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

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Date:	Wed, 27 Jan 2016 16:21:53 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
Cc:	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, 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.

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.