linux-kernel - Re: [PATCH] of: resolver: Add missing of_node

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Date:	Wed, 27 Jan 2016 20:48:00 +0100 (CET)
From:	Julia Lawall <julia.lawall@...6.fr>
To:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
cc:	Mark Rutland <mark.rutland@....com>,
	Amitoj Kaur Chawla <amitoj1606@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	Frank Rowand <frowand.list@...il.com>,
	Grant Likely <grant.likely@...aro.org>,
	Devicetree List <devicetree@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	julia.lawall@...6.fr
Subject: Re: [PATCH] of: resolver: Add missing of_node_put



On Wed, 27 Jan 2016, Pantelis Antoniou wrote:

> Hi Mark,
> 
> > On Jan 27, 2016, at 18:21 , Mark Rutland <mark.rutland@....com> wrote:
> > 
> > On Wed, Jan 27, 2016 at 06:14:00PM +0200, Pantelis Antoniou wrote:
> >> Hi Mark,
> >> 
> >>> On Jan 27, 2016, at 18:05 , Mark Rutland <mark.rutland@....com> wrote:
> >>> 
> >>> On Wed, Jan 27, 2016 at 08:50:17PM +0530, Amitoj Kaur Chawla wrote:
> >>>> for_each_child_of_node performs an of_node_get on each iteration, so
> >>>> to break out of the loop an of_node_put is required.
> >>>> 
> >>>> Found using Coccinelle. The semantic patch used for this is as follows:
> >>>> 
> >>>> // <smpl>
> >>>> @@
> >>>> expression e;
> >>>> local idexpression n;
> >>>> @@
> >>>> 
> >>>> for_each_child_of_node(..., n) {
> >>>>  ... when != of_node_put(n)
> >>>>      when != e = n
> >>>> (
> >>>>  return n;
> >>>> |
> >>>> +  of_node_put(n);
> >>>> ?  return ...;
> >>>> )
> >>>>  ...
> >>>> }
> >>>> // </smpl
> >>>> 
> >>>> Signed-off-by: Amitoj Kaur Chawla <amitoj1606@...il.com>
> >>>> ---
> >>>> drivers/of/resolver.c | 4 +++-
> >>>> 1 file changed, 3 insertions(+), 1 deletion(-)
> >>>> 
> >>>> diff --git a/drivers/of/resolver.c b/drivers/of/resolver.c
> >>>> index 640eb4c..e2a0143 100644
> >>>> --- a/drivers/of/resolver.c
> >>>> +++ b/drivers/of/resolver.c
> >>>> @@ -40,8 +40,10 @@ static struct device_node *__of_find_node_by_full_name(struct device_node *node,
> >>>> 
> >>>> 	for_each_child_of_node(node, child) {
> >>>> 		found = __of_find_node_by_full_name(child, full_name);
> >>>> -		if (found != NULL)
> >>>> +		if (found != NULL) {
> >>>> +			of_node_put(child);
> >>>> 			return found;
> >>>> +		}
> >>>> 	}
> >>>> 
> >>>> 	return NULL;
> >>> 
> >>> I don't think this is quite right. When child == found, this change will
> >>> leave it decremented.
> >>> 
> >> 
> >> 
> >> This patch is bogus. 
> >> 
> >> __of_find_node_by_full_name() is not taking a reference on the node if found. 
> >> This method relies on keeping the reference taken by the loop.
> > 
> > Sure. For the found node, that makes sense.
> > 
> > However, it also increments the refcount of all the parents, which does
> > not seem correct to me, given they're not put on the return path, and a
> > put of the found node won't decrement its parents refcounts, unless I
> > have missed something.
> > 
> 
> Hmm, yes. The parent refcounts must be decremented. 

So there should be if (found != child) of_node_put(child); ?  Another 
option would be to duplicate the test and avoid the recursive call.

julia

> > So I believe we are missing some of_node_put logic here.
> > 
> >> Taking this into account all of these conccinelle tests are bogus.
> >> 
> >> The DT internal method are not using the object model in an obvious manner
> >> and applying these patches without vetting each and everyone is bound to
> >> break things.
> > 
> > Agreed.
> > 
> > Thanks,
> > Mark.
> > --
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> 
>