netdev - Re: [PATCH v3 1/2] PM: runtime: Add pm_runtime_resume_and

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Date:   Fri, 27 Nov 2020 11:15:51 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Zhang Qilong <zhangqilong3@...wei.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Fugang Duan <fugang.duan@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
        Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [PATCH v3 1/2] PM: runtime: Add pm_runtime_resume_and_get to deal
 with usage counter

Hi Zhang,

On Tue, Nov 10, 2020 at 10:29 AM Zhang Qilong <zhangqilong3@...wei.com> wrote:
> In many case, we need to check return value of pm_runtime_get_sync, but
> it brings a trouble to the usage counter processing. Many callers forget
> to decrease the usage counter when it failed, which could resulted in
> reference leak. It has been discussed a lot[0][1]. So we add a function
> to deal with the usage counter for better coding.
>
> [0]https://lkml.org/lkml/2020/6/14/88
> [1]https://patchwork.ozlabs.org/project/linux-tegra/list/?series=178139
> Signed-off-by: Zhang Qilong <zhangqilong3@...wei.com>

Thanks for your patch, which is now commit dd8088d5a8969dc2 ("PM:
runtime: Add pm_runtime_resume_and_get to deal with usage counter") in
v5.10-rc5.

> --- a/include/linux/pm_runtime.h
> +++ b/include/linux/pm_runtime.h
> @@ -386,6 +386,27 @@ static inline int pm_runtime_get_sync(struct device *dev)
>         return __pm_runtime_resume(dev, RPM_GET_PUT);
>  }
>
> +/**
> + * pm_runtime_resume_and_get - Bump up usage counter of a device and resume it.
> + * @dev: Target device.
> + *
> + * Resume @dev synchronously and if that is successful, increment its runtime
> + * PM usage counter. Return 0 if the runtime PM usage counter of @dev has been
> + * incremented or a negative error code otherwise.
> + */
> +static inline int pm_runtime_resume_and_get(struct device *dev)

Perhaps this function should be called pm_runtime_resume_and_get_sync(),
to make it clear it does a synchronous get?

I had to look into the implementation to verify that a change like

-       ret = pm_runtime_get_sync(&pdev->dev);
+       ret = pm_runtime_resume_and_get(&pdev->dev);

in the follow-up patches is actually a valid change, maintaining
synchronous operation. Oh, pm_runtime_resume() is synchronous, too...

While I agree the old pm_runtime_get_sync() had confusing semantics, we
should go to great lengths to avoid making the same mistake while fixing
it.

> +{
> +       int ret;
> +
> +       ret = __pm_runtime_resume(dev, RPM_GET_PUT);
> +       if (ret < 0) {
> +               pm_runtime_put_noidle(dev);
> +               return ret;
> +       }
> +
> +       return 0;
> +}
> +
>  /**
>   * pm_runtime_put - Drop device usage counter and queue up "idle check" if 0.
>   * @dev: Target device.

Gr{oetje,eeting}s,

                        Geert


--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds