Re: [PATCH V3 2/2] vhost: handle polling errors

Michael S. Tsirkin
2013-01-14T06:57:12Z

On Mon, Jan 14, 2013 at 10:59:02AM +0800, Jason Wang wrote:
> On 01/13/2013 07:10 PM, Michael S. Tsirkin wrote:
> > On Mon, Jan 07, 2013 at 11:04:32PM +0800, Jason Wang wrote:
> >> On 01/07/2013 10:55 PM, Michael S. Tsirkin wrote:
> >>> On Mon, Jan 07, 2013 at 12:38:17PM +0800, Jason Wang wrote:
> >>>> On 01/06/2013 09:22 PM, Michael S. Tsirkin wrote:
> >>>>> On Sun, Jan 06, 2013 at 03:18:38PM +0800, Jason Wang wrote:
> >>>>>> Polling errors were ignored by vhost/vhost_net, this may lead to crash when
> >>>>>> trying to remove vhost from waitqueue when after the polling is failed. Solve
> >>>>>> this problem by:
> >>>>>>
> >>>>>> - checking the poll->wqh before trying to remove from waitqueue
> >>>>>> - report an error when poll() returns a POLLERR in vhost_start_poll()
> >>>>>> - report an error when vhost_start_poll() fails in
> >>>>>> vhost_vring_ioctl()/vhost_net_set_backend() which is used to notify the
> >>>>>> failure to userspace.
> >>>>>> - report an error in the data path in vhost_net when meet polling errors.
> >>>>>>
> >>>>>> After those changes, we can safely drop the tx polling state in vhost_net since
> >>>>>> it was replaced by the checking of poll->wqh.
> >>>>>>
> >>>>>> Signed-off-by: Jason Wang <jasowang@redhat.com>
> >>>>>> ---
> >>>>>> drivers/vhost/net.c | 74 ++++++++++++++++--------------------------------
> >>>>>> drivers/vhost/vhost.c | 31 +++++++++++++++-----
> >>>>>> drivers/vhost/vhost.h | 2 +-
> >>>>>> 3 files changed, 49 insertions(+), 58 deletions(-)
> >>>>>>
> >>>>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> >>>>>> index d10ad6f..125c1e5 100644
> >>>>>> --- a/drivers/vhost/net.c
> >>>>>> +++ b/drivers/vhost/net.c
> >>>>>> @@ -64,20 +64,10 @@ enum {
> >>>>>> VHOST_NET_VQ_MAX = 2,
> >>>>>> };
> >>>>>>
> >>>>>> -enum vhost_net_poll_state {
> >>>>>> - VHOST_NET_POLL_DISABLED = 0,
> >>>>>> - VHOST_NET_POLL_STARTED = 1,
> >>>>>> - VHOST_NET_POLL_STOPPED = 2,
> >>>>>> -};
> >>>>>> -
> >>>>>> struct vhost_net {
> >>>>>> struct vhost_dev dev;
> >>>>>> struct vhost_virtqueue vqs[VHOST_NET_VQ_MAX];
> >>>>>> struct vhost_poll poll[VHOST_NET_VQ_MAX];
> >>>>>> - /* Tells us whether we are polling a socket for TX.
> >>>>>> - * We only do this when socket buffer fills up.
> >>>>>> - * Protected by tx vq lock. */
> >>>>>> - enum vhost_net_poll_state tx_poll_state;
> >>>>>> /* Number of TX recently submitted.
> >>>>>> * Protected by tx vq lock. */
> >>>>>> unsigned tx_packets;
> >>>>>> @@ -155,24 +145,6 @@ static void copy_iovec_hdr(const struct iovec *from, struct iovec *to,
> >>>>>> }
> >>>>>> }
> >>>>>>
> >>>>>> -/* Caller must have TX VQ lock */
> >>>>>> -static void tx_poll_stop(struct vhost_net *net)
> >>>>>> -{
> >>>>>> - if (likely(net->tx_poll_state != VHOST_NET_POLL_STARTED))
> >>>>>> - return;
> >>>>>> - vhost_poll_stop(net->poll + VHOST_NET_VQ_TX);
> >>>>>> - net->tx_poll_state = VHOST_NET_POLL_STOPPED;
> >>>>>> -}
> >>>>>> -
> >>>>>> -/* Caller must have TX VQ lock */
> >>>>>> -static void tx_poll_start(struct vhost_net *net, struct socket *sock)
> >>>>>> -{
> >>>>>> - if (unlikely(net->tx_poll_state != VHOST_NET_POLL_STOPPED))
> >>>>>> - return;
> >>>>>> - vhost_poll_start(net->poll + VHOST_NET_VQ_TX, sock->file);
> >>>>>> - net->tx_poll_state = VHOST_NET_POLL_STARTED;
> >>>>>> -}
> >>>>>> -
> >>>>>> /* In case of DMA done not in order in lower device driver for some reason.
