netdev - Re: [PATCH v2] net/macvtap: fix reference counting

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Message-Id: <1265922577.28768.6.camel@w-sridhar.beaverton.ibm.com>
Date:	Thu, 11 Feb 2010 13:09:37 -0800
From:	Sridhar Samudrala <sri@...ibm.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Patrick McHardy <kaber@...sh.net>,
	Ed Swierk <eswierk@...stanetworks.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v2] net/macvtap: fix reference counting

On Thu, 2010-02-11 at 16:55 +0100, Arnd Bergmann wrote:
> The RCU usage in the original code was broken because
> there are cases where we possibly sleep with rcu_read_lock
> held. As a fix, change the macvtap_file_get_queue to
> get a reference on the socket and the netdev instead of
> taking the full rcu_read_lock.
> 
> Also, change macvtap_file_get_queue failure case to
> not require a subsequent macvtap_file_put_queue, as
> pointed out by Ed Swierk.

Looks good.

Acked-by: Sridhar Samudrala <sri@...ibm.com>

> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Cc: Ed Swierk <eswierk@...stanetworks.com>
> Cc: Sridhar Samudrala <sri@...ibm.com>
> ---
>  drivers/net/macvtap.c |   35 ++++++++++++++++++++++-------------
>  1 files changed, 22 insertions(+), 13 deletions(-)
> 
> Please disregard v1 of this patch, I accidentally sent Sridhar's
> patch...
> 
> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> index ad1f6ef..fe7656b 100644
> --- a/drivers/net/macvtap.c
> +++ b/drivers/net/macvtap.c
> @@ -70,7 +70,8 @@ static struct cdev macvtap_cdev;
>   * exists.
>   *
>   * The callbacks from macvlan are always done with rcu_read_lock held
> - * already, while in the file_operations, we get it ourselves.
> + * already. For calls from file_operations, we use the rcu_read_lock_bh
> + * to get a reference count on the socket and the device.
>   *
>   * When destroying a queue, we remove the pointers from the file and
>   * from the dev and then synchronize_rcu to make sure no thread is
> @@ -159,13 +160,21 @@ static void macvtap_del_queues(struct net_device *dev)
> 
>  static inline struct macvtap_queue *macvtap_file_get_queue(struct file *file)
>  {
> +	struct macvtap_queue *q;
>  	rcu_read_lock_bh();
> -	return rcu_dereference(file->private_data);
> +	q = rcu_dereference(file->private_data);
> +	if (q) {
> +		sock_hold(&q->sk);
> +		dev_hold(q->vlan->dev);
> +	}
> +	rcu_read_unlock_bh();
> +	return q;
>  }
> 
> -static inline void macvtap_file_put_queue(void)
> +static inline void macvtap_file_put_queue(struct macvtap_queue *q)
>  {
> -	rcu_read_unlock_bh();
> +	sock_put(&q->sk);
> +	dev_put(q->vlan->dev);
>  }
> 
>  /*
> @@ -314,8 +323,8 @@ static unsigned int macvtap_poll(struct file *file, poll_table * wait)
>  	     sock_writeable(&q->sk)))
>  		mask |= POLLOUT | POLLWRNORM;
> 
> +	macvtap_file_put_queue(q);
>  out:
> -	macvtap_file_put_queue();
>  	return mask;
>  }
> 
> @@ -366,8 +375,8 @@ static ssize_t macvtap_aio_write(struct kiocb *iocb, const struct iovec *iv,
> 
>  	result = macvtap_get_user(q, iv, iov_length(iv, count),
>  			      file->f_flags & O_NONBLOCK);
> +	macvtap_file_put_queue(q);
>  out:
> -	macvtap_file_put_queue();
>  	return result;
>  }
> 
> @@ -398,10 +407,8 @@ static ssize_t macvtap_aio_read(struct kiocb *iocb, const struct iovec *iv,
>  	struct sk_buff *skb;
>  	ssize_t len, ret = 0;
> 
> -	if (!q) {
> -		ret = -ENOLINK;
> -		goto out;
> -	}
> +	if (!q)
> +		return -ENOLINK;
> 
>  	len = iov_length(iv, count);
>  	if (len < 0) {
> @@ -437,7 +444,7 @@ static ssize_t macvtap_aio_read(struct kiocb *iocb, const struct iovec *iv,
>  	remove_wait_queue(q->sk.sk_sleep, &wait);
> 
>  out:
> -	macvtap_file_put_queue();
> +	macvtap_file_put_queue(q);
>  	return ret;
>  }
> 
> @@ -468,7 +475,7 @@ static long macvtap_ioctl(struct file *file, unsigned int cmd,
>  		if (!q)
>  			return -ENOLINK;
>  		memcpy(devname, q->vlan->dev->name, sizeof(devname));
> -		macvtap_file_put_queue();
> +		macvtap_file_put_queue(q);
> 
>  		if (copy_to_user(&ifr->ifr_name, q->vlan->dev->name, IFNAMSIZ) ||
>  		    put_user((TUN_TAP_DEV | TUN_NO_PI), &ifr->ifr_flags))
> @@ -485,8 +492,10 @@ static long macvtap_ioctl(struct file *file, unsigned int cmd,
>  			return -EFAULT;
> 
>  		q = macvtap_file_get_queue(file);
> +		if (!q)
> +			return -ENOLINK;
>  		q->sk.sk_sndbuf = u;
> -		macvtap_file_put_queue();
> +		macvtap_file_put_queue(q);
>  		return 0;
> 
>  	case TUNSETOFFLOAD:

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