From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [net PATCH v4 2/6] virtio_net: wrap rtnl_lock in test for
 calling with lock already held
Date: Tue, 17 Jan 2017 11:57:33 -0500 (EST)
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From: John Fastabend <john.fastabend@gmail.com>
Date: Sun, 15 Jan 2017 15:59:47 -0800

> @@ -2358,7 +2371,10 @@ static void remove_vq_common(struct virtnet_info *vi)
>  	/* Free unused buffers in both send and recv, if any. */
>  	free_unused_bufs(vi);
>  
> -	free_receive_bufs(vi);
> +	if (rtnl_is_locked())
> +		_free_receive_bufs(vi);
> +	else
> +		free_receive_bufs(vi);
>  
>  	free_receive_page_frags(vi);
>  

This doesn't work.  rtnl_is_locked() doesn't tell if _you_ own the mutex, it
just says that someone does.

So if we now execute this code without taking the RTNL lock just because some
other thread of control holds it, we introduce a race.