> >>>>>> * upend_idx is used to track end of used idx, done_idx is used to track head
> >>>>>> * of used idx. Once lower device DMA done contiguously, we will signal KVM
> >>>>>> @@ -227,6 +199,7 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
> >>>>>> static void handle_tx(struct vhost_net *net)
> >>>>>> {
> >>>>>> struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> >>>>>> + struct vhost_poll *poll = net->poll + VHOST_NET_VQ_TX;
> >>>>>> unsigned out, in, s;
> >>>>>> int head;
> >>>>>> struct msghdr msg = {
> >>>>>> @@ -252,7 +225,8 @@ static void handle_tx(struct vhost_net *net)
> >>>>>> wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> >>>>>> if (wmem >= sock->sk->sk_sndbuf) {
> >>>>>> mutex_lock(&vq->mutex);
> >>>>>> - tx_poll_start(net, sock);
> >>>>>> + if (vhost_poll_start(poll, sock->file))
> >>>>>> + vq_err(vq, "Fail to start TX polling\n");
> >>>>> s/Fail/Failed/
> >>>>>
> >>>>> A question though: how can this happen? Could you clarify please?
> >>>>> Maybe we can find a way to prevent this error?
> >>>> Two conditions I think this can happen:
> >>>>
> >>>> 1) a buggy userspace disable a queue through TUNSETQUEUE
> >>>> 2) the net device were gone
> >>>>
> >>>> For 1, looks like we can delay the disabling until the refcnt goes to
> >>>> zero. For 2 may needs more changes.
> >>> I'd expect keeping a socket reference would prevent both issues.
> >>> Doesn't it?
> >> Doesn't work for 2 I think, the socket didn't hold a refcnt of the
> >> device, so the device can go away at anytime. Although we can change
> >> this, but it's the behaviour before multiqueue support.
> > Hmm there's one scenario that does seem to
> > trigger this: queue can get disabled
> > and then poll fails.
> >
> > Is this the only issue?
>
> Another one I think we can trigger is:
>
> - start vhost thread
> - do ip link del link dev tap0 to delete the tap device
>
> In this case, the netdevice is unregistered but the file/socket still exist.

Yes but in this case poll_wait is called so apparently no issue
with existing code? We only have an issue if poll_wait is
not called right?

Maybe the simplest fix is to invoke poll_wait before
checking if (!tun) ?

> >
> >>>> Not sure it's worth to do this work,
> >>>> maybe a warning is enough just like other failure.
> >>> With other failures, you normally can correct the error then
> >>> kick to have it restart. This is soomething thagt would not
> >>> work here.
> >> If userspace is wrote correctly, (e.g passing a fd with correct state)
> >> it can also be corrected.
> >>>>>> mutex_unlock(&vq->mutex);
> >>>>>> return;
> >>>>>> }
> >>>>>> @@ -261,7 +235,7 @@ static void handle_tx(struct vhost_net *net)
> >>>>>> vhost_disable_notify(&net->dev, vq);
> >>>>>>
> >>>>>> if (wmem < sock->sk->sk_sndbuf / 2)
> >>>>>> - tx_poll_stop(net);
> >>>>>> + vhost_poll_stop(poll);
> >>>>>> hdr_size = vq->vhost_hlen;
> >>>>>> zcopy = vq->ubufs;
> >>>>>>
> >>>>>> @@ -283,8 +257,10 @@ static void handle_tx(struct vhost_net *net)
> >>>>>>
> >>>>>> wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> >>>>>> if (wmem >= sock->sk->sk_sndbuf * 3 / 4) {
> >>>>>> - tx_poll_start(net, sock);
> >>>>>> - set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
> >>>>>> + if (vhost_poll_start(poll, sock->file))
> >>>>>> + vq_err(vq, "Fail to start TX polling\n");
> >>>>>> + else
> >>>>>> + set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
> >>>>>> break;
> >>>>>> }
> >>>>>> /* If more outstanding DMAs, queue the work.
