Re: Usage of for_each_child_of_node()

[Guenter Roeck]

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@xxxxxxxxxxxx> 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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