2010-07-15 12:25:19

by Michael S. Tsirkin

Subject: [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 <[email protected]>
---
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

2010-07-15 12:43:49

by Michael S. Tsirkin

Subject: Re: [PATCH] vhost-net: avoid flush under lock

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 <[email protected]>

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

2010-07-15 18:18:07

by Sridhar Samudrala

Subject: Re: [PATCH] vhost-net: avoid flush under lock

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 <[email protected]>
> ---
> 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: