Messages in this thread | | | Subject | Re: [PATCH net 2/4] vhost_net: rework on the lock ordering for busy polling | From | Jason Wang <> | Date | Wed, 12 Dec 2018 11:03:57 +0800 |
| |
On 2018/12/11 下午12:04, Michael S. Tsirkin wrote: > On Tue, Dec 11, 2018 at 11:06:43AM +0800, Jason Wang wrote: >> 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. > Right but it's all slow path - it happens when queue is > otherwise empty. So this is what I am saying: let's drop the locks > we hold around this.
Is this really safe? I looks to me it can race with SET_VRING_ADDR. And the codes did more:
- access sock object
- access device IOTLB
- enable and disable notification
None of above is safe without the protection of vq mutex.
> > >>> 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 > I'd rather we kept the fine grained locking. E.g. people are > looking at splitting the tx and rx threads. But if not possible > let's fix it cleanly with a coarse-grained one. A mess here will > just create more trouble later. >
I believe we won't go back to coarse one. Looks like we can solve this by using mutex_trylock() for rxq during TX. And don't do polling for rxq is a IOTLB updating is pending.
Let me post V2.
Thanks
| |