netdev - Re: [PATCH] Revert "net: linkwatch: add check for netdevice being present to linkwatch_do

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Date:   Fri, 11 Sep 2020 08:32:55 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     David Miller <davem@...emloft.net>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>, Jakub Kicinski <kuba@...nel.org>,
        Gaku Inami <gaku.inami.xh@...esas.com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Revert "net: linkwatch: add check for netdevice being
 present to linkwatch_do_dev"

Hi David,

On Thu, Sep 10, 2020 at 9:20 PM David Miller <davem@...emloft.net> wrote:
> From: Geert Uytterhoeven <geert+renesas@...der.be>
> Date: Tue,  1 Sep 2020 17:02:37 +0200
>
> > This reverts commit 124eee3f6955f7aa19b9e6ff5c9b6d37cb3d1e2c.
> >
> > Inami-san reported that this commit breaks bridge support in a Xen
> > environment, and that reverting it fixes this.
> >
> > During system resume, bridge ports are no longer enabled, as that relies
> > on the receipt of the NETDEV_CHANGE notification.  This notification is
> > not sent, as netdev_state_change() is no longer called.
> >
> > Note that the condition this commit intended to fix never existed
> > upstream, as the patch triggering it and referenced in the commit was
> > never applied upstream.  Hence I can confirm s2ram on r8a73a4/ape6evm
> > and sh73a0/kzm9g works fine before/after this revert.
> >
> > Reported-by Gaku Inami <gaku.inami.xh@...esas.com>
> > Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
>
> Maybe you cannot reproduce it, but the problem is there and it still
> looks very real to me.
>
> netdev_state_change() does two things:
>
> 1) Emit the NETDEV_CHANGE notification
>
> 2) Emit an rtmsg_ifinfo() netlink message, which in turn tries to access
>    the device statistics via ->ndo_get_stats*().
>
> It is absolutely wrong to do #2 when netif_device_present() is false.
>
> So I cannot apply this patch as-is, sorry.

Thanks a lot for looking into this!

But doing #1 is still safe?  That is the part that calls into the bridge
code.  So would moving the netif_device_present() check from
linkwatch_do_dev() to netdev_state_change(), to prevent doing #2, be
acceptable?

Thanks again!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds