netdev - Re: [PATCH net-next] macvlan: silence RCU list debugging warning

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Message-ID: <643f9363-9e25-7e13-a5cf-fe189268b445@gmail.com>
Date:   Mon, 20 Apr 2020 08:48:36 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Wei Yongjun <weiyongjun1@...wei.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Taehee Yoo <ap420073@...il.com>,
        Richard Cochran <richardcochran@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net-next] macvlan: silence RCU list debugging warning



On 4/20/20 4:59 AM, Wei Yongjun wrote:
> macvlan_hash_lookup() uses list_for_each_entry_rcu() for traversing
> outside of an RCU read side critical section but under the protection
> of rtnl_mutex. Hence, add the corresponding lockdep expression to
> silence the following false-positive warning:
>


This changelog is misleading.

macvlan_hash_lookup() _can_ be used under RCU only, in its fast path.

So you can not claim that it is run with RTNL held.

Please be precise in the changelogs

Thanks.
 
> =============================
> WARNING: suspicious RCU usage
> 5.7.0-rc1-next-20200416-00003-ga3b8d28bc #1 Not tainted
> -----------------------------
> drivers/net/macvlan.c:126 RCU-list traversed in non-reader section!!
> 
> Signed-off-by: Wei Yongjun <weiyongjun1@...wei.com>
> ---
>  drivers/net/macvlan.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
> index e7289d67268f..654c1fa11826 100644
> --- a/drivers/net/macvlan.c
> +++ b/drivers/net/macvlan.c
> @@ -123,7 +123,8 @@ static struct macvlan_dev *macvlan_hash_lookup(const struct macvlan_port *port,
>  	struct macvlan_dev *vlan;
>  	u32 idx = macvlan_eth_hash(addr);
>  
> -	hlist_for_each_entry_rcu(vlan, &port->vlan_hash[idx], hlist) {
> +	hlist_for_each_entry_rcu(vlan, &port->vlan_hash[idx], hlist,
> +				 lockdep_rtnl_is_held()) {
>  		if (ether_addr_equal_64bits(vlan->dev->dev_addr, addr))
>  			return vlan;
>  	}
>