Linux Virtualization - Jul 2010 - [PATCH] vhost-net: avoid flush under lock

We flush under vq mutex when changing backends.
This creates a deadlock as workqueue being flushed
needs this lock as well.

https://bugzilla.redhat.com/show_bug.cgi?id=612421

Drop the vq mutex before flush: we have the device mutex
which is sufficient to prevent another ioctl from touching
the vq.

Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---
 drivers/vhost/net.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 28d7786..50df58e6 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -534,11 +534,16 @@ static long vhost_net_set_backend(struct vhost_net *n,
unsigned index, int fd)
 	rcu_assign_pointer(vq->private_data, sock);
 	vhost_net_enable_vq(n, vq);
 done:
+	mutex_unlock(&vq->mutex);
+
 	if (oldsock) {
 		vhost_net_flush_vq(n, index);
 		fput(oldsock->file);
 	}
 
+	mutex_unlock(&n->dev.mutex);
+	return 0;
+
 err_vq:
 	mutex_unlock(&vq->mutex);
 err:
-- 
1.7.2.rc0.14.g41c1c

On Thu, Jul 15, 2010 at 03:19:12PM +0300, Michael S. Tsirkin
wrote:
> We flush under vq mutex when changing backends.
> This creates a deadlock as workqueue being flushed
> needs this lock as well.
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=612421
> 
> Drop the vq mutex before flush: we have the device mutex
> which is sufficient to prevent another ioctl from touching
> the vq.
> 
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Dave, just to clarify, I'll send pull request to merge it through my tree,
there's no need for you to bother with this.
> ---
>  drivers/vhost/net.c |    5 +++++
>  1 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 28d7786..50df58e6 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -534,11 +534,16 @@ static long vhost_net_set_backend(struct vhost_net
*n, unsigned index, int fd)
>  	rcu_assign_pointer(vq->private_data, sock);
>  	vhost_net_enable_vq(n, vq);
>  done:
> +	mutex_unlock(&vq->mutex);
> +
>  	if (oldsock) {
>  		vhost_net_flush_vq(n, index);
>  		fput(oldsock->file);
>  	}
>  
> +	mutex_unlock(&n->dev.mutex);
> +	return 0;
> +
>  err_vq:
>  	mutex_unlock(&vq->mutex);
>  err:
> -- 
> 1.7.2.rc0.14.g41c1c

On Thu, 2010-07-15 at 15:19 +0300, Michael S. Tsirkin
wrote:
> We flush under vq mutex when changing backends.
> This creates a deadlock as workqueue being flushed
> needs this lock as well.
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=612421
> 
> Drop the vq mutex before flush: we have the device mutex
> which is sufficient to prevent another ioctl from touching
> the vq.
Why do we need to flush the vq when trying to set the backend and
we find that it is already set. Is this just an optimization?

Thanks
Sridhar
> 
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> ---
>  drivers/vhost/net.c |    5 +++++
>  1 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 28d7786..50df58e6 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -534,11 +534,16 @@ static long vhost_net_set_backend(struct vhost_net
*n, unsigned index, int fd)
>  	rcu_assign_pointer(vq->private_data, sock);
>  	vhost_net_enable_vq(n, vq);
>  done:
> +	mutex_unlock(&vq->mutex);
> +
>  	if (oldsock) {
>  		vhost_net_flush_vq(n, index);
>  		fput(oldsock->file);
>  	}
> 
> +	mutex_unlock(&n->dev.mutex);
> +	return 0;
> +
>  err_vq:
>  	mutex_unlock(&vq->mutex);
>  err: