Re: [PATCH net] Revert "net: phy: Correctly process PHY_HALTED in phy_stop_machine()"

[David Daney <ddaney.cavm@gmail.com>]

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() {
   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?

David.