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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 7 Jan 2013 16:55:59 +0200

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 <[email-masked]>
> >> ---
> >>  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?

> 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.

> >
> >>  		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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