netdev - Re: [PATCH v5 net-next 01/15] net: bridge: mst: Multiple Spanning Tree (MST) mode

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Message-ID: <20230109122908.dn2xlzlbcvfxgeii@skbuf>
Date:   Mon, 9 Jan 2023 14:29:08 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     Tobias Waldekranz <tobias@...dekranz.com>, davem@...emloft.net,
        kuba@...nel.org, Nikolay Aleksandrov <razor@...ckwall.org>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Ivan Vecera <ivecera@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Russell King <linux@...linux.org.uk>,
        Petr Machata <petrm@...dia.com>,
        Ido Schimmel <idosch@...dia.com>,
        Matt Johnston <matt@...econstruct.com.au>,
        Cooper Lees <me@...perlees.com>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, bridge@...ts.linux-foundation.org
Subject: Re: [PATCH v5 net-next 01/15] net: bridge: mst: Multiple Spanning
 Tree (MST) mode

On Mon, Jan 09, 2023 at 02:20:02PM +0200, Ido Schimmel wrote:
> On Mon, Jan 09, 2023 at 01:56:53PM +0200, Vladimir Oltean wrote:
> > On Mon, Jan 09, 2023 at 01:43:46PM +0200, Ido Schimmel wrote:
> > > OK, thanks for confirming. Will send a patch later this week if Tobias
> > > won't take care of it by then. First patch will probably be [1] to make
> > > sure we dump the correct MST state to user space. It will also make it
> > > easier to show the problem and validate the fix.
> > > 
> > > [1]
> > > diff --git a/net/bridge/br.c b/net/bridge/br.c
> > > index 4f5098d33a46..f02a1ad589de 100644
> > > --- a/net/bridge/br.c
> > > +++ b/net/bridge/br.c
> > > @@ -286,7 +286,7 @@ int br_boolopt_get(const struct net_bridge *br, enum br_boolopt_id opt)
> > >  	case BR_BOOLOPT_MCAST_VLAN_SNOOPING:
> > >  		return br_opt_get(br, BROPT_MCAST_VLAN_SNOOPING_ENABLED);
> > >  	case BR_BOOLOPT_MST_ENABLE:
> > > -		return br_opt_get(br, BROPT_MST_ENABLED);
> > > +		return br_mst_is_enabled(br);
> > 
> > Well, this did report the correct MST state despite the incorrect static
> > branch state, no? The users of br_mst_is_enabled(br) are broken, not
> > those of br_opt_get(br, BROPT_MST_ENABLED).
> 
> I should have said "actual"/"effective" instead of "correct". IMO, it's
> better to use the same conditional in the both the data and control
> paths to eliminate discrepancies. Without the patch, a user will see
> that MST is supposedly enabled when it is actually disabled in the data
> path.

The discussion is about to get philosophical, but I don't know if it's
necessary to make a bug more widespread before fixing it..
The br_mst_used is an optimization to avoid calling br_opt_get() when
surely MST is not enabled. There should be no discrepancy between using
and not using it, if the static branch works correctly (not the case here).
I would also expect that consolidation to be part of net-next though.

> > Anyway, I see there's a br_mst_is_enabled() and also a br_mst_enabled()?!
> > One is used in the fast path and the other in the slow path. They should
> > probably be merged, I guess. They both exist probably because somebody
> > thought that the "if (!netif_is_bridge_master(dev))" test is redundant
> > in the fast path.
> 
> The single user of br_mst_enabled() (DSA) is not affected by the bug
> (only the SW data path is), so I suggest making this consolidation in
> net-next after the bug is fixed. OK?

Yes, net-next, sure.