linux-kernel - Re: [PATCH] clk: Fix race conditions between clk_set_parent() and clk

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Date:	Tue, 15 May 2012 21:42:45 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Saravana Kannan <skannan@...eaurora.org>
Cc:	Mike Turquette <mturquette@...aro.org>,
	Arnd Bergman <arnd.bergmann@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
	Rob Herring <rob.herring@...xeda.com>,
	Russell King <linux@....linux.org.uk>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Walmsley <paul@...an.com>,
	Shawn Guo <shawn.guo@...escale.com>,
	Jamie Iles <jamie@...ieiles.com>,
	Richard Zhao <richard.zhao@...aro.org>,
	Magnus Damm <magnus.damm@...il.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Linus Walleij <linus.walleij@...ricsson.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Deepak Saxena <dsaxena@...aro.org>,
	Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [PATCH] clk: Fix race conditions between clk_set_parent() and
 clk_enable()

Hi Saravana,

On Fri, May 11, 2012 at 09:59:56PM -0700, Saravana Kannan wrote:
> Without this patch, the following race conditions are possible.
> 
> Race condition 1:
> * clk-A has two parents - clk-X and clk-Y.
> * All three are disabled and clk-X is current parent.
> * Thread A: clk_set_parent(clk-A, clk-Y).
> * Thread A: <snip execution flow>
> * Thread A: Grabs enable lock.
> * Thread A: Sees enable count of clk-A is 0, so doesn't enable clk-Y.
> * Thread A: Releases enable lock.
> * Thread B: Calls clk_enable(clk-A), which in turn enables clk-X.
> * Thread A: Switches clk-A's parent to clk-Y in hardware.
> 
> clk-A is now enabled in software, but not clocking in hardware.
> 
> Race condition 2:
> * clk-A has two parents - clk-X and clk-Y.
> * All three are disabled and clk-X is current parent.
> * Thread A: clk_set_parent(clk-A, clk-Y).
> * Thread A: <snip execution flow>
> * Thread A: Switches parent in hardware to clk-Y.
> * Thread A: Grabs enable lock.
> * Thread A: Sees enable count of clk-A is 0, so doesn't disable clk-X.
> * Thread A: Releases enable lock.
> * Thread B: Calls clk_enable(clk-A)
> * Thread B: Software state still says parent is clk-X.
> * Thread B: So, enables clk-X and then itself.
> * Thread A: Updates parent in software state to clk-Y.

Had a look at this and I can follow your reasoning and you patch seems
to fix this. However, there is a problem,

>  
>  	/* migrate prepare and enable */
> @@ -1092,23 +1098,23 @@ static int __clk_set_parent(struct clk *clk, struct clk *parent)
>  
>  	/* FIXME replace with clk_is_enabled(clk) someday */
>  	spin_lock_irqsave(&enable_lock, flags);
> +
>  	if (clk->enable_count)
>  		__clk_enable(parent);
> -	spin_unlock_irqrestore(&enable_lock, flags);
>  
>  	/* change clock input source */
>  	ret = clk->ops->set_parent(clk->hw, i);

You call ->set_parent while holding a spinlock. This won't work with i2c
clocks.

Sascha

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