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

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Date:   Thu, 31 Aug 2017 20:12:42 +0200
From:   Mason <slash.tmp@...e.fr>
To:     Florian Fainelli <f.fainelli@...il.com>,
        David Daney <ddaney.cavm@...il.com>
Cc:     Marc Gonzalez <marc_gonzalez@...madesigns.com>,
        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>
Subject: Re: [PATCH net] Revert "net: phy: Correctly process PHY_HALTED in
 phy_stop_machine()"

On 31/08/2017 19:53, Florian Fainelli wrote:
> On 08/31/2017 10:49 AM, Mason wrote:
>> On 31/08/2017 18:57, Florian Fainelli wrote:
>>> And the race is between phy_detach() setting phydev->attached_dev = NULL
>>> and phy_state_machine() running in PHY_HALTED state and calling
>>> netif_carrier_off().
>>
>> I must be missing something.
>> (Since a thread cannot race against itself.)
>>
>> phy_disconnect calls phy_stop_machine which
>> 1) stops the work queue from running in a separate thread
>> 2) calls phy_state_machine *synchronously*
>>      which runs the PHY_HALTED case with everything well-defined
>> end of phy_stop_machine
>>
>> phy_disconnect only then calls phy_detach()
>> which makes future calls of phy_state_machine perilous.
>>
>> This all happens in the same thread, so I'm not yet
>> seeing where the race happens?
> 
> The race is as described in David's earlier email, so let's recap:
> 
> Thread 1			Thread 2
> phy_disconnect()
> phy_stop_interrupts()
> phy_stop_machine()
> phy_state_machine()
>  -> queue_delayed_work()
> phy_detach()
> 				phy_state_machine()
> 				-> netif_carrier_off()
> 
> If phy_detach() finishes earlier than the workqueue had a chance to be
> scheduled and process PHY_HALTED again, then we trigger the NULL pointer
> de-reference.
> 
> workqueues are not tasklets, the CPU scheduling them gets no guarantee
> they will run on the same CPU.

Something does not add up.

The synchronous call to phy_state_machine() does:

	case PHY_HALTED:
		if (phydev->link) {
			phydev->link = 0;
			netif_carrier_off(phydev->attached_dev);
			phy_adjust_link(phydev);
			do_suspend = true;
		}

then sets phydev->link = 0; therefore subsequent calls to
phy_state_machin() will be no-op.

Also, queue_delayed_work() is only called in polling mode.
David stated that he's using interrupt mode.

Regards.