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Date:   Sat, 5 May 2018 20:51:45 +0200
From:   Andrew Lunn <andrew@lunn.ch>
To:     Kees Cook <keescook@chromium.org>
Cc:     Salvatore Mesoraca <s.mesoraca16@gmail.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Vivien Didelot <vivien.didelot@savoirfairelinux.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Network Development <netdev@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH] net: dsa: drop some VLAs in switch.c
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> > You could make the bitmap part of the dsa_switch structure. This is
> > allocated by dsa_switch_alloc() and is passed the number of ports.
> > Doing the allocation there means you don't need to worry about it
> > failing in dsa_switch_mdb_add() or dsa_switch_vlan_add().
> 
> Are dsa_switch_mdb_add() and dsa_switch_vlan_add() guaranteed to be
> single-threaded?

Yes, that is the interesting question here.... against each other, or
themselves?

They are called from a notifier chain. It is the same notifier chain
for both dsa_switch_mdb_add() and dsa_switch_vlan_add().

notifier_call_chain() itself appears to not provide any guarantees
about the same handler being called in parallel.

It is dsa_port_notify() which is calling the notifier_call_chain().
This is being called by both dsa_port_vlan_add() and
dsa_port_mdb_add() in dsa_slave_port_obj_add(). This is a switchdev
op. switchdev_port_obj_add_now() does have ASSERT_RTNL(); So that
should serialize everything.

     Andrew