netdev - Re: [PATCH net] net: dsa: ocelot: call dsa_tag_8021q_unregister() under rtnl

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Date: Fri, 4 Aug 2023 14:10:45 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Andrew Lunn <andrew@...n.ch>,
	Florian Fainelli <f.fainelli@...il.com>,
	Claudiu Manoil <claudiu.manoil@....com>,
	Alexandre Belloni <alexandre.belloni@...tlin.com>,
	UNGLinuxDriver@...rochip.com,
	Colin Foster <colin.foster@...advantage.com>
Subject: Re: [PATCH net] net: dsa: ocelot: call dsa_tag_8021q_unregister()
 under rtnl_lock() on driver remove

On Thu, Aug 03, 2023 at 09:29:38PM +0200, Simon Horman wrote:
> On Thu, Aug 03, 2023 at 04:42:53PM +0300, Vladimir Oltean wrote:
> > diff --git a/drivers/net/dsa/ocelot/felix.c b/drivers/net/dsa/ocelot/felix.c
> > index fd7eb4a52918..9a3e5ec16972 100644
> > --- a/drivers/net/dsa/ocelot/felix.c
> > +++ b/drivers/net/dsa/ocelot/felix.c
> > @@ -1619,8 +1619,10 @@ static void felix_teardown(struct dsa_switch *ds)
> >  	struct felix *felix = ocelot_to_felix(ocelot);
> >  	struct dsa_port *dp;
> >  
> > +	rtnl_lock();
> >  	if (felix->tag_proto_ops)
> >  		felix->tag_proto_ops->teardown(ds);
> > +	rtnl_unlock();
> 
> Hi Vladimir,
> 
> I am curious to know if RTNL could be taken in
> felix_tag_8021q_teardown() instead.

Negative. This call path also exists:

dsa_tree_change_tag_proto()
-> rtnl_trylock()
-> dsa_tree_notify(dst, DSA_NOTIFIER_TAG_PROTO, &info)
   -> dsa_switch_change_tag_proto()
      -> ds->ops->change_tag_protocol()
         -> felix_change_tag_protocol()
            -> old_proto_ops->teardown()
               -> felix_tag_8021q_teardown()

where the rtnl_mutex is already held.