linux-kernel - Re: [PATCH v3 1/3] PM / OPP: add dev_pm_opp_get_suspend

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Date:	Mon, 7 Sep 2015 11:26:11 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Cc:	Thomas Abraham <thomas.ab@...sung.com>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Kukjin Kim <kgene@...nel.org>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Michael Turquette <mturquette@...libre.com>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Lukasz Majewski <l.majewski@...sung.com>,
	Heiko Stuebner <heiko@...ech.de>,
	Chanwoo Choi <cw00.choi@...sung.com>,
	Kevin Hilman <khilman@...aro.org>,
	Javier Martinez Canillas <javier@....samsung.com>,
	Tobias Jakobi <tjakobi@...h.uni-bielefeld.de>,
	Anand Moon <linux.amoon@...il.com>,
	linux-samsung-soc@...r.kernel.org, linux-clk@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] PM / OPP: add dev_pm_opp_get_suspend_opp() helper

On 03-09-15, 20:11, Bartlomiej Zolnierkiewicz wrote:
> Add dev_pm_opp_get_suspend_opp() helper to obtain suspend opp.
> 
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Cc: Thomas Abraham <thomas.ab@...sung.com>
> Cc: Javier Martinez Canillas <javier@....samsung.com>
> Cc: Krzysztof Kozlowski <k.kozlowski@...sung.com>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Tobias Jakobi <tjakobi@...h.uni-bielefeld.de>
> Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
> ---
>  drivers/base/power/opp.c | 28 ++++++++++++++++++++++++++++
>  include/linux/pm_opp.h   |  6 ++++++
>  2 files changed, 34 insertions(+)
> 
> diff --git a/drivers/base/power/opp.c b/drivers/base/power/opp.c
> index eb25449..eaafca7 100644
> --- a/drivers/base/power/opp.c
> +++ b/drivers/base/power/opp.c
> @@ -341,6 +341,34 @@ unsigned long dev_pm_opp_get_max_clock_latency(struct device *dev)
>  EXPORT_SYMBOL_GPL(dev_pm_opp_get_max_clock_latency);
>  
>  /**
> + * dev_pm_opp_get_suspend_opp() - Get suspend opp
> + * @dev:	device for which we do this operation
> + *
> + * Return: This function returns pointer to the suspend opp if it is
> + * defined, otherwise it returns NULL.
> + *
> + * Locking: This function takes rcu_read_lock().
> + */
> +struct dev_pm_opp *dev_pm_opp_get_suspend_opp(struct device *dev)
> +{
> +	struct device_opp *dev_opp;
> +	struct dev_pm_opp *suspend_opp;
> +
> +	rcu_read_lock();

This is wrong. Please have a look at other exported APIs that return
an OPP. RCU protected structures must be accessed from within RCU
locks _ONLY_. But your function returns the pointer after dropping
the locks. Which essentially means that the OPP can be freed by core,
by the time you start using it. :)

So, in such cases, its upto the caller to ensure that locks are taken
properly.

You can look at dev_pm_opp_find_freq_exact() as an example.

-- 
viresh
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