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

From: Jason Wang <[email-masked]>
Date: Mon, 14 Jan 2013 10:59:02 +0800

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.
>
>>>> 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
>>>>> --
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