linux-kernel - Re: [PATCH] PM: runtime: clk: Fix clk_pm_runtime

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Message-ID: <e9130ce8-fd22-c871-c089-585585c7133f@samsung.com>
Date:   Fri, 22 May 2020 07:19:28 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH] PM: runtime: clk: Fix clk_pm_runtime_get() error path

Hi Rafael,

On 21.05.2020 19:08, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> clk_pm_runtime_get() assumes that the PM-runtime usage counter will
> be dropped by pm_runtime_get_sync() on errors, which is not the case,
> so PM-runtime references to devices acquired by the former are leaked
> on errors returned by the latter.
>
> Fix this by modifying clk_pm_runtime_get() to drop the reference if
> pm_runtime_get_sync() returns an error.
>
> Fixes: 9a34b45397e5 clk: Add support for runtime PM
> Cc: 4.15+ <stable@...r.kernel.org> # 4.15+
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Frankly, I would rather fix the runtime_get_sync() instead of fixing the 
return path everywhere in the kernel. The current behavior of the 
pm_runtime_get_sync() is completely counter-intuitive then. I bet that 
in the 99% of the places where it is being called assume that no special 
fixup is needed in case of failure. This is one of the most common 
runtime PM related function and it is really a common pattern in the 
drivers to call:

pm_runtime_get_sync()

do something with the hardware

pm_runtime_put()

Do you really want to fix the error paths of the all such calls?


> ---
>   drivers/clk/clk.c |    6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
>
> Index: linux-pm/drivers/clk/clk.c
> ===================================================================
> --- linux-pm.orig/drivers/clk/clk.c
> +++ linux-pm/drivers/clk/clk.c
> @@ -114,7 +114,11 @@ static int clk_pm_runtime_get(struct clk
>   		return 0;
>   
>   	ret = pm_runtime_get_sync(core->dev);
> -	return ret < 0 ? ret : 0;
> +	if (ret < 0) {
> +		pm_runtime_put_noidle(core->dev);
> +		return ret;
> +	}
> +	return 0;
>   }
>   
>   static void clk_pm_runtime_put(struct clk_core *core)
>
>
>
>
Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland