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

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Date:   Thu, 31 Aug 2017 11:29:12 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Mason <slash.tmp@...e.fr>, 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 08/31/2017 11:12 AM, Mason wrote:
> 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.

Actually you are right, once phydev->link is set to 0 these would become
no-ops. Still scratching my head as to what happens for David then...

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

Right that's confusing too now. David can you check if you tree has:

49d52e8108a21749dc2114b924c907db43358984 ("net: phy: handle state
correctly in phy_stop_machine")
-- 
Florian