netdev - Re: [PATCH net 1/4] igc: Report speed and duplex as unknown when device is runtime suspended

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Date:   Sat, 30 Jan 2021 10:12:47 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Neftin, Sasha" <sasha.neftin@...el.com>
Cc:     Tony Nguyen <anthony.l.nguyen@...el.com>, davem@...emloft.net,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        netdev@...r.kernel.org, sassmann@...hat.com,
        "Lifshits, Vitaly" <vitaly.lifshits@...el.com>
Subject: Re: [PATCH net 1/4] igc: Report speed and duplex as unknown when
 device is runtime suspended

On Sat, 30 Jan 2021 16:00:06 +0200 Neftin, Sasha wrote:
> On 1/30/2021 08:22, Jakub Kicinski wrote:
> > On Thu, 28 Jan 2021 13:38:48 -0800 Tony Nguyen wrote:  
> >> From: Kai-Heng Feng <kai.heng.feng@...onical.com>
> >>
> >> Similar to commit 165ae7a8feb5 ("igb: Report speed and duplex as unknown
> >> when device is runtime suspended"), if we try to read speed and duplex
> >> sysfs while the device is runtime suspended, igc will complain and
> >> stops working:  
> >   
> >> The more generic approach will be wrap get_link_ksettings() with begin()
> >> and complete() callbacks, and calls runtime resume and runtime suspend
> >> routine respectively. However, igc is like igb, runtime resume routine
> >> uses rtnl_lock() which upper ethtool layer also uses.
> >>
> >> So to prevent a deadlock on rtnl, take a different approach, use
> >> pm_runtime_suspended() to avoid reading register while device is runtime
> >> suspended.  
> > 
> > Is someone working on the full fix to how PM operates?
> > 
> > There is another rd32(IGC_STATUS) in this file which I don't think
> > is protected either.  
>
> What is another rd32(IGC_STATUS) you meant? in  igc_ethtool_get_regs? 

Yes.

> While the device in D3 state there is no configuration space registers 
> access.

That's to say similar stack trace will be generated to the one fixed
here, if someone runs ethtool -d, correct? I don't see anything
checking runtime there either.

To be clear I'm not asking for this to be addressed in this series.
Rather for a strong commitment that PM handling will be restructured.
It seems to me you should depend on refcounting / locking that the PM
subsystem does more rather than involving rtnl_lock.