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 <swb...@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. 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);
>
