->vq->poll);
-			mutex_unlock(&node->vq->mutex);
-

In fact vhost_poll_queue is generally lockless so it's
safe to call without any locks.

Right, I'll remove the comment

Thanks,
Jean

quoted

 			list_del(&node->node);
 			kfree(node);

quoted

 		}
-- 
2.19.1

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