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

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Date:   Wed, 6 Sep 2017 10:53:40 -0700
From:   David Daney <ddaney@...iumnetworks.com>
To:     Mason <slash.tmp@...e.fr>, Florian Fainelli <f.fainelli@...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>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>
Subject: Re: [PATCH net] Revert "net: phy: Correctly process PHY_HALTED in
 phy_stop_machine()"

On 09/06/2017 07:33 AM, Mason wrote:
> On 31/08/2017 20:29, Florian Fainelli wrote:
>> 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")
> 
> Hello David,
> 
> A week ago, you wrote about my patch:
> "This is broken.  Please revert."
> 
> I assume you tested the revert locally, and that reverting did make
> the crash disappear. Is that correct?
> 

Yes, I always test things before making this type of assertion.


> The reason I ask is because the analysis you provided contains some
> flaws, as noted above. But, if reverting my patch did fix your issue,
> then perhaps understanding *why* is unimportant.

I didn't want to take the time to generate calling sequence traces to 
verify each step of my analysis, but I believe the overall concept is 
essentially correct.

Once the polling work is canceled and we set a bunch of essential 
pointers to NULL, you cannot go blindly restarting the polling.

> 
> I'm a bit baffled that it took less than 90 minutes for your request
> to be approved, and the patch reverted in all branches, before I even
> had a chance to comment.
> 

o The last chance for patches to v4.13 was fast approaching.

o There were multiple reports of failures caused by the patch.

o The patch was clearly stand-alone.

The kernel maintainers are a model of efficiency, there was no reason to 
delay.