netdev - Re: [PATCH net] dsa: mv88e6xxx: Ensure all pending interrupts are handled prior to exit

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Date:   Tue, 12 Feb 2019 22:55:05 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        John David Anglin <dave.anglin@...l.net>,
        Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
        Florian Fainelli <f.fainelli@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net] dsa: mv88e6xxx: Ensure all pending interrupts are
 handled prior to exit

On Tue, Feb 12, 2019 at 09:54:55PM +0100, Heiner Kallweit wrote:
> On 12.02.2019 17:30, Russell King - ARM Linux admin wrote:
> > On Tue, Feb 12, 2019 at 07:51:05AM +0100, Heiner Kallweit wrote:
> >> On 12.02.2019 04:58, Andrew Lunn wrote:
> >>> That change means we don't check the PHY device if it caused an
> >>> interrupt when its state is less than UP.
> >>>
> >>> What i'm seeing is that the PHY is interrupting pretty early on after
> >>> a reboot when the previous boot had the interface up.
> >>>
> >> So this means that when going down for reboot the interrupts are not
> >> properly masked / disabled? Because (at least for net-next) we enable
> >> interrupts in phy_start() only.
> > 
> [..]
> > In looking at this, I came across this chunk of code:
> > 
> > static inline bool __phy_is_started(struct phy_device *phydev)
> > {
> >         WARN_ON(!mutex_is_locked(&phydev->lock));
> > 
> >         return phydev->state >= PHY_UP;
> > }
> > 
> > /**
> >  * phy_is_started - Convenience function to check whether PHY is started
> >  * @phydev: The phy_device struct
> >  */
> > static inline bool phy_is_started(struct phy_device *phydev)
> > {
> >         bool started;
> > 
> >         mutex_lock(&phydev->lock);
> >         started = __phy_is_started(phydev);
> >         mutex_unlock(&phydev->lock);
> > 
> >         return started;
> > }
> > 
> > which looks to me like over-complication.  The mutex locking there is
> > completely pointless - what are you trying to achieve with it?
> > 
> > Let's go through this.  The above is exactly equivalent to:
> > 
> > bool phy_is_started(phydev)
> > {
> > 	int state;
> > 
> > 	mutex_lock(&phydev->lock);
> > 	state = phydev->state;
> > 	mutex_unlock(&phydev->lock);
> > 
> > 	return state >= PHY_UP;
> > }
> > 
> > since when we do the test is irrelevant.  Architectures that Linux
> > runs on are single-copy atomic, which means that reading phydev->state
> > itself is an atomic operation.  So, the mutex locking around that
> > doesn't add to the atomicity of the entire operation.
> > 
> > How, depending on what you do with the rest of this function depends
> > whether the entire operation is safe or not.  For example, let's take
> > this code at the end of phy_state_machine():
> > 
> >         if (phy_polling_mode(phydev) && phy_is_started(phydev))
> >                 phy_queue_state_machine(phydev, PHY_STATE_TIME);
> > 
> > state = PHY_UP
> > 		thread 0			thread 1
> > 						phy_disconnect()
> > 						+-phy_is_started()
> > 		phy_is_started()                |
> > 						`-phy_stop()
> > 						  +-phydev->state = PHY_HALTED
> > 						  `-phy_stop_machine()
> > 						    `-cancel_delayed_work_sync()
> > 		phy_queue_state_machine()
> > 		`-mod_delayed_work()
> > 
> > At this point, the phydev->state_queue() has been added back onto the
> > system workqueue despite phy_stop_machine() having been called and
> > cancel_delayed_work_sync() called on it.
> > 
> > The original code in 4.20 did not have this race condition.
> > 
> > Basically, the lock inside phy_is_started() does nothing useful, and
> > I'd say is dangerously misleading.
> > 
> Then idea would be to first remove the locking from phy_is_started()
> and in a second step do the following to prevent the described race
> (phy_stop() takes phydev->lock too).
> 
> diff --git a/drivers/net/phy/phy.c b/drivers/net/phy/phy.c
> index c1ed03800..69dc64a4d 100644
> --- a/drivers/net/phy/phy.c
> +++ b/drivers/net/phy/phy.c
> @@ -957,8 +957,10 @@ void phy_state_machine(struct work_struct *work)
>          * state machine would be pointless and possibly error prone when
>          * called from phy_disconnect() synchronously.
>          */
> +       mutex_lock(&phydev->lock);
>         if (phy_polling_mode(phydev) && phy_is_started(phydev))
>                 phy_queue_state_machine(phydev, PHY_STATE_TIME);
> +       mutex_unlock(&phydev->lock);
>  }

Yep, that approach would certainly be better.  I didn't exhaustively
audit the 5.0-rc code though.

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