linux-kernel - Re: [PATCH 1/5] input: twl6040-vibra: fix DT node memory management

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Message-Id: <25D4CEE0-DEDE-4E12-8D7A-B0E4EBD1772B@goldelico.com>
Date:	Tue, 19 Apr 2016 09:43:08 +0200
From:	"H. Nikolaus Schaller" <hns@...delico.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	Fabio Estevam <fabio.estevam@...escale.com>,
	Peter Ujfalusi <peter.ujfalusi@...com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel@...a-handheld.com, letux-kernel@...nphoenux.org
Subject: Re: [PATCH 1/5] input: twl6040-vibra: fix DT node memory management


> Am 18.04.2016 um 23:22 schrieb Dmitry Torokhov <dmitry.torokhov@...il.com>:
> 
> On Mon, Apr 18, 2016 at 09:55:37PM +0200, H. Nikolaus Schaller wrote:
>> commit e7ec014a47e4 ("Input: twl6040-vibra - update for device tree support")
>> 
>> made the separate vibra DT node to a subnode of the twl6040.
>> 
>> It now calls of_find_node_by_name() to locate the "vibra" subnode.
>> This function has a side effect to call of_node_put on() for the twl6040
>> parent node passed in as a parameter. This causes trouble later on.
>> 
>> Solution: we must call of_node_get() before of_find_node_by_name()
> 
> God, what messed up API.

Yes, indeed. It is opposite to the usual object ownership rule that the code
fragment that asks for a handle has to release it.

Usually it does not become obvious because often CONFIG_OF_DYNAMIC=n.
This disables all of_node refcounting completely so such bugs remain unnoticed.

> Any chance we can make it a bit more sane and
> not drop the reference inside it instead?

Well, if you want to change ~2000 files, test on all platforms and ask Linus
for agreement?

> 
>> 
>> Signed-off-by: H. Nikolaus Schaller <hns@...delico.com>
>> ---
>> drivers/input/misc/twl6040-vibra.c | 1 +
>> 1 file changed, 1 insertion(+)
>> 
>> diff --git a/drivers/input/misc/twl6040-vibra.c b/drivers/input/misc/twl6040-vibra.c
>> index ea63fad..7221a00 100644
>> --- a/drivers/input/misc/twl6040-vibra.c
>> +++ b/drivers/input/misc/twl6040-vibra.c
>> @@ -262,6 +262,7 @@ static int twl6040_vibra_probe(struct platform_device *pdev)
>> 	int vddvibr_uV = 0;
>> 	int error;
>> 
>> +	of_node_get(twl6040_core_dev->of_node);
>> 	twl6040_core_node = of_find_node_by_name(twl6040_core_dev->of_node,
>> 						 "vibra");
>> 	if (!twl6040_core_node) {
>> -- 
>> 2.7.3
>> 
> 
> -- 
> Dmitry