linux-kernel - Re: [1/3] powerpc/pmac: Fix DT refcount imbalance in pmac_pic_probe

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Date:	Tue, 03 Feb 2015 12:12:32 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Geert Uytterhoeven <geert+renesas@...der.be>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Bryan Wu <cooloney@...il.com>,
	Richard Purdie <rpurdie@...ys.net>,
	Jingoo Han <jg1.han@...sung.com>,
	Lee Jones <lee.jones@...aro.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [1/3] powerpc/pmac: Fix DT refcount imbalance in
 pmac_pic_probe_oldstyle

On Fri, 2015-01-30 at 10:00 +0100, Geert Uytterhoeven wrote:
> Hi Michael,
> 
> On Fri, Jan 30, 2015 at 5:09 AM, Michael Ellerman <mpe@...erman.id.au> wrote:
> > On Wed, 2015-14-01 at 13:51:57 UTC, Geert Uytterhoeven wrote:
> >> of_find_node_by_name() calls of_node_put() on its "from" parameter,
> >> which must not be done on "master", as it's still in use, and will be
> >> released manually later.  This may cause a zero kref refcount.
> >> Use of_get_child_by_name() instead to fix this.
> >
> > But of_find_node_by_name() searches *all* nodes, not just the children of the
> > parameter.
> 
> That's correct. However, I guess the second mac-io will just be a direct child.

Yeah OK, I don't have a system or an example device tree to check.

> > So this is a logic change AFAICS, and I have no idea what machines we'd need to
> > test on to check it.
> 
> Originally it comes from arch/ppc/platforms/pmac_pic.c, added in 2002 in
> full-history-linux commit 5ea3254844ae344a
> ("Import arch/ppc and include/asm-ppc changes from linuxppc_2_5 tree").
> 
> I've also checked my linuxppc mail archives from 1997-2002, but couldn't find
> the actual patch and a description.
> 
> So I don't know on which machines it's needed.

Yep. Ben or Paul might know, but even then their memory may not be perfect :)
 
> > So I think an of_node_get(master) would be safer and also fix the refcounting.
> 
> If no one can confirm the above, that may indeed be the best solution.

I think so. Given how few of these machines are around it's easy to break them
with an inadvertent change like this, so I think it's better to be safe.

Wanna send a patch for that?

cheers



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