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

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Message-ID: <CAGETcx-9XSdXcuGQFSoS-mMPwp=UJ3_FfTJ1Cx+9jddyjYTKEg@mail.gmail.com>
Date: Mon, 2 Dec 2024 12:46:06 -0800
From: Saravana Kannan <saravanak@...gle.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
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 12:16 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> 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?

Not sure what you mean by "go away"? But get_device() is going to keep
a non-zero refcount on the parent so that struct doesn't get freed.
The parent itself can still "go away" in terms of unbinding or
removing the children from the dpm_list(). But that's what the
device_pm_initialized() check is for. When a device_del() is called,
it's removed from the dpm_list. The actual freeing comes later. But we
aren't/weren't checking for that anyway.

>
> > > 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);

Parent struct device being there isn't the same as whether this device
is in the dpm_list? So, shouldn't we still check this?

Also, is it really racy anymore with the new algorithm? We don't kick
off the subordinates until after the parent is done.

> >
> > and please update the comment above this.

Will do.

Thanks,
Saravana
> >
> > >         put_device(parent);
> > >
> > >         dpm_wait_for_suppliers(dev, async);
> > > --