linux-kernel - device_node lifetime (was: Re: [PATCH 1/7] phy: brcmstb-sata: add missing of_node

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Message-ID: <20151118190500.GE140057@google.com>
Date:	Wed, 18 Nov 2015 11:05:00 -0800
From:	Brian Norris <computersforpeace@...il.com>
To:	Julia Lawall <julia.lawall@...6.fr>
Cc:	Kishon Vijay Abraham I <kishon@...com>,
	kernel-janitors@...r.kernel.org,
	Gregory Fong <gregory.0xf0@...il.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Andrew Lunn <andrew@...n.ch>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Jason Cooper <jason@...edaemon.net>, devicetree@...r.kernel.org
Subject: device_node lifetime (was: Re: [PATCH 1/7] phy: brcmstb-sata: add
 missing of_node_put)

(changing subject, add devicetree@...r.kernel.org)

On Tue, Nov 17, 2015 at 11:33:25PM +0100, Julia Lawall wrote:
> On Tue, 17 Nov 2015, Brian Norris wrote:
> > On Tue, Nov 17, 2015 at 06:48:39PM +0100, Julia Lawall wrote:
> > > Is this something that should be checked for elsewhere?
> > 
> > I expect the same sort of problem shows up plenty of other places. I
> > don't think many people use CONFIG_OF_DYNAMIC, so the effects of these
> > failures probably aren't felt by many.
> 
> I tried the following semantic patch:
> 
> @@
> struct device_node *e;
> expression e1;
> identifier fld;
> @@
> 
>  ... when != of_node_get(...)
> *(<+...e1->fld...+>) = e
>  ... when != of_node_get(...)
>  return e1;
> 
> basically, this says that a structure field is initilized to a device node 
> value, the structure is returned by the containing function, and the 
> containing function contains no of_node_get at all.  Certainly this is 
> quite constrained, but it does produce a number of examples.
> 
> I looked at a few of them:
> 
> drivers/clk/ingenic/cgu.c, ingenic_cgu_new
> clk/pistachio/clk.c, pistachio_clk_alloc_provider

It looks like the clock core (drivers/clk/clk.c) initially grabs the clk
provider node in of_clk_init(), then drops it after it's initialized,
but most of these providers use of_clk_add_provider(), which seems to
manage the device_node lifetime for the user. So I think these are OK.

> drivers/mfd/syscon.c, of_syscon_register

This one looks potentially suspect. Syscon nodes aren't usually directly
managed by a single driver, and the device_node pointer is used for
lookups later...so I think it should keep a kref, and it doesn't.

> drivers/of/pdt.c, function of_pdt_create_node

Not real sure about this one.

> Any idea whether these need of_node_get?  In all cases the device node 
> value comes in as a parameter.

I'm really not an expert on this stuff. I just saw a potential problem
that I happen to be looking at in other subsystems, and I wanted to know
what others thought. I think this discussion should include the DT folks
and the subsystems in question. For one, I'm as interested as anyone in
getting this todo clarified:

Documentation/devicetree/todo.txt
- Document node lifecycle for CONFIG_OF_DYNAMIC

Regards,
Brian
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