linux-kernel - Re: [PATCH v4 4/6] gpio: introduce lock mechanism for gpiochip

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Message-Id: <20120530063325.0F6AA3E065C@localhost>
Date:	Wed, 30 May 2012 14:33:25 +0800
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Dong Aisheng <dongas86@...il.com>
Cc:	Dong Aisheng <b29396@...escale.com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linus.walleij@...ricsson.com,
	swarren@...dotorg.org, devicetree-discuss@...ts.ozlabs.org,
	rob.herring@...xeda.com
Subject: Re: [PATCH v4 4/6] gpio: introduce lock mechanism for gpiochip_find

On Wed, 30 May 2012 12:10:58 +0800, Dong Aisheng <dongas86@...il.com> wrote:
> Hi Grant,
> 
> On Fri, May 25, 2012 at 06:25:00PM -0600, Grant Likely wrote:
> > On Fri, 25 May 2012 21:36:18 +0800, Dong Aisheng <b29396@...escale.com> wrote:
> > > From: Dong Aisheng <dong.aisheng@...aro.org>
> > > 
> > > The module lock will be automatically claimed for gpiochip_find function
> > > in case the gpio module is removed during the using of gpiochip instance.
> > > Users are responsible to call gpiochip_put to release the lock after
> > > the using.
> > > 
> > > Signed-off-by: Dong Aisheng <dong.aisheng@...aro.org>
> > 
> ...
> > Also, it doesn't do anything to protect against the gpio_chip being
> > removed after the gpio number is resolved, which means the gpio number
> > may no longer be valid, or may no longer point to the same gpio chip.
> > It looks like the locking protection needs to be wider to be useful.
> > 
> I understand the issue now.
> It's correct that we did not lock gpio_chip before calling gpio_request
> after the gpio number is resolved.
> 
> I thought about adding a new API called of_gpio_request to hide the lock
> to users like:
> int of_gpio_request(..)
> {
> 	spin_lock_irqsave(&gpio_lock, flags);
> 	ret = of_get_named_gpio(..);
> 	if (ret < 0)
> 		do_err..
> 	ret = gpio_request(..)
> 
> 	spin_unlock_irqrestore(&gpio_lock, flags);
> 	return ret;
> }
> But it seems it does not work since the gpio_request may sleep and we may
> need a new sleepable lock rather using the exist gpio_lock.
> 
> In the same time, i'm also thinking about a question that do we really
> need to do this to protect gpio_chip being removed afer gpio number is
> resolved?

Probably not (for this series) because it shouldn't cause an oops if
it happens.  Ultimately however it would be good to have proper
reference counting on gpio chips to prevent any problems.

g.
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