linux-kernel - Re: [PATCH 5.14 309/849] net: dsa: lantiq_gswip: serialize access to the PCE table

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Date:   Mon, 15 Nov 2021 23:32:34 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Hauke Mehrtens <hauke@...ke-m.de>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.14 309/849] net: dsa: lantiq_gswip: serialize access to
 the PCE table

On Mon, Nov 15, 2021 at 07:28:14PM +0100, Hauke Mehrtens wrote:
> On 11/15/21 5:56 PM, Greg Kroah-Hartman wrote:
> > From: Vladimir Oltean <vladimir.oltean@....com>
> > 
> > [ Upstream commit 49753a75b9a32de4c0393bb8d1e51ea223fda8e4 ]
> > 
> > Looking at the code, the GSWIP switch appears to hold bridging service
> > structures (VLANs, FDBs, forwarding rules) in PCE table entries.
> > Hardware access to the PCE table is non-atomic, and is comprised of
> > several register reads and writes.
> > 
> > These accesses are currently serialized by the rtnl_lock, but DSA is
> > changing its driver API and that lock will no longer be held when
> > calling ->port_fdb_add() and ->port_fdb_del().
> > 
> > So this driver needs to serialize the access to the PCE table using its
> > own locking scheme. This patch adds that.
> > 
> > Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> > Reviewed-by: Florian Fainelli <f.fainelli@...il.com>
> > Acked-by: Hauke Mehrtens <hauke@...ke-m.de>
> > Signed-off-by: David S. Miller <davem@...emloft.net>
> > Signed-off-by: Sasha Levin <sashal@...nel.org>
> > ---
> >   drivers/net/dsa/lantiq_gswip.c | 28 +++++++++++++++++++++++-----
> >   1 file changed, 23 insertions(+), 5 deletions(-)
> 
> Hi Greg and Vladimir,
> 
> I understood this is only needed when we apply the complete patch series
> from Vladimir. This would only be needed when we also apply this patch:
> > commit 5cdfde49a07f38663c277ddf2e56345ea1706fc2
> > Author: Vladimir Oltean <vladimir.oltean@....com>
> > Date:   Fri Oct 22 21:43:10 2021 +0300
> >
> >     net: dsa: drop rtnl_lock from dsa_slave_switchdev_event_work
> This was added in v5.16-rc1.
> 
> Without this patch the sections are protected by rtnl_lock().

This is correct, this patch does not need to be backported, and unless
it is needed for purposes other than fixing a bug, it should be dropped.