linux-kernel - Re: [PATCH v2 net 1/2] net: phylink: add lock for serializing concurrent pl->phydev writes with resolver

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Message-ID: <aLhkKVsbrkXmFbgK@shell.armlinux.org.uk>
Date: Wed, 3 Sep 2025 16:52:09 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Vladimir Oltean <vladimir.oltean@....com>
Cc: netdev@...r.kernel.org, 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>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net 1/2] net: phylink: add lock for serializing
 concurrent pl->phydev writes with resolver

On Wed, Sep 03, 2025 at 06:31:20PM +0300, Vladimir Oltean wrote:
> On Wed, Sep 03, 2025 at 04:26:35PM +0100, Russell King (Oracle) wrote:
> > On Wed, Sep 03, 2025 at 06:23:47PM +0300, Vladimir Oltean wrote:
> > > @@ -2305,6 +2314,7 @@ void phylink_disconnect_phy(struct phylink *pl)
> > >  
> > >  	phy = pl->phydev;
> > >  	if (phy) {
> > > +		mutex_lock(&pl->phy_lock);
> > 
> > If we can, I think it would be better to place this a couple of lines
> > above and move the unlock.
> 
> Sorry for potentially misunderstanding, do you mean like this?
> 
> 	mutex_lock(&pl->phy_lock);
> 	phy = pl->phydev;
> 	if (phy) {
> 		mutex_lock(&phy->lock);
> 		mutex_lock(&pl->state_mutex);
> 		pl->phydev = NULL;
> 		pl->phy_enable_tx_lpi = false;
> 		pl->mac_tx_clk_stop = false;
> 		mutex_unlock(&pl->state_mutex);
> 		mutex_unlock(&phy->lock);
> 		mutex_unlock(&pl->phy_lock);
> 		flush_work(&pl->resolve);
> 
> 		phy_disconnect(phy);
> 	} else {
> 		mutex_unlock(&pl->phy_lock);
> 	}
> 
> move the unlock where? because flush_work(&pl->resolve) needs to happen
> unlocked, otherwise we'll deadlock with phylink_resolve().
> 
> Additionally, dereferincing pl->phydev under rtnl_lock() is already safe,
> and doesn't need the secondary clock.

The reason I'm making the suggestion is for consistency. If the lock
is there to ensure that reading pl->phydev is done safely, having one
site where we read it and then take the lock makes it look confusing.
I've also been thinking that it should be called pl->phydev_mutex
(note that phylink uses _mutex for mutexes.)

To avoid it looking weird, what about this:

	mutex_lock(&pl->phy_lock);
	phy = pl->phydev;
	if (phy) {
		mutex_lock(&phy->lock);
		mutex_lock(&pl->state_mutex);
		pl->phydev = NULL;
		pl->phy_enable_tx_lpi = false;
		pl->mac_tx_clk_stop = false;
		mutex_unlock(&pl->state_mutex);
		mutex_unlock(&phy->lock);
	}
	mutex_unlock(&pl->phy_lock);

	if (phy) 
 		flush_work(&pl->resolve);
 
 		phy_disconnect(phy);
 	}


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