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

Michael S. Tsirkin
2013-01-06T13:22:01Z

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?

> 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.

> 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