linux-kernel - Re: [PATCH 1/8] sta2x11-mfd : add apb-soc regs driver and factor out common code

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Date:	Thu, 27 Sep 2012 15:13:14 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Davide Ciminaghi <ciminaghi@...dd.com>
Cc:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	sameo@...ux.intel.com, rubini@...dd.com, giancarlo.asnaghi@...com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] sta2x11-mfd : add apb-soc regs driver and factor
 out common code

> as far as I know, nested locks are fine provided that you always take them in
> the same order and release them in the opposite order (lock A, lock B,
> unlock B, unlock A). So my conclusion is that nested spinlocks require
> potential regmap users of sta2x11 registers to take the sta2x11-mfd spinlock
> first. The pattern would be (sctl registers for instance):

The release order does not matter. Taking AB and releasing AB or BA is
fine. Taking AB and dropping B and retaking B is fine. Taking AB and
somewhere else taking BA is not. There are performance reasons in some
cases why taking AB releasing A is best with locks, but thats generally
with sleepable locks.

It's a bit more subtle because you often have other interactions. In
particular people often come unstuck on del_timer_sync or waiting for
workqueues and IRQs to finish (even in free_irq) while holding a lock
that the handler needs to take.

Alan
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