linux-kernel - Re: Usage of for_each_child_of

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Date:	Thu, 24 Oct 2013 09:50:58 +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 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
if statement the node could also be assigned to some field within a
structure or similar, in which case this might still be valid. So it
really needs to be evaluated on a case by case basis.

If the above is actually verbatim, then yes, that's certainly an error.

> or even
> 	if (found node)
> 		return of_node_get(node);
> 
> in the loop.

Yeah, I think all of those are probably wrong too.

Thierry

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