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

On 17-01-13 08:34 AM, Stephen Hemminger wrote:

On Thu, 12 Jan 2017 18:51:00 -0800
John Fastabend [off-list ref] wrote:

quoted

 
-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:

OK I'll use the normal form.

void _foo(some args)
{
	ASSERT_RTNL();

	...
}

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