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

Jason Wang
2013-01-07T04:38:17Z

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. Not sure it's worth to do this work,
maybe a warning is enough just like other failure.
>
>> 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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