> >>>>>> @@ -294,8 +270,10 @@ static void handle_tx(struct vhost_net *net)
> >>>>>> (vq->upend_idx - vq->done_idx) :
> >>>>>> (vq->upend_idx + UIO_MAXIOV - vq->done_idx);
> >>>>>> if (unlikely(num_pends > VHOST_MAX_PEND)) {
> >>>>>> - tx_poll_start(net, sock);
> >>>>>> - set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
> >>>>>> + if (vhost_poll_start(poll, sock->file))
> >>>>>> + vq_err(vq, "Fail to start TX polling\n");
> >>>>>> + else
> >>>>>> + set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
> >>>>>> break;
> >>>>>> }
> >>>>>> if (unlikely(vhost_enable_notify(&net->dev, vq))) {
> >>>>>> @@ -360,7 +338,8 @@ static void handle_tx(struct vhost_net *net)
> >>>>>> }
> >>>>>> vhost_discard_vq_desc(vq, 1);
> >>>>>> if (err == -EAGAIN || err == -ENOBUFS)
> >>>>>> - tx_poll_start(net, sock);
> >>>>>> + if (vhost_poll_start(poll, sock->file))
> >>>>>> + vq_err(vq, "Fail to start TX polling\n");
> >>>>>> break;
> >>>>>> }
> >>>>>> if (err != len)
> >>>>>> @@ -623,7 +602,6 @@ static int vhost_net_open(struct inode *inode, struct file *f)
> >>>>>>
> >>>>>> vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, dev);
> >>>>>> vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, dev);
> >>>>>> - n->tx_poll_state = VHOST_NET_POLL_DISABLED;
> >>>>>>
> >>>>>> f->private_data = n;
> >>>>>>
> >>>>>> @@ -633,29 +611,25 @@ static int vhost_net_open(struct inode *inode, struct file *f)
> >>>>>> static void vhost_net_disable_vq(struct vhost_net *n,
> >>>>>> struct vhost_virtqueue *vq)
> >>>>>> {
> >>>>>> + struct vhost_poll *poll = n->poll + (vq - n->vqs);
> >>>>>> +
> >>>>>> if (!vq->private_data)
> >>>>>> return;
> >>>>>> - if (vq == n->vqs + VHOST_NET_VQ_TX) {
> >>>>>> - tx_poll_stop(n);
> >>>>>> - n->tx_poll_state = VHOST_NET_POLL_DISABLED;
> >>>>>> - } else
> >>>>>> - vhost_poll_stop(n->poll + VHOST_NET_VQ_RX);
> >>>>>> + vhost_poll_stop(poll);
> >>>>>> }
> >>>>>>
> >>>>>> -static void vhost_net_enable_vq(struct vhost_net *n,
> >>>>>> +static int vhost_net_enable_vq(struct vhost_net *n,
> >>>>>> struct vhost_virtqueue *vq)
> >>>>>> {
> >>>>>> struct socket *sock;
> >>>>>> + struct vhost_poll *poll = n->poll + (vq - n->vqs);
> >>>>>>
> >>>>>> sock = rcu_dereference_protected(vq->private_data,
> >>>>>> lockdep_is_held(&vq->mutex));
> >>>>>> if (!sock)
> >>>>>> - return;
> >>>>>> - if (vq == n->vqs + VHOST_NET_VQ_TX) {
> >>>>>> - n->tx_poll_state = VHOST_NET_POLL_STOPPED;
> >>>>>> - tx_poll_start(n, sock);
> >>>>>> - } else
> >>>>>> - vhost_poll_start(n->poll + VHOST_NET_VQ_RX, sock->file);
> >>>>>> + return 0;
> >>>>>> +
> >>>>>> + return vhost_poll_start(poll, sock->file);
> >>>>>> }
> >>>>>>
> >>>>>> static struct socket *vhost_net_stop_vq(struct vhost_net *n,
> >>>>>> @@ -833,7 +807,9 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> >>>>>> r = vhost_init_used(vq);
> >>>>>> if (r)
> >>>>>> goto err_used;
> >>>>>> - vhost_net_enable_vq(n, vq);
> >>>>>> + r = vhost_net_enable_vq(n, vq);
> >>>>>> + if (r)
> >>>>>> + goto err_used;
> >>>>>>
> >>>>>> oldubufs = vq->ubufs;
> >>>>>> vq->ubufs = ubufs;
> >>>>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> >>>>>> index 34389f7..5c7a466 100644
