netdev - Re: [PATCH RFC net-next 03/14] net: phylink: add support for PCS link change notifications

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Message-ID: <ZbAch9ZlbDrZqzpw@shell.armlinux.org.uk>
Date: Tue, 23 Jan 2024 20:07:35 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Sean Anderson <sean.anderson@...o.com>
Cc: Landen.Chao@...iatek.com, UNGLinuxDriver@...rochip.com,
	alexandre.belloni@...tlin.com, andrew@...n.ch,
	angelogioacchino.delregno@...labora.com, arinc.unal@...nc9.com,
	claudiu.manoil@....com, daniel@...rotopia.org, davem@...emloft.net,
	dqfext@...il.com, edumazet@...gle.com, f.fainelli@...il.com,
	hkallweit1@...il.com, kuba@...nel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org, matthias.bgg@...il.com,
	netdev@...r.kernel.org, olteanv@...il.com, pabeni@...hat.com,
	sean.wang@...iatek.com
Subject: Re: [PATCH RFC net-next 03/14] net: phylink: add support for PCS
 link change notifications

On Tue, Jan 23, 2024 at 02:46:15PM -0500, Sean Anderson wrote:
> Hi Russell,
> 
> Does there need to be any locking when calling phylink_pcs_change? I
> noticed that you call it from threaded IRQ context in [1]. Can that race
> with phylink_major_config?

What kind of scenario are you thinking may require locking?

I guess the possibility would be if pcs->phylink changes and the
compiler reads it multiple times - READ_ONCE() should solve that.

However, in terms of the mechanics, there's no race.

During the initial bringup, the resolve worker isn't started until
after phylink_major_config() has completed (it's started at
phylink_enable_and_run_resolve().) So, if phylink_pcs_change()
gets called while in phylink_major_config() there, it'll see
that pl->phylink_disable_state is non-zero, and won't queue the
work.

The next one is within the worker itself - and there can only
be one instance of the worker running in totality. So, if
phylink_pcs_change() gets called while phylink_major_config() is
running from this path, the only thing it'll do is re-schedule
the resolve worker to run another iteration which is harmless
(whether or not the PCS is still current.)

The last case is phylink_ethtool_ksettings_set(). This runs under
the state_mutex, which locks out the resolve worker (since it also
takes that mutex).

So calling phylink_pcs_change() should be pretty harmless _unless_
the compiler re-reads pcs->phylink multiple times inside
phylink_pcs_change(), which I suppose with modern compilers is
possible. Hence my suggestion above about READ_ONCE() for that.

Have you encountered an OOPS because pcs->phylink has become NULL?
Or have you spotted another issue?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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