netdev - Re: [net PATCH v4 2/6] virtio_net: wrap rtnl_lock in test for calling with lock already held

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Message-Id: <20170117.115733.428297564258724055.davem@davemloft.net>
Date:   Tue, 17 Jan 2017 11:57:33 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     john.fastabend@...il.com
Cc:     jasowang@...hat.com, mst@...hat.com, john.r.fastabend@...el.com,
        netdev@...r.kernel.org, alexei.starovoitov@...il.com,
        daniel@...earbox.net
Subject: Re: [net PATCH v4 2/6] virtio_net: wrap rtnl_lock in test for
 calling with lock already held

From: John Fastabend <john.fastabend@...il.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.