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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 14 Jan 2013 08:57:12 +0200

On Mon, Jan 14, 2013 at 10:59:02AM +0800, Jason Wang wrote:
> On 01/13/2013 07:10 PM, Michael S. Tsirkin wrote:
> > 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. 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?
> 
> Another one I think we can trigger is:
> 
> - start vhost thread
> - do ip link del link dev tap0 to delete the tap device
> 
> In this case, the netdevice is unregistered but the file/socket still exist.

Yes but in this case poll_wait is called so apparently no issue
with existing code? We only have an issue if poll_wait is
not called right?

Maybe the simplest fix is to invoke poll_wait before
checking if (!tun) ?

> >
> >>>> 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
> >>>>> --
> >>>>> To unsubscribe from this list: send the line "unsubscribe netdev" in
> >>>>> the body of a message to [email-masked]
> >>>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >>> --
> >>> To unsubscribe from this list: send the line "unsubscribe kvm" in
> >>> the body of a message to [email-masked]
> >>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > --
> > To unsubscribe from this list: send the line "unsubscribe kvm" in
> > the body of a message to [email-masked]
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html