On Monday, 7 of January 2008, Alan Stern wrote: > Let's try to summarize the main issues here: > > 1. We want the PM core to lock all devices during suspend and > hibernation. This implies that registration and unregistration > at such times can't work, because they need to lock the > device sem in order to make probe and remove method calls. > > 2. Registration calls can be failed, with an error message in the > system log. However unregistration calls cannot fail. They > _can_ block until the system resumes, but if the unregistration > call was made from within a suspend or resume method it will > deadlock. This seems inescapable, but at least we should print > an error in the log so the offending driver can be identified. > > 3. In response to 2, the PM core should have a special routine for > unregistering devices while a suspend is in progress. Rafael > proposed that the core should unlock the device to permit the > call to go through. This seems dangerous to me; I would prefer > to leave the locks in place and defer the unregistration until > after the system is back up and the locks have all been > dropped. This would avoid all sorts of locking, deadlock, and > mutual exclusion problems. > > (As a side note: destroy_suspended_device() has a rather limited > interface anyway, since it can handle only devices that were created by > create_device().) > > 4. Rafael pointed out that unregistration can occur concurrently > with system suspend. When this happens we can end up trying to > suspend a device which has already been through > bus_remove_device(), because it hasn't yet been removed from > the dpm_active list. He proposes we make unregistration block > system suspend, just as registration does. > > I don't see 4 as a real problem. Starting an unregistration before > the suspend and finishing it afterward should be okay. Once a device > has gone through bus_remove_device() it hasn't got a suspend method any > more, so trying to suspend it won't do anything at all -- the tests in > suspend_device() will all fail. Conversely, if bus_remove_device() > hasn't run yet then we would end up calling the driver's suspend method > before the device_del() call returns. As Johannes pointed out, this is > a normal race that would exist anyway. > > On the other hand, having unregistration block system suspend wouldn't > actually be wrong. I simply don't think it is necessary. But note > that making the two mutually exclusive would complicate Rafael's > synchronous approach for destroy_suspended_device(). > > 5. All the discussion about pm_sleep_rwsem and so on is > implementation details. Once we have settled on the correct > approach for 1-4, the implementation should be relatively easy. Please see the patch at: http://lkml.org/lkml/2008/1/6/298 . It represents my current idea about how to do that. 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
