On Tue, 24 Jul 2007, Rafael J. Wysocki wrote: > > Now here's an idea which might work. Can we require every caller of > > device_add() to hold some existing device's semaphore? Normally it > > would be the semaphore of the new device's parent, but it could be a > > higher ancestor. There even could be a single "root" semaphore for > > drivers registering a top-level device with no parent. > > > > (Some testing shows that during startup things like ACPI and IDE don't > > fulfill this requirement, so maybe we should require it only after > > userspace has begun running. After all, the system can't suspend > > until then.) > > > > It seems like a reasonable sort of thing to do. Hotplugged devices > > tend to be registered as they are discovered by their parent's driver, > > so it shouldn't be too much to ask that the parent's semaphore be held > > when the new device is registered. Static devices generally aren't > > quite so nice; the serial and floppy drivers in particular would need a > > little work (and probably some other drivers too). > > > > If we do this, then once the PM core has acquired the semaphore for > > every device it will be guaranteed that no new devices can be added. > > It would be a simple solution to a rather nasty problem. > > Hmm, in device_pm_add() and device_pm_remove() we acquire dpm_list_mtx which > also is acquired by device_suspend() and device_resume(). Thus, every attempt > to register a new device or unregister an existing one will be blocked while > either device_suspend() or device_resume() is running. > > If we arrange things so that dpm_list_mtx is acquired, but not released, by > device_suspend() and released, but not acquired, by device_resume(), then it > won't be possible to register/unregister a device during a suspend-resume > cycle. As with Oliver's suggestion, this would create a locking order violation. Drivers registering children (and thus acquiring dpm_list_mtx) will often already hold the parent's sem. But device_suspend() needs to acquire device sems while holding dpm_list_mtx. Alan Stern _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
