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Date:   Mon, 25 Apr 2022 08:36:09 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Eric Dumazet <edumazet@google.com>
Cc:     Jann Horn <jannh@google.com>, Lukas Wunner <lukas@wunner.de>,
        Paolo Abeni <pabeni@redhat.com>,
        Oliver Neukum <oneukum@suse.com>,
        "David S. Miller" <davem@davemloft.net>,
        Oleksij Rempel <o.rempel@pengutronix.de>,
        netdev <netdev@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>,
        Andrew Lunn <andrew@lunn.ch>,
        Jacky Chou <jackychou@asix.com.tw>, Willy Tarreau <w@1wt.eu>,
        Lino Sanfilippo <LinoSanfilippo@gmx.de>,
        Philipp Rosenberger <p.rosenberger@kunbus.com>,
        Heiner Kallweit <hkallweit1@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH] net: linkwatch: ignore events for unregistered netdevs
Message-ID: <20220425083609.0c4bf0d4@kernel.org>
In-Reply-To: <CANn89iJPs13ndN3PCs6KDAetMUg7N5RzG9_ixvQCmswmcN28mw@mail.gmail.com>
References: <18b3541e5372bc9b9fc733d422f4e698c089077c.1650177997.git.lukas@wunner.de>
        <9325d344e8a6b1a4720022697792a84e545fef62.camel@redhat.com>
        <20220423160723.GA20330@wunner.de>
        <20220425074146.1fa27d5f@kernel.org>
        <CAG48ez3ibQjhs9Qxb0AAKE4-UZiZ5UdXG1JWcPWHAWBoO-1fVw@mail.gmail.com>
        <20220425080057.0fc4ef66@kernel.org>
        <CANn89iLwvqUJHBNifLESJyBQ85qjK42sK85Fs=QV4M7HqUXmxQ@mail.gmail.com>
        <20220425082804.209e3676@kernel.org>
        <CANn89iJPs13ndN3PCs6KDAetMUg7N5RzG9_ixvQCmswmcN28mw@mail.gmail.com>
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On Mon, 25 Apr 2022 08:31:23 -0700 Eric Dumazet wrote:
> > Jann described a case where someone does
> >
> >     CPU 0      CPU 1     CPU 2
> >
> >   dev_hold()
> >    ------  #unregister -------
> >              dev_hold()
> >                          dev_put()
> >
> > Our check for refcount == 0 goes over the CPUs one by one,
> > so if it sums up CPUs 0 and 1 at the "unregister" point above
> > and CPU2 after the CPU1 hold and CPU2 release it will "miss"
> > one refcount.
> >
> > That's a problem unless doing a dev_hold() on a netdev we only have
> > a reference on is illegal.  
> 
> What is 'illegal' is trying to keep using the device after #unregister.
> 
> We have barriers to prevent that.
> 
> Somehow a layer does not care about the barriers and pretends the
> device is still good to use.
> 
> It is of course perfectly fine to stack multiple dev_hold() from one
> path (if these do not leak, but this is a different issue)

So we'd need something like

WARN_ON(dev->reg_state != NETREG_REGISTERED && !rtnl_held())

in dev_hold()?