Re: [PATCH net 2/4] vhost_net: rework on the lock ordering for busy polling

[Jason Wang]

On 2018/12/11 äå9:34, Michael S. Tsirkin wrote:
> On Mon, Dec 10, 2018 at 05:44:52PM +0800, Jason Wang wrote:
> > When we try to do rx busy polling in tx path in commit 441abde4cd84
> > ("net: vhost: add rx busy polling in tx path"), we lock rx vq mutex
> > after tx vq mutex is held. This may lead deadlock so we try to lock vq
> > one by one in commit 78139c94dc8c ("net: vhost: lock the vqs one by
> > one"). With this commit, we avoid the deadlock with the assumption
> > that handle_rx() and handle_tx() run in a same process. But this
> > commit remove the protection for IOTLB updating which requires the
> > mutex of each vq to be held.
> >
> > To solve this issue, the first step is to have a exact same lock
> > ordering for vhost_net. This is done through:
> >
> > - For handle_rx(), if busy polling is enabled, lock tx vq immediately.
> > - For handle_tx(), always lock rx vq before tx vq, and unlock it if
> >    busy polling is not enabled.
> > - Remove the tricky locking codes in busy polling.
> >
> > With this, we can have a exact same lock ordering for vhost_net, this
> > allows us to safely revert commit 78139c94dc8c ("net: vhost: lock the
> > vqs one by one") in next patch.
> >
> > The patch will add two more atomic operations on the tx path during
> > each round of handle_tx(). 1 byte TCP_RR does not notice such
> > overhead.
> >
> > Fixes: commit 78139c94dc8c ("net: vhost: lock the vqs one by one")
> > Cc: Tonghao Zhang<xiangxia.m.yue@xxxxxxxxx>
> > Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
> > ---
> >   drivers/vhost/net.c | 18 +++++++++++++++---
> >   1 file changed, 15 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index ab11b2bee273..5f272ab4d5b4 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -513,7 +513,6 @@ static void vhost_net_busy_poll(struct vhost_net *net,
> >   	struct socket *sock;
> >   	struct vhost_virtqueue *vq = poll_rx ? tvq : rvq;
> >   
> > -	mutex_lock_nested(&vq->mutex, poll_rx ? VHOST_NET_VQ_TX: VHOST_NET_VQ_RX);
> >   	vhost_disable_notify(&net->dev, vq);
> >   	sock = rvq->private_data;
> >   
> > @@ -543,8 +542,6 @@ static void vhost_net_busy_poll(struct vhost_net *net,
> >   		vhost_net_busy_poll_try_queue(net, vq);
> >   	else if (!poll_rx) /* On tx here, sock has no rx data. */
> >   		vhost_enable_notify(&net->dev, rvq);
> > -
> > -	mutex_unlock(&vq->mutex);
> >   }
> >   
> >   static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
> > @@ -913,10 +910,16 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
> >   static void handle_tx(struct vhost_net *net)
> >   {
> >   	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
> > +	struct vhost_net_virtqueue *nvq_rx = &net->vqs[VHOST_NET_VQ_RX];
> >   	struct vhost_virtqueue *vq = &nvq->vq;
> > +	struct vhost_virtqueue *vq_rx = &nvq_rx->vq;
> >   	struct socket *sock;
> >   
> > +	mutex_lock_nested(&vq_rx->mutex, VHOST_NET_VQ_RX);
> >   	mutex_lock_nested(&vq->mutex, VHOST_NET_VQ_TX);
> > +	if (!vq->busyloop_timeout)
> > +		mutex_unlock(&vq_rx->mutex);
> > +
> >   	sock = vq->private_data;
> >   	if (!sock)
> >   		goto out;
> > @@ -933,6 +936,8 @@ static void handle_tx(struct vhost_net *net)
> >   		handle_tx_copy(net, sock);
> >   
> >   out:
> > +	if (vq->busyloop_timeout)
> > +		mutex_unlock(&vq_rx->mutex);
> >   	mutex_unlock(&vq->mutex);
> >   }
> >   
> So rx mutex taken on tx path now.  And tx mutex is on rc path ...  This
> is just messed up. Why can't tx polling drop rx lock before
> getting the tx lock and vice versa?


Because we want to poll both tx and rx virtqueue at the same time 
(vhost_net_busy_poll()).

ÂÂÂ while (vhost_can_busy_poll(endtime)) {
ÂÂ ÂÂÂ Âif (vhost_has_work(&net->dev)) {
ÂÂ ÂÂÂ ÂÂÂ Â*busyloop_intr = true;
ÂÂ ÂÂÂ ÂÂÂ Âbreak;
ÂÂ ÂÂÂ Â}

ÂÂ ÂÂÂ Âif ((sock_has_rx_data(sock) &&
ÂÂ ÂÂÂ ÂÂÂÂÂ !vhost_vq_avail_empty(&net->dev, rvq)) ||
ÂÂ ÂÂÂ ÂÂÂÂ !vhost_vq_avail_empty(&net->dev, tvq))
ÂÂ ÂÂÂ ÂÂÂ Âbreak;

ÂÂ ÂÂÂ Âcpu_relax();

ÂÂ Â}


And we disable kicks and notification for better performance.


> Or if we really wanted to force everything to be locked at
> all times, let's just use a single mutex.

We could, but it might requires more changes which could be done for 
-next I believe.


Thanks