Message-ID: <CAGETcx_5oRA-kTzU7WtWSV2=Bp0cKcT=2kPa0+ZkwkHTA+rwOQ@mail.gmail.com>
Date: Mon, 2 Dec 2024 15:27:33 -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 1:15 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Mon, Dec 2, 2024 at 9:46 PM Saravana Kannan <saravanak@...gle.com> wrote:
> >
> > 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.
>
> That's after it has returned.
>
> There is nothing to prevent dev from being freed in get_device()
> itself before the kobject_get(&dev->kobj) call.
>
> So when get_device() is called, there needs to be an active ref on the
> device already or something else to prevent the target device object
> from being freed.
>
> In this particular case it is the lock along with the
> device_pm_initialized(dev) check on the child.

Ugh... my head hurts whenever I think about get_device(). Yeah, I
think you are right.

Hmm... I wanted to have this function be replaced by a call to a
generic helper function dpm_for_each_superior() in the next patch. But
that helper function could be called from places where the dpm_list
lock is held. Also, I was planning on making it even more generic (not
specific to dpm) in the future. So, depending on dpm_list lock didn't
make sense.

Any other ideas on how I could do this without grabbing the dpm_list mutex?

Actually, with the rewrite and at the end of this series, we don't
have to worry about this race because each device's suspend/resume is
only started after all the dependencies are done. So, if the parent
deletes a child and itself, the child device's suspend wouldn't have
been triggered at all.

So, another option is to just squash the series and call it a day. Or
add a comment to the commit text that this adds a race that's fixed by
the time we get to the end of the series.

Thoughts?

Thanks,
Saravana


>
> > 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);
> > > > > --

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