linux-kernel - Re: Usage of for_each_child_of

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Message-ID: <20131024142143.GC25061@ulmo.nvidia.com>
Date:	Thu, 24 Oct 2013 16:21:43 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Guenter Roeck <linux@...ck-us.net>
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 Thu, Oct 24, 2013 at 06:31:21AM -0700, Guenter Roeck wrote:
> 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 ;-).

Well, that's better than boring, isn't it? =)

Thierry

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