linux-kernel - Re: [PATCH] clocksource: Do not drop unheld reference on device node

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Date:	Sun, 24 Nov 2013 22:28:15 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
CC:	Thierry Reding <thierry.reding@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnd Bergmann <arnd@...db.de>,
	Rob Herring <rob.herring@...xeda.com>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Brown <davidb@...eaurora.org>
Subject: Re: [PATCH] clocksource: Do not drop unheld reference on device node

On 11/22/2013 08:22 PM, Uwe Kleine-König wrote:
> On Fri, Nov 22, 2013 at 05:31:46PM +0100, Daniel Lezcano wrote:
>> On 11/22/2013 05:16 PM, Thierry Reding wrote:
>>> On Sat, Oct 19, 2013 at 12:49:48AM +0200, Thierry Reding wrote:
>>>> When booting a recent kernel on ARM with OF_DYNAMIC enabled, the kernel
>>>> warns about the following:
>>>>
>>>> 	[    0.000000] ERROR: Bad of_node_put() on /timer@...04600
>>>> 	[    0.000000] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.12.0-rc5-next-20131017-00077-gedfd827-dirty #406
>>>> 	[    0.000000] [<c0015b68>] (unwind_backtrace+0x0/0xf4) from [<c00117e4>] (show_stack+0x10/0x14)
>>>> 	[    0.000000] [<c00117e4>] (show_stack+0x10/0x14) from [<c055f734>] (dump_stack+0x9c/0xc8)
>>>> 	[    0.000000] [<c055f734>] (dump_stack+0x9c/0xc8) from [<c03b47d4>] (of_node_release+0x90/0x9c)
>>>> 	[    0.000000] [<c03b47d4>] (of_node_release+0x90/0x9c) from [<c03b5084>] (of_find_matching_node_and_match+0x78/0xb4)
>>>> 	[    0.000000] [<c03b5084>] (of_find_matching_node_and_match+0x78/0xb4) from [<c07887c8>] (clocksource_of_init+0x60/0x70)
>>>> 	[    0.000000] [<c07887c8>] (clocksource_of_init+0x60/0x70) from [<c076e99c>] (start_kernel+0x1f4/0x33c)
>>>> 	[    0.000000] [<c076e99c>] (start_kernel+0x1f4/0x33c) from [<80008074>] (0x80008074)
>>>>
>>>> This is caused by clocksource_of_init() dropping a reference on the
>>>> device node that it never took. The reference taken by the loop is
>>>> implicitly dropped on subsequent iterations. See the implementation of
>>>> and the comment on top of the of_find_matching_node_and_match()
>>>> function for reference (no pun intended).
> I only saw the end of the comment saying:
>
> 	Returns a node pointer with refcount incremented, use
> 	of_node_put() on it when done.
>
> Maybe that could be improved to something like:
>
> /**
>   * of_find_matching_node_and_match - Find a node based on an of_device_id
>   *                                   match table
>   * @from: The node to start searching from or NULL, the node you pass
>   *        will not be searched, only the next one will; typically, you
>   *        pass what the previous call returned.
>   * @matches: ...
>   * @match: ...
>   *
>   * Reference counting: Puts a reference on @from and increases the
>   * reference count of the returned node.
>   */
>
>>>>
>>>> Signed-off-by: Thierry Reding <treding@...dia.com>
>>>> ---
>>>>   drivers/clocksource/clksrc-of.c | 1 -
>>>>   1 file changed, 1 deletion(-)
>>>
>>> Can someone look at this?
>>
>> Yes. Sounds like I missed it.
>>
>> This regression has been introduced by:
>>
>> commit 326e31eebe61dc838e031ea16968b2cfb43443e3
>> Author: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
>> Date:   Tue Oct 1 11:00:53 2013 +0200
>>
>>      clocksource: Put nodes passed to CLOCKSOURCE_OF_DECLARE
>> callbacks centrally
>>
>>      Instead of letting each driver call of_node_put do it centrally in the
>>      loop that also calls the CLOCKSOURCE_OF_DECLARE callbacks. This is less
>>      prone to error and also moves getting and putting the references
>> into the
>>      same function.
>>
>>      Consequently all respective of_node_put calls in drivers are removed.
>>
>>      Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
>>      Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
>>      Acked-by: David Brown <davidb@...eaurora.org>
> Still all but the hook in clocksource_of_init of this commit was
> correct, right? (Well, but this buggy hunk makes the commit log wrong.)

I don't understand your comment, can you elaborate ?

Thanks
   -- Daniel

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