On Wednesday, January 26, 2011, Alan Stern wrote: > On Tue, 25 Jan 2011, Rafael J. Wysocki wrote: > > > From: Rafael J. Wysocki <rjw@xxxxxxx> > > > > The memory barrier in wakeup_source_deactivate() is supposed to > > prevent the callers of pm_wakeup_pending() and pm_get_wakeup_count() > > from seeing the new value of events_in_progress (0, in particular) > > and the old value of event_count at the same time. However, if > > wakeup_source_deactivate() is executed by CPU0 and, for instance, > > pm_wakeup_pending() is executed by CPU1, where both processors can > > reorder operations, the memory barrier in wakeup_source_deactivate() > > doesn't affect CPU1 which can reorder reads. In that case CPU1 may > > very well decide to fetch event_count before it's modified and > > events_in_progress after it's been updated, so pm_wakeup_pending() > > may fail to detect a wakeup event. This issue can be addressed by > > adding a read memory barrier in pm_wakeup_pending() that will enforce > > events_in_progress to be read before event_count. > > > > For similar reason, a read memory barrier should be added to > > pm_get_wakeup_count(). > > How come this is implemented using memory barriers rather than a lock? > Is it because this is potentially a fairly hot path? Yes, that's the reason. > New memory barriers are supposed to have comments present in the code, > explaining why they are needed. Of course I can add them. > Ideally you could do away with the need for synchronization entirely. > For example, events_in_progress and event_count could be stored as two > 16-bit values stuffed into a single atomic variable. Then they could > both be read or updated simultaneously. I thought about that too, but didn't actually implement it. Well, I guess it would be better. Thanks, Rafael _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
