linux-kernel - Re: [RFC net-next] net: dsa: rtl8366rb: support bridge offloading

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Message-ID: <20210302161140.l3jtvkcm3tvlv5q3@skbuf>
Date:   Tue, 2 Mar 2021 18:11:40 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     DENG Qingfang <dqfext@...il.com>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        netdev <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC net-next] net: dsa: rtl8366rb: support bridge offloading

On Tue, Mar 02, 2021 at 05:05:00PM +0100, Linus Walleij wrote:
> On Tue, Mar 2, 2021 at 4:58 AM DENG Qingfang <dqfext@...il.com> wrote:
> > On Mon, Mar 1, 2021 at 9:48 PM Linus Walleij <linus.walleij@...aro.org> wrote:
> > > With my minor changes:
> > > Tested-by: Linus Walleij <linus.walleij@...aro.org>
> >
> > How about using a mutex lock in port_bridge_{join,leave} ?
> > In my opinion all functions that access multiple registers should be
> > synchronized.
> 
> That's one way, in some cases the framework (DSA) serialize
> the accesses so I don't know if that already happens on a
> higher level? Since it is accessed over a slow bus we should go
> for mutex in that case indeed.

DSA does not serialize this. The .port_bridge_join and
.port_bridge_leave calls are initiated from the NETDEV_CHANGEUPPER net
device event, which is called under rtnl_mutex (see call_netdevice_notifiers).
This is pretty fundamental and I don't think it will ever change.

However, if you still want to add an extra layer of locking (with code
paths that for some reason are not under the rtnl_mutex), then go ahead,
I suppose. It will be challenging to make sure they do something that
isn't snake oil, though.