linux-kernel - Re: [PATCH] PM / wakeup: Register wakeup class kobj after device is added

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Message-ID: <CANA+-vDZqXS-yac9Q_K54oVrqZLtJojBSKa=ToJ9ZkfYHRcqbQ@mail.gmail.com>
Date:   Fri, 16 Aug 2019 15:38:07 -0700
From:   Tri Vo <trong@...roid.com>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>, Qian Cai <cai@....pw>
Subject: Re: [PATCH] PM / wakeup: Register wakeup class kobj after device is added

On Fri, Aug 16, 2019 at 2:46 PM Stephen Boyd <swboyd@...omium.org> wrote:
>
> Quoting Tri Vo (2019-08-16 14:27:35)
> > On Fri, Aug 16, 2019 at 7:56 AM Stephen Boyd <swboyd@...omium.org> wrote:
> > > diff --git a/drivers/base/power/sysfs.c b/drivers/base/power/sysfs.c
> > > index 1b9c281cbe41..27ee00f50bd7 100644
> > > --- a/drivers/base/power/sysfs.c
> > > +++ b/drivers/base/power/sysfs.c
> > > @@ -5,6 +5,7 @@
> > >  #include <linux/export.h>
> > >  #include <linux/pm_qos.h>
> > >  #include <linux/pm_runtime.h>
> > > +#include <linux/pm_wakeup.h>
> > >  #include <linux/atomic.h>
> > >  #include <linux/jiffies.h>
> > >  #include "power.h"
> > > @@ -661,14 +662,21 @@ int dpm_sysfs_add(struct device *dev)
> > >                 if (rc)
> > >                         goto err_runtime;
> > >         }
> > > +       if (dev->power.wakeup) {
> >
> > This conditional checks for the situation when wakeup source
> > registration have been previously attempted, but failed at
> > wakeup_source_sysfs_add(). My concern is that it's not easy to
> > understand what this check does without knowing exactly what
> > device_wakeup_enable() does to dev->power.wakeup before we reach this
> > point.
>
> Oh, actually this is wrong. It should be a check for
> dev->power.wakeup->dev being non-NULL. That's the variable that's set by
> wakeup_source_sysfs_add() upon success. So I should make it:
>
>         if (dev->power.wakeup && !dev->power.wakeup->dev)

Oh ok, this makes more sense now :)
>
> And there's the problem that CONFIG_PM_SLEEP could be unset. Let me fix
> it up with a new inline function like device_has_wakeup_dev().
>
> >
> > > +               rc = wakeup_source_sysfs_add(dev, dev->power.wakeup);
> > > +               if (rc)
> > > +                       goto err_wakeup;
> > > +       }
> > >         if (dev->power.set_latency_tolerance) {
> > >                 rc = sysfs_merge_group(&dev->kobj,
> > >                                        &pm_qos_latency_tolerance_attr_group);
> > >                 if (rc)
> > > -                       goto err_wakeup;
> > > +                       goto err_wakeup_source;
> > >         }
> > >         return 0;
> > >
> > > + err_wakeup_source:
> > > +       wakeup_source_sysfs_remove(dev->power.wakeup);
> > >   err_wakeup:
> > >         sysfs_unmerge_group(&dev->kobj, &pm_wakeup_attr_group);
> > >   err_runtime:
> > > diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
> > > index f7925820b5ca..5817b51d2b15 100644
> > > --- a/drivers/base/power/wakeup.c
> > > +++ b/drivers/base/power/wakeup.c
> > > @@ -220,10 +220,12 @@ struct wakeup_source *wakeup_source_register(struct device *dev,
> > >
> > >         ws = wakeup_source_create(name);
> > >         if (ws) {
> > > -               ret = wakeup_source_sysfs_add(dev, ws);
> > > -               if (ret) {
> > > -                       wakeup_source_free(ws);
> > > -                       return NULL;
> > > +               if (!dev || device_is_registered(dev)) {
> >
> > Is there a possible race condition here? If dev->power.wakeup check in
> > dpm_sysfs_add() is done at the same time as device_is_registered(dev)
> > check here, then wakeup_source_sysfs_add() won't ever be called?
>
> The same race exists for device_set_wakeup_capable() so I didn't bother
> to try to avoid it. I suppose wakeup_source_sysfs_add() could run
> completely, allocate the device and set the name, etc., but not call
> device_add() and then we can set ws->dev and call device_add() under a
> mutex so that we keep a very small window where the wakeup class is
> published to sysfs. Or just throw a big mutex around the whole wakeup
> class creation path so that there isn't a chance of a race. But really,
> is anyone going to call device_set_wakeup_*() on a device that is also
> being added to the system? Seems unlikely.

True. I don't have a strong opinion.
>
> >
> > > +                       ret = wakeup_source_sysfs_add(dev, ws);
> > > +                       if (ret) {
> > > +                               wakeup_source_free(ws);