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

Michael S. Tsirkin
2013-01-13T11:10:35Z

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?

> >
> >> 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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