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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 13 Jan 2013 13:04:55 +0200

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 <[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?
> 
> 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.

Hmm I don't really understand.
All we care about is that socket is around no?
Could you please show how a problematic case can
be triggered?

> Although we can change
> this, but it's the behaviour before multiqueue support.
> >
> >> 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.

What I mean is that since we don't poll
the backend, there appears to be no way to
recover. So I'd prefer that we prevent
userspace from creating this broken configuration
when ring is running.

> >
> >>>>  		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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