linux-kernel - Re: ABBA deadlock in Common Clock Framework

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Date:	Wed, 2 Jul 2014 14:49:24 +0300
From:	Peter De Schrijver <pdeschrijver@...dia.com>
To:	Tomasz Figa <t.figa@...sung.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tomasz Stanisławski <t.stanislaws@...sung.com>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Mike Turquette <mturquette@...aro.org>,
	Wolfram Sang <wsa@...-dreams.de>,
	Andrzej Hajda <a.hajda@...sung.com>,
	Mark Brown <broonie@...nel.org>,
	Lee Jones <lee.jones@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: ABBA deadlock in Common Clock Framework

On Wed, Jul 02, 2014 at 12:59:04PM +0200, Tomasz Figa wrote:
> Hi All,
> 
> While testing linux-next (next-20140625) on Exynos4412-based TRATS2
> board, from time to time I hit a deadlock between clk_disable_unused()
> of Common Clock Framework and parallel clk_prepare() from s3c24xx-i2c
> driver.
> 
> I believe the following is happening (in processes 1 and 2):
>  1: clk_disable_unused()
>  1: clk_prepare_lock()
>  1: mutex_lock(clk prepare mutex) // lock A
>  1: max77686_clk_is_prepared()
>  1: regmap_read()
>  2: regulator_set_voltage()
>  2: regmap_read()
>  2: mutex_lock(max77686 regmap mutex) // lock B
>  2: i2c_transfer()
>  2: s3c24xx_i2c_xfer()
>  2: clk_unprepare()
>  2: clk_prepare_lock()
>  2: mutex_lock(clk prepare mutex) // lock A
>  1: mutex_lock(max77686 regmap mutex) // lock B
>  1,2: BOOM!
> 
> I suppose this is a general problem affecting any MFD device that is
> also a clock provider, but I clearly don't have a good idea what to do
> about it.
> 

Or if you use notifiers which use i2c... See also
http://comments.gmane.org/gmane.linux.kernel/1553699. One workaround is to
always leave the clock of the i2c controller in a prepared state.

Cheers,

Peter.
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