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

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Date:   Wed, 23 Sep 2020 15:42:17 -0700
From:   Saeed Mahameed <saeed@...nel.org>
To:     Heiner Kallweit <hkallweit1@...il.com>,
        David Miller <davem@...emloft.net>
Cc:     geert+renesas@...der.be, f.fainelli@...il.com, andrew@...n.ch,
        kuba@...nel.org, gaku.inami.xh@...esas.com,
        yoshihiro.shimoda.uh@...esas.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "net: linkwatch: add check for netdevice being
 present to linkwatch_do_dev"

On Wed, 2020-09-23 at 22:44 +0200, Heiner Kallweit wrote:
> On 23.09.2020 22:15, David Miller wrote:
> > From: Heiner Kallweit <hkallweit1@...il.com>
> > Date: Wed, 23 Sep 2020 21:58:59 +0200
> > 
> > > On 23.09.2020 20:35, Saeed Mahameed wrote:
> > > > Why would a driver detach the device on ndo_stop() ?
> > > > seems like this is the bug you need to be chasing ..
> > > > which driver is doing this ? 
> > > > 
> > > Some drivers set the device to PCI D3hot at the end of ndo_stop()
> > > to save power (using e.g. Runtime PM). Marking the device as
> > > detached
> > > makes clear to to the net core that the device isn't accessible
> > > any
> > > longer.
> > 
> > That being the case, the problem is that IFF_UP+!present is not a
> > valid netdev state.
> > 
> If this combination is invalid, then netif_device_detach() should
> clear IFF_UP? At a first glance this should be sufficient to avoid
> the issue I was dealing with.
> 

Feels like a work around and would conflict with the assumption that 
netif_device_detach() should only be called when !IFF_UP

Maybe we need to clear IFF_UP before calling ops->ndo_stop(dev),
instead of after on __dev_close_many(). Assuming no driver is checking
IFF_UP state on its own ndo_stop(), other than this, the order
shouldn't really matter, since clearing the flag and calling ndo_stop()
should be considered as one atomic operation.

> > Is it simply the issue that, upon resume, IFF_UP is marked true
> > before
> > the device is brought out from D3hot state and thus marked as
> > present
> > again?
> > 
> I can't really comment on that. The issue I was dealing with at the
> time I submitted this change was about an async linkwatch event
> (caused by powering down the PHY in ndo_stop) trying to access the
> device when it was powered down already.