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

Stephen Hemminger
2017-01-13T16:35:10Z

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