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

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Message-ID: <20120523091617.GZ8730@tbergstrom-lnx.Nvidia.com>
Date:	Wed, 23 May 2012 12:16:17 +0300
From:	Peter De Schrijver <pdeschrijver@...dia.com>
To:	"Turquette, Mike" <mturquette@...com>
CC:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Saravana Kannan <skannan@...eaurora.org>,
	Andrew Lunn <andrew@...n.ch>,
	Grant Likely <grant.likely@...retlab.ca>,
	Jamie Iles <jamie@...ieiles.com>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Magnus Damm <magnus.damm@...il.com>,
	Deepak Saxena <dsaxena@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Arnd Bergman <arnd.bergmann@...aro.org>,
	Stephen Warren <swarren@...dotorg.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Rob Herring <rob.herring@...xeda.com>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Zhao <richard.zhao@...aro.org>,
	Shawn Guo <shawn.guo@...escale.com>,
	Paul Walmsley <paul@...an.com>,
	Linus Walleij <linus.walleij@...ricsson.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Amit Kucheria <amit.kucheria@...aro.org>
Subject: Re: [PATCH] clk: Fix race conditions between clk_set_parent() and
 clk_enable()

On Tue, May 22, 2012 at 08:06:45PM +0200, Turquette, Mike wrote:
> On Tue, May 22, 2012 at 6:58 AM, Peter De Schrijver
> <pdeschrijver@...dia.com> wrote:
> > On Tue, May 15, 2012 at 08:20:44PM +0200, Saravana Kannan wrote:
> >> On 05/11/2012 09:59 PM, 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.
> >> >
> >
> > This looks correct to me. Is there any usecase where enabling/disabling a
> > clock would require sleeping but changing the parent would not?
> >
> 
> clk_enable & clk_disable must never sleep.  clk_prepare and
> clk_unprepare may sleep.
> 

In that case the clock is actually enabled in clk_prepare and disabled in
clk_unprepare I guess (and clk_enable/clk_disable are dummy functions)?

What I'm trying to say is that I don't think there are clocks which can be
enabled/disabled using non blocking operations, but where a parent change
would require a blocking operation.

Cheers,

Peter.
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