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

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Date:	Thu, 15 Jul 2010 15:37:44 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sridhar Samudrala <samudrala@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Arnd Bergmann <arnd@...db.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	kvm@...r.kernel.org, virtualization@...ts.osdl.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
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 <mst@...hat.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
--
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