linux-kernel - Re: Locking in the clk API, part 2: clk_prepare/clk

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Message-ID: <20110201131512.GH31216@n2100.arm.linux.org.uk>
Date:	Tue, 1 Feb 2011 13:15:12 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Cc:	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Dima Zavin <dmitriyz@...gle.com>,
	Saravana Kannan <skannan@...eaurora.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	linux-sh@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-kernel@...r.kernel.org, Paul Mundt <lethal@...ux-sh.org>,
	Ben Dooks <ben-linux@...ff.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	Nicolas Pitre <nicolas.pitre@...aro.org>
Subject: Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare

On Tue, Feb 01, 2011 at 11:54:49AM +0100, Uwe Kleine-König wrote:
> Alternatively don't force the sleep in clk_prepare (e.g. by protecting
> prepare_count by a spinlock (probably enable_lock)) and call clk_prepare
> before calling clk->ops->enable?

That's a completely bad idea.  I assume you haven't thought about this
very much.

There's two ways I can think of doing what you're suggesting:

int clk_prepare(struct clk *clk)
{
	unsigned long flags;
	int ret = 0;

	might_sleep();

	spin_lock_irqsave(&clk->enable_lock, flags);
	if (clk->prepare_count++ == 0)
		ret = clk->ops->prepare(clk);
	spin_unlock_irqrestore(&clk->enable_clock, flags);

	return ret;
}

The problem is that clk->ops->prepare() is called in a non-sleepable
context.  So this breaks the whole idea of clk_prepare(), and so isn't
a solution.

The other solution is:

int clk_prepare(struct clk *clk)
{
	unsigned long flags;
	int ret = 0;
	bool first;

	might_sleep();

	spin_lock_irqsave(clk->enable_lock, flags);
	first = clk->prepare_count++ == 0;
	spin_unlock_irqrestore(clk->enable_clock, flags);

	if (first)
		ret = clk->ops->prepare(clk);

	return ret;
}

The problem with this is that you now don't have any sane locking on
the prepare callback, and the circumstances under which it's called
are very indefinite.  For example, consider a preempt-enabled system:

	thread 1		thread 2		prepare_count
	clk_prepare					0
	clk->prepare_count++				1
	<thread switch>
				clk_prepare		1
				clk->prepare_count++	2
				clk_prepare returns	2

				clk_enable		2
				<explodes as clock is not prepared>
				<thread switch>
	clk->ops->prepare(clk)

So really, what you're suggesting is completely broken.
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