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

From: Jason Wang <[email-masked]>
Date: Mon, 14 Jan 2013 15:37:58 +0800

On 01/14/2013 02:57 PM, Michael S. Tsirkin wrote:
> 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?

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

True, but we this depends on the assumption that each kind of backend
must make sure poll_wait() is called in .poll(). And we need to change
macvtap_poll() as well. And we'd better also pass those polling errors
to notify user about this wrong state.
>
>>>>>> 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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