linux-kernel - Re: [PATCH v5] PM / wakeup: show wakeup sources stats in sysfs

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Date:   Wed, 31 Jul 2019 10:13:57 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Tri Vo <trong@...roid.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        kbuild test robot <lkp@...el.com>
Subject: Re: [PATCH v5] PM / wakeup: show wakeup sources stats in sysfs

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@...omium.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. 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?

> -       ret = device_wakeup_attach(dev, ws);
> -       if (ret)
> -               wakeup_source_unregister(ws);
> +       spin_lock_irq(&dev->power.lock);
>  
> -       return ret;
> +       dev->power.wakeup = ws;
> +       if (dev->power.wakeirq)
> +               device_wakeup_attach_irq(dev, dev->power.wakeirq);
> +
> +       spin_unlock_irq(&dev->power.lock);
> +
> +       return 0;
>  }
>  EXPORT_SYMBOL_GPL(device_wakeup_enable);
>