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

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Message-ID: <CAMuHMdUwQ=K7SnFvpaHzL2dCyNXGR3w-_SzFOEWb3cYCiQJfHg@mail.gmail.com>
Date:	Fri, 30 Jan 2015 10:00:23 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Michael Ellerman <mpe@...erman.id.au>
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

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.

> 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.

> 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.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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