Messages in this thread | | | Subject | Re: [PATCH net 2/4] vhost_net: rework on the lock ordering for busy polling | From | Jason Wang <> | Date | Tue, 11 Dec 2018 11:06:43 +0800 |
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On 2018/12/11 上午9:34, 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@gmail.com> >> Signed-off-by: Jason Wang<jasowang@redhat.com> >> --- >> 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()).
while (vhost_can_busy_poll(endtime)) { if (vhost_has_work(&net->dev)) { *busyloop_intr = true; break; }
if ((sock_has_rx_data(sock) && !vhost_vq_avail_empty(&net->dev, rvq)) || !vhost_vq_avail_empty(&net->dev, tvq)) break;
cpu_relax();
}
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
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