On Monday, 7 of January 2008, Alan Stern wrote: > On Mon, 7 Jan 2008, Rafael J. Wysocki wrote: > > > On Monday, 7 of January 2008, Alan Stern wrote: > > > On Mon, 7 Jan 2008, Rafael J. Wysocki wrote: > > > > > > > Please see the patch at: http://lkml.org/lkml/2008/1/6/298 . It represents my > > > > current idea about how to do that. > > > > > > It has some problems. > > > > > > First, note that the list manipulations in dpm_suspend(), > > > device_power_down(), and so on aren't protected by dpm_list_mtx. So > > > your patch could corrupt the list pointers. > > > > Yes, they need the locking. I have overlooked that, mostly because the locking > > was removed by gregkh-driver-pm-acquire-device-locks-prior-to-suspending.patch > > too (because you assumed there woundn't be any need to remove a device during > > a suspend, right?). > > Right. > > > > Are you assuming that no other threads can be running at this time? > > > > No, I'm not. > > > > > Note also that device_pm_destroy_suspended() does up(&dev->sem), but it > > > doesn't know whether or not dev->sem was locked to begin with. > > > > Do you mean it might have been released already by another thread > > calling device_pm_destroy_suspended() on the same device? > > I was thinking that it might be called before lock_all_devices(). I've added pm_sleep_start_end_mtx and the locking dance in device_pm_destroy_suspended() specifically to prevent this from happening. > However let's ignore that possibility and simplify the discussion by > assuming that destroy_suspended_device() is never called except by a > suspend or resume method for that device or one of its ancestors. It may also be called by one of the CPU hotplug notifiers. > (This still leaves the possibility that it might get called by mistake > during a runtime suspend or resume...) > > > > Do you want to rule out the possibility of a driver's suspend or remove > > > methods calling destroy_suspended_device() on its own device? With > > > your synchronous approach, this would mean that the suspend/resume > > > method would indirectly end up calling the remove method. This is > > > dangerous at best; with USB it would be a lockdep violation. With an > > > asynchronous approach, on the other hand, this wouldn't be a problem. > > > > Well, the asynchronous apprach has the problem that the device may end up > > on a wrong list when removed by one of the .suspend() callbacks (and I don't > > see how to avoid that without extra complexity). Perhaps that's something we > > can live with, though. > > The same problem affects the synchronous approach. No, it doesn't as of the $subject patch (the list_empty() tests should help). > We can fix it by having dpm_suspend() do the list_move() before calling > suspend_device(). Then if the suspend fails move the device back. Yes, we can. > > One more question: is there any particular reason not to call > > device_pm_remove() at the beginning of device_del()? > > I think it's done this way to avoid having a window where the device > isn't on a PM list and is still owned by the bus and the driver. But > if a suspend occurs during that window, it shouldn't matter that the > device will be left unsuspended. After all, the same thing would have > happened if the suspend occurred after bus_remove_device(). > > So no, there shouldn't be a problem with moving the call. Okay, well, now I'm leaning towards the asynchronous approach. I'll prepare a new patch and send it later today. Thanks, Rafael - To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
