netdev - Re: [RFC net] Revert "net: phy: Fix race condition on link status change"

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Message-ID: <b5ae4bc5-20cb-470b-988c-86353592f1c9@lunn.ch>
Date: Wed, 16 Aug 2023 21:35:57 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Serge Semin <fancer.lancer@...il.com>
Cc: Heiner Kallweit <hkallweit1@...il.com>,
	Russell King <linux@...linux.org.uk>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Francesco Dolcini <francesco.dolcini@...adex.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC net] Revert "net: phy: Fix race condition on link status
 change"

On Wed, Aug 16, 2023 at 09:09:40PM +0300, Serge Semin wrote:
> Protecting the phy_driver.drv->handle_interrupt() callback invocation by
> the phy_device.lock mutex causes all the IRQ-capable PHY drivers to lock
> the mutex twice thus deadlocking on the next calls thread:
> IRQ: phy_interrupt()
>      +-> mutex_lock(&phydev->lock); <-------------+
>          drv->handle_interrupt()                  | Deadlock due to the
>          +-> phy_error()                          + nested PHY-device
>              +-> phy_process_error()              | mutex lock
>                  +-> mutex_lock(&phydev->lock); <-+
>                      phydev->state = PHY_ERROR;
>                      mutex_unlock(&phydev->lock);
>          mutex_unlock(&phydev->lock);
> 
> The problem can be easily reproduced just by calling phy_error() from the
> any PHY-device interrupt handler.

https://elixir.bootlin.com/linux/v6.5-rc6/source/drivers/net/phy/phy.c#L1201

/**
 * phy_error - enter ERROR state for this PHY device
 * @phydev: target phy_device struct
 *
 * Moves the PHY to the ERROR state in response to a read
 * or write error, and tells the controller the link is down.
 * Must not be called from interrupt context, or while the
 * phydev->lock is held.
 */
void phy_error(struct phy_device *phydev)
{
	WARN_ON(1);
	phy_process_error(phydev);
}
EXPORT_SYMBOL(phy_error);

It is clearly documented you should not do this.

[Goes and looks]

Ah, there are lots of examples of

micrel.c-	irq_status = phy_read(phydev, LAN8814_INTS);
micrel.c-	if (irq_status < 0) {
micrel.c:		phy_error(phydev);
micrel.c-		return IRQ_NONE;
micrel.c-	}

I actually think phy_error() is broken here. The general pattern is
that the mutex is locked before calling into the driver. So we
actually want phy_error() to be safe to use with the lock already
taken. The exceptions when the lock is not held is stuff outside of
PHY operation, like HWMON, and suspend and resume, plus probe.

So i suggest you change phy_process_error() to remove the lock. Maybe
add a test to ensure the lock is actually held, and do a phydev_err()
if not.

The comment about interrupt context is also probably bogus. phylib
only uses threaded interrupts, and it is safe to block in this
context.

	Andrew