linux-kernel - Re: [PATCH 2/3] of: irq: make callers of of_irq_parse

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Message-ID: <CAMuHMdVdZatMYsc=367OuGotzYuo2-XVe5MAZdzh+kBs31=t5A@mail.gmail.com>
Date:   Sat, 4 Mar 2023 15:47:11 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Jean-Jacques Hiblot <jjhiblot@...phandler.com>
Cc:     saravanak@...gle.com, clement.leger@...tlin.com,
        Magnus Damm <magnus.damm@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        zajec5@...il.com, Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        Marc Zyngier <maz@...nel.org>, afaerber@...e.de,
        Manivannan Sadhasivam <mani@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Nishanth Menon <nm@...com>, ssantosh@...nel.org,
        mathias.nyman@...el.com, gregkh@...uxfoundation.org,
        thierry.reding@...il.com, jonathanh@...dia.com,
        linux-renesas-soc@...r.kernel.org,
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        linux-tegra@...r.kernel.org
Subject: Re: [PATCH 2/3] of: irq: make callers of of_irq_parse_one() release
 the device node

Hi Jean-Jacques,

On Sat, Mar 4, 2023 at 11:34 AM Jean-Jacques Hiblot
<jjhiblot@...phandler.com> wrote:
> On 02/03/2023 08:49, Geert Uytterhoeven wrote:
> > On Wed, Mar 1, 2023 at 7:53 PM Jean-Jacques Hiblot
> > <jjhiblot@...phandler.com> wrote:
> >> of_irq_parse_one() does a get() on the device node returned in out_irq->np.
> >> Callers of of_irq_parse_one() must do a put() when they are done with it.
> >
> > What does "be done with it" really mean here?
> >
> >> Signed-off-by: Jean-Jacques Hiblot <jjhiblot@...phandler.com>
> >
> >> --- a/arch/arm/mach-shmobile/regulator-quirk-rcar-gen2.c
> >> +++ b/arch/arm/mach-shmobile/regulator-quirk-rcar-gen2.c
> >> @@ -184,6 +184,7 @@ static int __init rcar_gen2_regulator_quirk(void)
> >>                          kfree(quirk);
> >>                          continue;
> >>                  }
> >> +               of_node_put(argsa->np);
> >
> > The quirk object, which is a container of argsa, is still used below,
> > and stored in a linked list.  I agree argsa->np is not dereferenced,
> > but the pointer itself is still compared to other pointers.
>
> I fail to see when the pointers are compared. It looks to me that only
> the args are compared. Am I missing something ?

You're right, in upstream, there is no such check.
In my local tree, I have converted the comparisons below to use a new
helper of_phandle_args_eq() (which does compare the np member, too),
but that change never went upstream, as the other user of that helper
was rejected.

> In any case, looking more closely at the code, I guess that indeed the
> of_node_put() shouldn't be added here because this code expects that the
> nodes never go away. That is probably a good assertion in case of PMICs

OK.

> > IIUIC, calling of_node_put() might cause the reference count to drop to
> > zero, and the underlying struct node object to be deallocated.
> > So when a future reference to the same DT node will be taken, a new
> > struct node object will be allocated, and the pointer comparison below
> > will fail?

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