Subject: Re: [PATCH] vhost: fix IOTLB locking

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 30 Nov 2018 08:32:53 -0500

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

> 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 <[email-masked]>

Acked-by: Michael S. Tsirkin <[email-masked]>

but see below for a minor comment.

I guess we now need this on stable?

> ---
>  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.

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

>  		}
> -- 
> 2.19.1