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

From: Jason Wang <[email-masked]>
Date: Mon, 07 Jan 2013 12:38:17 +0800

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. Not sure it's worth to do this work,
maybe a warning is enough just like other failure.
>
>>  		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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