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

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Date:   Sat, 12 Sep 2020 14:33:59 +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 Sat, Sep 12, 2020 at 2:44 AM David Miller <davem@...emloft.net> wrote:
> From: Geert Uytterhoeven <geert@...ux-m68k.org>
> Date: Fri, 11 Sep 2020 08:32:55 +0200
>
> > 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?
>
> I have a better question.  Why is a software device like the bridge,
> that wants to effectively exist and still receive netdev event
> notifications, marking itself as not present?
>
> That's seems like the real bug here.

"dev" is not the bridge device, but the physical Ethernet interface, which
may already be suspended during s2ram.

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