Subject: Re: [net PATCH v3 2/5] net: virtio: wrap rtnl_lock in test for calling with lock already held

From: Stephen Hemminger <[email-masked]>
Date: Fri, 13 Jan 2017 08:34:08 -0800

On Thu, 12 Jan 2017 18:51:00 -0800
John Fastabend <[email-masked]> wrote:

>  
> -static void free_receive_bufs(struct virtnet_info *vi)
> +static void free_receive_bufs(struct virtnet_info *vi, bool need_lock)
>  {
>  	struct bpf_prog *old_prog;
>  	int i;
>  
> -	rtnl_lock();
> +	if (need_lock)
> +		rtnl_lock();
>  	for (i = 0; i < vi->max_queue_pairs; i++) {
>  		while (vi->rq[i].pages)
>  			__free_pages(get_a_page(&vi->rq[i], GFP_KERNEL), 0);
> @@ -1879,7 +1880,8 @@ static void free_receive_bufs(struct virtnet_info *vi)
>  		if (old_prog)
>  			bpf_prog_put(old_prog);
>  	}
> -	rtnl_unlock();
> +	if (need_lock)
> +		rtnl_unlock();
>  }

Conditional locking is bad idea; sparse complains about it and is later source
of bugs. The more typical way of doing this in kernel is:

void _foo(some args)
{
	ASSERT_RTNL();

	...
}

void foo(some args)
{
	rtnl_lock();
	_foo(some args)
	rtnl_unlock();
}