netdev - Re: [PATCH net-next] net: pcs: lynxi: fully reconfigure if link is down

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Date: Thu, 17 Aug 2023 14:30:51 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Denis Kirjanov <dkirjanov@...e.de>
Cc: Daniel Golle <daniel@...rotopia.org>,
	Alexander Couzens <lynxis@...0.eu>, Andrew Lunn <andrew@...n.ch>,
	Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH net-next] net: pcs: lynxi: fully reconfigure if link is
 down

On Thu, Aug 17, 2023 at 04:13:44PM +0300, Denis Kirjanov wrote:
> btw is it a thread-safe to check the mpcs->interface member?
> I've quick checked and phylink_pcs_config can be invoked from different places
> and the code below does the following assignment:
> mpcs->interface = interface;

pcs_config is called from:
- phylink_change_inband_advert() while holding pl->state_mutex
- phylink_major_config()

phylink_major_config() is called from:
- phylink_resolve() while holding pl->state_mutex
- phylink_ethtool_ksettings_set() while holding pl->state_mutex
- phylink_mac_initial_config()

phylink_mac_initial_config() is called from:
- phylink_resume()
- phylink_start()

In both cases, these are called without holding pl->state_mutex, so
one may think that they could be unsafe. In both cases, however,
pl->phylink_disable_state is non-zero.

phylink_change_inband_advert() will check whether
PHYLINK_DISABLE_STOPPED is set, and not proceed to call pcs_config()
in that case. In the phylink_resume() case, userspace isn't running,
so there can't be a call to phylink_change_inband_advert().

So, one can assume (as must be the case of course, we're fiddling
with hardware) that pcs_config() will always be run without another
thread also running it for the same hardware. Two over-lapping
pcs_config() calls would lead to an indeterminant hardware state.

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