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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Wed, 31 Jul 2019 23:19:04 +0200
Message-ID: <CAJZ5v0gdmmgBuSQU7FWaBmdTq7diToKO=5F5e5vRt=Yqvn9C2Q@mail.gmail.com>
Subject: Re: [PATCH v5] PM / wakeup: show wakeup sources stats in sysfs
To:     Stephen Boyd <swboyd@chromium.org>
Cc:     "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Tri Vo <trong@android.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Hridya Valsaraju <hridya@google.com>,
        Sandeep Patil <sspatil@google.com>,
        Kalesh Singh <kaleshsingh@google.com>,
        Ravi Chandra Sadineni <ravisadineni@chromium.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        "Cc: Android Kernel" <kernel-team@android.com>,
        kbuild test robot <lkp@intel.com>
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On Wed, Jul 31, 2019 at 7:14 PM Stephen Boyd <swboyd@chromium.org> wrote:
>
> Quoting Rafael J. Wysocki (2019-07-31 04:58:36)
> > On Wednesday, July 31, 2019 10:34:11 AM CEST Rafael J. Wysocki wrote:
> > > On Wed, Jul 31, 2019 at 1:41 AM Stephen Boyd <swboyd@chromium.org> wrote:
> > > >
> > >
> > > > We can run into the same problem when two buses name their devices the
> > > > same name and then we attempt to attach a wakeup source to those two
> > > > devices. Or we can have a problem where a virtual wakeup is made with
> > > > the same name, and again we'll try to make a duplicate named device.
> > > > Using something like 'event' or 'wakeup' or 'ws' as the prefix avoids this
> > > > problem and keeps things clean.
> > >
> > > Or suffix, like "<devname-wakeup>.
> > >
> > > But if prefixes are used by an existing convention, I would prefer
> > > "ws-" as it is concise enough and should not be confusing.
>
> Another possibility is 'eventN', so it reads as /sys/class/wakeup/event0
>
> > >
> > > > We should probably avoid letting the same virtual wakeup source be made
> > > > with the same name anyway, because userspace will be confused about what
> > > > virtual wakeup it is otherwise. I concede that using the name of the
> > > > wakeup source catches this problem without adding extra code.
> > > >
> > > > Either way, I'd like to see what you outline implemented so that we
> > > > don't need to do more work than is necessary when userspace writes to
> > > > the file.
> > >
> > > Since we agree here, let's make this change first.  I can cut a patch
> > > for that in a reasonable time frame I think if no one else beats me to
> > > that.
> >
> > So maybe something like the patch below (untested).
> >
> > Index: linux-pm/drivers/base/power/wakeup.c
> > ===================================================================
> > --- linux-pm.orig/drivers/base/power/wakeup.c
> > +++ linux-pm/drivers/base/power/wakeup.c
> > @@ -265,15 +244,29 @@ int device_wakeup_enable(struct device *
> >         if (pm_suspend_target_state != PM_SUSPEND_ON)
> >                 dev_dbg(dev, "Suspicious %s() during system transition!\n", __func__);
> >
> > +       spin_lock_irq(&dev->power.lock);
> > +
> > +       if (dev->power.wakeup) {
> > +               spin_unlock_irq(&dev->power.lock);
> > +               return -EEXIST;
> > +       }
> > +       dev->power.wakeup = ERR_PTR(-EBUSY);
> > +
> > +       spin_unlock_irq(&dev->power.lock);
> > +
> >         ws = wakeup_source_register(dev_name(dev));
> >         if (!ws)
> >                 return -ENOMEM;
> >
>
> Let's say that device_wakeup_enable() is called twice at around the same
> time. First thread gets to wakeup_source_register() and it fails, we
> return -ENOMEM.

The return is premature.  dev->power.wakeup should be reset back to
NULL if the wakeup source creation fails.

> dev->power.wakeup is assigned to ERR_PTR(-EBUSY). Second
> thread is at the spin_lock_irq() above, it grabs the lock and sees
> dev->power.wakeup is ERR_PTR(-EBUSY) so it bails out with return
> -EEXIST. I'd think we would want to try to create the wakeup source
> instead.
>
>     CPU0                                   CPU1
>     ----                                   ----
>     spin_lock_irq(&dev->power.lock)
>     ...
>     dev->power.wakeup = ERR_PTR(-EBUSY)
>     spin_unlock_irq(&dev->power.lock)
>     ws = wakeup_source_register(...)
>     if (!ws)
>         return -ENOMEM;                 spin_lock_irq(&dev->power.lock)
>                                         if (dev->power.wakeup)
>                                             return -EEXIST; // Bad
>
>
> Similar problems probably exist with wakeup destruction racing with
> creation. I think it might have to be a create and then publish pointer
> style of code to keep the spinlock section small?

There is a problem when there are two concurrent callers of
device_wakeup_enable() running in parallel with a caller of
device_wakeup_disable(), but that can be prevented by an extra check
in the latter.

Apart from that I missed a few if (dev->power.wakeup) checks to convert.

I'll update the patch and resend it.