linux-kernel - Re: [PATCH v1 2/5] PM: sleep: Remove unnecessary mutex lock when waiting on parent

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Message-ID: <CAJZ5v0iLq9L5nMp13BrBmbWavFs1ZEAtJ-WeyRzv3D2GXPNuag@mail.gmail.com>
Date: Mon, 2 Dec 2024 21:16:34 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Saravana Kannan <saravanak@...gle.com>
Cc: Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, Geert Uytterhoeven <geert@...ux-m68k.org>, Marek Vasut <marex@...x.de>, 
	Bird@...gle.com, Tim <Tim.Bird@...y.com>, kernel-team@...roid.com, 
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/5] PM: sleep: Remove unnecessary mutex lock when
 waiting on parent

On Mon, Dec 2, 2024 at 9:11 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> Sorry for the delay.
>
> On Thu, Nov 14, 2024 at 11:09 PM Saravana Kannan <saravanak@...gle.com> wrote:
> >
> > Locking is not needed to do get_device(dev->parent). We either get a NULL
> > (if the parent was cleared) or the actual parent. Also, when a device is
> > deleted (device_del()) and removed from the dpm_list, its completion
> > variable is also complete_all()-ed. So, we don't have to worry about
> > waiting indefinitely on a deleted parent device.
>
> The device_pm_initialized(dev) check before get_device(dev->parent)
> doesn't make sense without the locking and that's the whole point of
> it.

Hmm, not really.

How is the parent prevented from going away in get_device() right
after the initial dev check without the locking?

> > Signed-off-by: Saravana Kannan <saravanak@...gle.com>
> > ---
> >  drivers/base/power/main.c | 13 ++-----------
> >  1 file changed, 2 insertions(+), 11 deletions(-)
> >
> > diff --git a/drivers/base/power/main.c b/drivers/base/power/main.c
> > index 86e51b9fefab..9b9b6088e56a 100644
> > --- a/drivers/base/power/main.c
> > +++ b/drivers/base/power/main.c
> > @@ -284,18 +284,9 @@ static bool dpm_wait_for_superior(struct device *dev, bool async)
> >          * counting the parent once more unless the device has been deleted
> >          * already (in which case return right away).
> >          */
> > -       mutex_lock(&dpm_list_mtx);
> > -
> > -       if (!device_pm_initialized(dev)) {
> > -               mutex_unlock(&dpm_list_mtx);
> > -               return false;
> > -       }
> > -
> >         parent = get_device(dev->parent);
> > -
> > -       mutex_unlock(&dpm_list_mtx);
> > -
> > -       dpm_wait(parent, async);
> > +       if (device_pm_initialized(dev))
> > +               dpm_wait(parent, async);
>
> This is racy, so what's the point?
>
> You can just do
>
> parent = get_device(dev->parent);
> dpm_wait(parent, async);
>
> and please update the comment above this.
>
> >         put_device(parent);
> >
> >         dpm_wait_for_suppliers(dev, async);
> > --