linux-kernel - Re: [PATCH] irqchip/renesas-rzg2l: Fix missing put

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Message-ID: <861q0t6xdl.wl-maz@kernel.org>
Date: Sun, 06 Oct 2024 16:53:42 +0100
From: Marc Zyngier <maz@...nel.org>
To: Fabrizio Castro <fabrizio.castro.jz@...esas.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Geert Uytterhoeven
	<geert+renesas@...der.be>,
	Prabhakar Mahadev Lad
	<prabhakar.mahadev-lad.rj@...renesas.com>,
	"linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>,
	Chris Paterson <Chris.Paterson2@...esas.com>,
	Biju Das <biju.das.jz@...renesas.com>,
	"linux-renesas-soc@...r.kernel.org"
	<linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH] irqchip/renesas-rzg2l: Fix missing put_device

On Tue, 01 Oct 2024 12:54:30 +0100,
Fabrizio Castro <fabrizio.castro.jz@...esas.com> wrote:
> 
> Hi Marc,
> 
> thank you for your reply.
> 
> > From: Marc Zyngier <maz@...nel.org>
> > Sent: Monday, September 30, 2024 8:15 PM
> > To: Fabrizio Castro <fabrizio.castro.jz@...esas.com>
> > Subject: Re: [PATCH] irqchip/renesas-rzg2l: Fix missing put_device
> > 
> > On Mon, 30 Sep 2024 17:36:20 +0100,
> > Fabrizio Castro <fabrizio.castro.jz@...esas.com> wrote:
> > >
> > > Hi Marc,
> > >
> > > Thanks for your feedback.
> > >
> > > > From: Marc Zyngier <maz@...nel.org>
> > > > Sent: Monday, September 30, 2024 4:50 PM
> > > > To: Fabrizio Castro <fabrizio.castro.jz@...esas.com>
> > > > Subject: Re: [PATCH] irqchip/renesas-rzg2l: Fix missing put_device
> > > >
> > > > On Mon, 30 Sep 2024 15:55:39 +0100,
> > > > Fabrizio Castro <fabrizio.castro.jz@...esas.com> wrote:
> > > > >
> > > > > rzg2l_irqc_common_init calls of_find_device_by_node, but the
> > > > > corresponding put_device call is missing.
> > > > >
> > > > > Make sure we call put_device both when failing and when succeeding.
> > > >
> > > > What sort of lifetime are you trying to enforce?
> > >
> > > Function rzg2l_irqc_common_init uses pdev->dev until its very end.
> > > My understanding is that we should decrement the reference counter
> > > once we are fully done with it. Is my understanding correct?
> > 
> > "done with it" is what scares me. Specially when I see code like this:
> > 
> > 	rzg2l_irqc_data = devm_kzalloc(&pdev->dev, sizeof(*rzg2l_irqc_data), GFP_KERNEL);
> > 	if (!rzg2l_irqc_data)
> > 		return -ENOMEM;
> > 
> > 	rzg2l_irqc_data->irqchip = irq_chip;
> > 
> > 	rzg2l_irqc_data->base = devm_of_iomap(&pdev->dev, pdev->dev.of_node, 0, NULL);
> > 	if (IS_ERR(rzg2l_irqc_data->base))
> > 		return PTR_ERR(rzg2l_irqc_data->base);
> > 
> > If you drop the reference on the device, you are allowing it to be
> > removed, and everything the driver cares about to disappear behind
> > its back.
> 
> Thanks for the explanation. I think this means that we don't need to
> put the device on the successful path, but we still need to put the
> device on the error path.

That I would agree.

> If I take out the put_device for the successful path, and I run make
> coccicheck, I get the below:
> drivers/irqchip/irq-renesas-rzg2l.c:601:1-7: ERROR: missing
> put_device; call of_find_device_by_node on line 538, but without a
> corresponding object release within this function.
> 
> Can I just ignore it?

My general approach is that these scripts are not a substitute for
reasoning, and in this instance, the advise seems pretty misplaced.

I would suggest you add a comment to keep the next script kiddie at
bay.

	M.

-- 
Without deviation from the norm, progress is not possible.