From: Mason <slash.tmp@free.fr>
To: Florian Fainelli <f.fainelli@gmail.com>,
David Daney <ddaney.cavm@gmail.com>
Cc: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>,
netdev <netdev@vger.kernel.org>,
Geert Uytterhoeven <geert+renesas@glider.be>,
David Miller <davem@davemloft.net>, Andrew Lunn <andrew@lunn.ch>,
Mans Rullgard <mans@mansr.com>,
Thibaud Cornic <thibaud_cornic@sigmadesigns.com>
Subject: Re: [PATCH net] Revert "net: phy: Correctly process PHY_HALTED in phy_stop_machine()"
Date: Wed, 6 Sep 2017 16:33:54 +0200 [thread overview]
Message-ID: <6721135d-8c3f-57a0-f423-9d18cd6e0947@free.fr> (raw)
In-Reply-To: <4ea8b432-4968-1616-eff9-48a2689dd3ce@gmail.com>
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?
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'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.
Regards.
next prev parent reply other threads:[~2017-09-06 14:34 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-31 0:49 [PATCH net] Revert "net: phy: Correctly process PHY_HALTED in phy_stop_machine()" Florian Fainelli
2017-08-31 1:47 ` David Miller
2017-08-31 12:29 ` Marc Gonzalez
2017-08-31 14:21 ` Marc Gonzalez
2017-08-31 16:36 ` David Daney
2017-08-31 16:57 ` Florian Fainelli
2017-08-31 17:49 ` Mason
2017-08-31 17:53 ` Florian Fainelli
2017-08-31 18:12 ` Mason
2017-08-31 18:29 ` Florian Fainelli
2017-09-06 14:33 ` Mason [this message]
2017-09-06 17:53 ` David Daney
2017-09-06 18:00 ` David Daney
2017-09-06 18:59 ` Florian Fainelli
2017-09-06 20:49 ` David Daney
2017-09-06 22:51 ` David Daney
2017-09-06 23:14 ` Florian Fainelli
2017-09-07 0:10 ` David Daney
2017-09-07 1:41 ` Florian Fainelli
2017-09-06 19:14 ` Mason
2017-08-31 17:35 ` Mason
2017-08-31 17:03 ` Florian Fainelli
2017-08-31 19:09 ` Mason
2017-08-31 19:18 ` Florian Fainelli
2017-09-06 14:55 ` Mason
2017-09-06 19:28 ` Florian Fainelli
2017-09-06 15:51 ` Mason
2017-09-06 19:42 ` Florian Fainelli
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