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Date: Wed, 6 Sep 2017 16:14:22 -0700
From: Florian Fainelli <f.fainelli@...il.com>
To: David Daney <ddaney.cavm@...il.com>,
David Daney <ddaney@...iumnetworks.com>,
Mason <slash.tmp@...e.fr>
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 09/06/2017 03:51 PM, David Daney wrote:
> On 09/06/2017 01:49 PM, David Daney wrote:
>> On 09/06/2017 11:59 AM, Florian Fainelli wrote:
>>> On 09/06/2017 11:00 AM, David Daney wrote:
>>>> On 08/31/2017 11:29 AM, 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.
>>>>
>>>> Did you see what I wrote?
>>>
>>> Still not following, see below.
>>>
>>>>
>>>> phy_disconnect() calls phy_stop_interrupts() which puts it into polling
>>>> mode. So the polling work gets queued unconditionally.
>>>
>>> What part of phy_stop_interrupts() is responsible for changing
>>> phydev->irq to PHY_POLL? free_irq() cannot touch phydev->irq otherwise
>>> subsequent request_irq() calls won't work anymore.
>>> phy_disable_interrupts() only calls back into the PHY driver to
>>> acknowledge and clear interrupts.
>>>
>>> If we were using a PHY with PHY_POLL, as Marc said, the first
>>> synchronous call to phy_state_machine() would have acted on PHY_HALTED
>>> and even if we incorrectly keep re-scheduling the state machine from
>>> PHY_HALTED to PHY_HALTED the second time around nothing can happen.
>>>
>>> What are we missing here?
>>>
>>
>> OK, I am now as confused as you guys are. I will go back and get an
>> ftrace log out of the failure.
>>
> OK, let's forget about the PHY_HALTED discussion.
>
>
> Consider instead the case of a Marvell phy with no interrupts connected
> on a v4.9.43 kernel, single CPU:
>
>
> 0) | phy_disconnect() {
> 0) | phy_stop_machine() {
> 0) | cancel_delayed_work_sync() {
> 0) + 23.986 us | } /* cancel_delayed_work_sync */
> 0) | phy_state_machine() {
> 0) | phy_start_aneg_priv() {
Thanks for providing the trace, I think I have an idea of what's going
on, see below.
> 0) | marvell_config_aneg() {
> 0) ! 240.538 us | } /* marvell_config_aneg */
> 0) ! 244.971 us | } /* phy_start_aneg_priv */
> 0) | queue_delayed_work_on() {
> 0) + 18.016 us | } /* queue_delayed_work_on */
> 0) ! 268.184 us | } /* phy_state_machine */
> 0) ! 297.394 us | } /* phy_stop_machine */
> 0) | phy_detach() {
> 0) | phy_suspend() {
> 0) | phy_ethtool_get_wol() {
> 0) 0.677 us | } /* phy_ethtool_get_wol */
> 0) | genphy_suspend() {
> 0) + 71.250 us | } /* genphy_suspend */
> 0) + 74.197 us | } /* phy_suspend */
> 0) + 80.302 us | } /* phy_detach */
> 0) ! 380.072 us | } /* phy_disconnect */
> .
> .
> .
> 0) | process_one_work() {
> 0) | find_worker_executing_work() {
> 0) 0.688 us | } /* find_worker_executing_work */
> 0) | set_work_pool_and_clear_pending() {
> 0) 0.734 us | } /* set_work_pool_and_clear_pending */
> 0) | phy_state_machine() {
> 0) | genphy_read_status() {
> 0) ! 205.721 us | } /* genphy_read_status */
> 0) | netif_carrier_off() {
> 0) | do_page_fault() {
>
>
> The do_page_fault() at the end indicates the NULL pointer dereference.
>
> That added call to phy_state_machine() turns the polling back on
> unconditionally for a phy that should be disconnected. How is that
> correct?
It is not fundamentally correct and I don't think there was any
objection to that to begin with. In fact there is a bug/inefficiency
here in that if we have entered the PHY state machine with PHY_HALTED we
should not re-schedule it period, only applicable to PHY_POLL cases
*and* properly calling phy_stop() followed by phy_disconnect().
What I now think is happening in your case is the following:
phy_stop() was not called, so nothing does set phydev->state to
PHY_HALTED in the first place so we have:
phy_disconnect()
-> phy_stop_machine()
-> cancel_delayed_work_sync() OK
phydev->state is probably RUNNING so we have:
-> phydev->state = PHY_UP
phy_state_machine() is called synchronously
-> PHY_UP -> needs_aneg = true
-> phy_restart_aneg()
-> queue_delayed_work_sync()
-> phydev->adjust_link = NULL
-> phy_deatch() -> boom
Can you confirm whether the driver you are using does call phy_stop()
prior to phy_disconnect()? If that is the case then this whole theory
falls apart, if not, then this needs fixing in both the driver and PHYLIB.
Thanks
--
Florian
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