linux-kernel - Re: Usage of for_each_child_of

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Date:	Thu, 24 Oct 2013 06:31:21 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Thierry Reding <thierry.reding@...il.com>
CC:	Rob Herring <robherring2@...il.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Usage of for_each_child_of_node()

On 10/24/2013 12:50 AM, Thierry Reding wrote:
> On Wed, Oct 23, 2013 at 09:16:44AM -0700, Guenter Roeck wrote:
>> On Wed, Oct 23, 2013 at 09:10:07AM +0200, Thierry Reding wrote:
>>> On Sat, Oct 12, 2013 at 10:15:03PM -0500, Rob Herring wrote:
>>>> On Sat, Oct 12, 2013 at 3:54 PM, Guenter Roeck <linux@...ck-us.net> wrote:
>>>>> Hi all,
>>>>>
>>>>> for_each_child_of_node() and similar functions increase the refcount
>>>>> on each returned node and expect the caller to release the node by
>>>>> calling of_node_put() when done.
>>>>>
>>>>> Looking through the kernel code, it appears this is hardly ever done,
>>>>> if at all. Some code even calls of_node_get() on returned nodes again.
>>>>>
>>>>> I guess this doesn't matter in cases where devicetree is a static entity.
>>>>> However, this is not (or no longer) the case with devicetree overlays,
>>>>> or more generically in cases where devicetree nodes are added and
>>>>> removed dynamically.
>>>>>
>>>>> Fundamental question: Would patches to fix this problem be accepted upstream
>>>>> ?
>>>>
>>>> Certainly.
>>>>
>>>>> Or, of course, stepping a bit back: Am I missing something essential ?
>>>>
>>>> No. I think this is frequently wrong since it typically doesn't matter
>>>> for static entries as you mention.
>>>
>>> Actually, I think it actually happens to be correct most of the time.
>>> The reason is that for_each_child_of_node() internally calls the
>>> of_get_next_child() to iterate over all children. And that function
>>> already calls of_node_put() on the "previous" node. So if all the code
>>> does is to iterate over all nodes to query them, then all should be
>>> fine.
>>>
>> Good, that reduces the scope of the problem significantly.
>>
>>> The only case where you actually need to drop the reference on a node is
>>> if you break out of the loop (so that of_get_next_child() will not be
>>> called). But that's usually the case when you need to perform some
>>> operation on the node, in which case it is the right thing to hold on to
>>> a reference until you're done with the node.
>>>
>> Unfortunately, there are many cases with code such as
>>
>> 	if (error)
>> 		return;	/* or break; */
>
> Well, a break isn't necessarily bad, since you could be using the node
> subsequently. I imagine that depending on the exact block following the

Correct, but I meant the error case. Randomly looking through several
drivers, most of them get error return handling wrong. "Winner" so far
is of_regulator_match(), which doesn't release the node on error return,
but does not acquire references for use afterwards either.

Something to do with my non-existing free time ;-).

Guenter

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