netdev - Re: [PATCH net] Revert "net: phy: Correctly process PHY_HALTED in phy_stop

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Date:   Thu, 31 Aug 2017 14:29:36 +0200
From:   Marc Gonzalez <marc_gonzalez@...madesigns.com>
To:     Florian Fainelli <f.fainelli@...il.com>,
        David Daney <ddaney.cavm@...il.com>
CC:     netdev <netdev@...r.kernel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        David Miller <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>, Mans Rullgard <mans@...sr.com>,
        Mason <slash.tmp@...e.fr>
Subject: Re: [PATCH net] Revert "net: phy: Correctly process PHY_HALTED in
 phy_stop_machine()"

On 31/08/2017 02:49, Florian Fainelli wrote:

> This reverts commit 7ad813f208533cebfcc32d3d7474dc1677d1b09a ("net: phy:
> Correctly process PHY_HALTED in phy_stop_machine()") because it is
> creating the possibility for a NULL pointer dereference.
> 
> David Daney provide the following call trace and diagram of events:
> 
> When ndo_stop() is called we call:
> 
>  phy_disconnect()
>     +---> phy_stop_interrupts() implies: phydev->irq = PHY_POLL;

What does this mean?

On the contrary, phy_stop_interrupts() is only called when *not* polling.

	if (phydev->irq > 0)
		phy_stop_interrupts(phydev);

>     +---> phy_stop_machine()
>     |      +---> phy_state_machine()
>     |              +----> queue_delayed_work(): Work queued.

You're referring to the fact that, at the end of phy_state_machine()
(in polling mode) the code reschedules itself through:

	if (phydev->irq == PHY_POLL)
		queue_delayed_work(system_power_efficient_wq, &phydev->state_queue, PHY_STATE_TIME * HZ);

>     +--->phy_detach() implies: phydev->attached_dev = NULL;
> 
> Now at a later time the queued work does:
> 
>  phy_state_machine()
>     +---->netif_carrier_off(phydev->attached_dev): Oh no! It is NULL:

I tested a sequence of 500 link up / link down in polling mode,
and saw no such issue. Race condition?

For what case in phy_state_machine() is netif_carrier_off()
being called? Surely not PHY_HALTED?


> The original motivation for this change originated from Marc Gonzales
> indicating that his network driver did not have its adjust_link callback
> executing with phydev->link = 0 while he was expecting it.

I expect the core to call phy_adjust_link() for link changes.
This used to work back in 3.4 and was broken somewhere along
the way.

> PHYLIB has never made any such guarantees ever because phy_stop() merely
> just tells the workqueue to move into PHY_HALTED state which will happen
> asynchronously.

My original proposal was to fix the issue in the driver.
I'll try locating it in my archives.

Regards.