linux-kernel - Re: [PATCH 3/3] of/gpio: Introduce of_put_gpio(), add ref counting for OF GPIO chips

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Date:	Tue, 9 Feb 2010 22:14:27 +0300
From:	Anton Vorontsov <avorontsov@...mvista.com>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	David Brownell <dbrownell@...rs.sourceforge.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Michal Simek <monstr@...str.eu>, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org, devicetree-discuss@...ts.ozlabs.org,
	microblaze-uclinux@...e.uq.edu.au
Subject: Re: [PATCH 3/3] of/gpio: Introduce of_put_gpio(), add ref counting
 for OF GPIO chips

On Tue, Feb 09, 2010 at 10:28:15AM -0700, Grant Likely wrote:
[...]
> Rather than having a lock at the device tree data pointer level which
> mixes usage with potentially many other drivers; wouldn't it make more
> sense to use a mutex at the of_gc subsystem context?

I don't think so.

of_gc = np->data;
lock(of_gc); (or lock(devtree))
<do something with of_gc>

doesn't provide us what we need, i.e. it doesn't guarantee that
np->data (of_gc) is still alive.

And here:

lock(np->data); (or lock(devtree))
of_gc = np->data;
lock(of_gc);
<do something with of_gc>

The second lock becomes useless (unless you also refcount np->data
usage and can drop the devtree/np->data lock, and grab some other
kind of lock, e.g. mutex, but this is silly).

Thanks,

-- 
Anton Vorontsov
email: cbouatmailru@...il.com
irc://irc.freenode.net/bd2
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