linux-kernel - Re: [PATCH 2/2] regmap: Make regmap-mmio usable from different contexts

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Date:	Thu, 23 May 2013 16:20:27 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Mark Brown <broonie@...nel.org>
CC:	Davide Ciminaghi <ciminaghi@...dd.com>,
	Stephen Warren <swarren@...dotorg.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] regmap: Make regmap-mmio usable from different contexts

On 05/23/2013 04:05 PM, Mark Brown wrote:
> On Thu, May 23, 2013 at 03:06:16PM +0200, Lars-Peter Clausen wrote:
> 
>> This patch updates the adds a flags parameter to the regmap lock and unlock
>> callbacks and uses spin_lock_irqsave() and spin_unlock_restore() for the mmio
>> case. This allows us to use regmap-mmio from different contexts.
> 
> This seems really invasive, why not just have the lock that gets passed
> in point to a struct which has both the lock and the flags?  As far as
> the core is concerned the lock is just whatever data is required to do
> the locking, the fact that it's actually two values is an implementation
> detail of this locking implementation.

I think that won't work. spin_lock_irqsave() will write to the flags
parameter before it has successfully taken the look. So if a process running
on another CPU tries to acquire the the lock while it is already held we'll
end up overwriting the flags. E.g:

CPU0				CPU1
spin_lock_irqsave()
  - write flags

				spin_lock_irqsave()
				  - overwrite flags

spin_unlock_irqrestore()
  - restore wrong flags

Hence flags needs to go onto the stack.

- Lars
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