> >>>>>> --- a/drivers/vhost/vhost.c
> >>>>>> +++ b/drivers/vhost/vhost.c
> >>>>>> @@ -77,26 +77,41 @@ void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
> >>>>>> init_poll_funcptr(&poll->table, vhost_poll_func);
> >>>>>> poll->mask = mask;
> >>>>>> poll->dev = dev;
> >>>>>> + poll->wqh = NULL;
> >>>>>>
> >>>>>> vhost_work_init(&poll->work, fn);
> >>>>>> }
> >>>>>>
> >>>>>> +/* Stop polling a file. After this function returns, it becomes safe to drop the
> >>>>>> + * file reference. You must also flush afterwards. */
> >>>>>> +void vhost_poll_stop(struct vhost_poll *poll)
> >>>>>> +{
> >>>>>> + if (poll->wqh) {
> >>>>>> + remove_wait_queue(poll->wqh, &poll->wait);
> >>>>>> + poll->wqh = NULL;
> >>>>>> + }
> >>>>>> +}
> >>>>>> +
> >>>>>> /* Start polling a file. We add ourselves to file's wait queue. The caller must
> >>>>>> * keep a reference to a file until after vhost_poll_stop is called. */
> >>>>>> -void vhost_poll_start(struct vhost_poll *poll, struct file *file)
> >>>>>> +int vhost_poll_start(struct vhost_poll *poll, struct file *file)
> >>>>>> {
> >>>>>> unsigned long mask;
> >>>>>> + int ret = 0;
> >>>>>> +
> >>>>>> + if (poll->wqh)
> >>>>>> + return -EBUSY;
> >>>>>>
> >>>>> I think this should return success: we are already polling.
> >>>>> Otherwise this would trigger a bug below I think.
> >>>> Ok.
> >>>>>> mask = file->f_op->poll(file, &poll->table);
> >>>>>> if (mask)
> >>>>>> vhost_poll_wakeup(&poll->wait, 0, 0, (void *)mask);
> >>>>>> -}
> >>>>>>
> >>>>>> -/* Stop polling a file. After this function returns, it becomes safe to drop the
> >>>>>> - * file reference. You must also flush afterwards. */
> >>>>>> -void vhost_poll_stop(struct vhost_poll *poll)
> >>>>>> -{
> >>>>>> - remove_wait_queue(poll->wqh, &poll->wait);
> >>>>>> + if (mask & POLLERR) {
> >>>>>> + ret = -EINVAL;
> >>>>>> + vhost_poll_stop(poll);
> >>>>>> + }
> >>>>>> +
> >>>>>> + return ret;
> >>>>>> }
> >>>>>>
> >>>>>> static bool vhost_work_seq_done(struct vhost_dev *dev, struct vhost_work *work,
> >>>>>> @@ -792,7 +807,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
> >>>>>> fput(filep);
> >>>>>>
> >>>>>> if (pollstart && vq->handle_kick)
> >>>>>> - vhost_poll_start(&vq->poll, vq->kick);
> >>>>>> + r = vhost_poll_start(&vq->poll, vq->kick);
> >>>>>>
> >>>>>> mutex_unlock(&vq->mutex);
> >>>>>>
> >>>>>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> >>>>>> index 2639c58..17261e2 100644
> >>>>>> --- a/drivers/vhost/vhost.h
> >>>>>> +++ b/drivers/vhost/vhost.h
> >>>>>> @@ -42,7 +42,7 @@ void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work);
> >>>>>>
> >>>>>> void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
> >>>>>> unsigned long mask, struct vhost_dev *dev);
> >>>>>> -void vhost_poll_start(struct vhost_poll *poll, struct file *file);
> >>>>>> +int vhost_poll_start(struct vhost_poll *poll, struct file *file);
> >>>>>> void vhost_poll_stop(struct vhost_poll *poll);
> >>>>>> void vhost_poll_flush(struct vhost_poll *poll);
> >>>>>> void vhost_poll_queue(struct vhost_poll *poll);
> >>>>>> --
> >>>>>> 1.7.1
> >>>>> --
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