linux-kernel - Re: [PATCH 1/2] PM-runtime: Fix __pm_runtime_set

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Date:   Tue, 12 Feb 2019 17:17:39 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Lukas Wunner <lukas@...ner.de>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Lucas Stach <l.stach@...gutronix.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH 1/2] PM-runtime: Fix __pm_runtime_set_status() race with
 runtime resume

On Tue, 12 Feb 2019 at 13:10, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> Commit 4080ab083000 ("PM-runtime: Take suppliers into account in
> __pm_runtime_set_status()") introduced a race condition that may
> trigger if __pm_runtime_set_status() is used incorrectly (that is,
> if it is called when PM-runtime is enabled for the target device
> and working).
>
> In that case, if the original PM-runtime status of the device is
> RPM_SUSPENDED, a runtime resume of the device may occur after
> __pm_runtime_set_status() has dropped its power.lock spinlock
> and before deactivating its suppliers, so the suppliers may be
> deactivated while the device is PM-runtime-active which may lead
> to functional issues.
>
> To avoid that, modify __pm_runtime_set_status() to check whether
> or not PM-runtime is enabled for the device before activating its
> suppliers (if the new status is RPM_ACTIVE) and either return an
> error if that's the case or increment the device's disable_depth
> counter to prevent PM-runtime from being enabled for it while
> the remaining part of the function is running (disable_depth is
> then decremented on the way out).
>
> Fixes: 4080ab083000 ("PM-runtime: Take suppliers into account in __pm_runtime_set_status()")
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/base/power/runtime.c |   24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
>
> Index: linux-pm/drivers/base/power/runtime.c
> ===================================================================
> --- linux-pm.orig/drivers/base/power/runtime.c
> +++ linux-pm/drivers/base/power/runtime.c
> @@ -1129,6 +1129,22 @@ int __pm_runtime_set_status(struct devic
>         if (status != RPM_ACTIVE && status != RPM_SUSPENDED)
>                 return -EINVAL;
>
> +       spin_lock_irq(&dev->power.lock);
> +
> +       /*
> +        * Prevent PM-runtime from being enabled for the device or return an
> +        * error if it is enabled already and working.
> +        */
> +       if (dev->power.runtime_error || dev->power.disable_depth)
> +               dev->power.disable_depth++;
> +       else
> +               error = -EAGAIN;
> +
> +       spin_unlock_irq(&dev->power.lock);
> +
> +       if (error)
> +               return error;
> +
>         /*
>          * If the new status is RPM_ACTIVE, the suppliers can be activated
>          * upfront regardless of the current status, because next time
> @@ -1147,12 +1163,6 @@ int __pm_runtime_set_status(struct devic
>
>         spin_lock_irq(&dev->power.lock);
>
> -       if (!dev->power.runtime_error && !dev->power.disable_depth) {
> -               status = dev->power.runtime_status;
> -               error = -EAGAIN;
> -               goto out;
> -       }
> -
>         if (dev->power.runtime_status == status || !parent)
>                 goto out_set;
>
> @@ -1205,6 +1215,8 @@ int __pm_runtime_set_status(struct devic
>                 device_links_read_unlock(idx);
>         }
>
> +       pm_runtime_enable(dev);

pm_runtime_enable() uses spin_lock_irqsave(), rather than
spin_lock_irq() - is there a reason to why you want to allow that
here, but not earlier in the function?

> +
>         return error;
>  }
>  EXPORT_SYMBOL_GPL(__pm_runtime_set_status);
>

Other than the above comment, this looks good to me.

Kind regards
Uffe