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

[Stephen Hemminger]

On Thu, 12 Jan 2017 18:51:00 -0800
John Fastabend <john.fastab...@gmail.com> 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();
}