linux-kernel - Re: [RFC PATCH 1/2] regmap: add configurable lock class key for lockdep

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Date:	Thu, 25 Jun 2015 06:21:43 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Nicolas Boichat <drinkcat@...omium.org>,
	Mark Brown <broonie@...nel.org>
CC:	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Antti Palosaari <crope@....fi>, Ingo Molnar <mingo@...hat.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, Bard Liao <bardliao@...ltek.com>,
	Oder Chiou <oder_chiou@...ltek.com>,
	Liam Girdwood <lgirdwood@...il.com>,
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.de>,
	alsa-devel@...a-project.org,
	Anatol Pomozov <anatol.pomozov@...il.com>
Subject: Re: [RFC PATCH 1/2] regmap: add configurable lock class key for lockdep

On 6/25/2015 2:35 AM, Nicolas Boichat wrote:
> From: Antti Palosaari <crope@....fi>
>
> Lockdep validator complains about recursive locking and deadlock
> when two different regmap instances are called in a nested order.
> That happens anytime a regmap read/write call needs to access
> another regmap.
>
> This is because, for performance reason, lockdep groups all locks
> initialized by the same mutex_init() in the same lock class.
> Therefore all regmap mutexes are in the same lock class, leading
> to lockdep "nested locking" warnings if a regmap accesses another
> regmap. However, depending on the specifics of the driver, this
> can be perfectly safe (e.g. if there is a clear hierarchy between
> a "master" regmap that uses another "slave" regmap). In these
> cases, the warning is false and should be silenced.
>
> As a solution, add configuration option to pass custom lock class
> key for lockdep validator, to be used in the regmap that needs to
> access another regmap. This removes the need for uglier workarounds
> in drivers, just to silence this warning (e.g. add custom mutex
> lock/unlock functions).

wouldn't it be better to use the mutex_lock_nested() and co to explicitly express your hierarchy?

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