On 02-12-2007 20:45, Alan Stern wrote: > Ingo: > > I ran into a lockdep reporting issue just now with some new code under > development. I think it's a false positive; the question is how best > to deal with it. > > Here's the situation. The new code runs during a system sleep (i.e., > suspend or hibernation). Certain activities have to be deferred during > a system sleep, so I defined an rwsem: system_sleep_in_progress_rwsem. > > Subroutines carrying out these activities acquire a read lock on the > rwsem, so normally they proceed with no hindrance. During a sleep > transition, I acquire a write lock -- this is done via a PM-notifier > callout routine. That is, during a PM_HIBERNATION_PREPARE or > PM_SUSPEND_PREPARE notification the routine does down_write(), and > during a PM_POST_HIBERNATION or PM_POST_SUSPEND notification the > routine does up_write(). > > The problem is that the notifier chain itself is under the control of > an rwsem (to prevent the chain from being modified while it is in use). > The resulting actions look like this: > > System sleep start: > down_read(notifier-chain rwsem); > call the notifier routine > down_write(&system_sleep_in_progress_rwsem); > up_read(notifier-chain rwsem); > > System sleep end: > down_read(notifier-chain rwsem); > call the notifier routine > up_write(&system_sleep_in_progress_rwsem); > up_read(notifier-chain rwsem); > > This creates a lockdep violation; each rwsem in turn is locked while > the other is being held. However the only way this could lead to > deadlock would be if there was already a bug in the system Power > Management code (overlapping notifications). Actually, IMHO, there is no reason for any lockdep violation: thread #1: has down_read(A); waits for #2 to down_write(B) thread #2: has down_write(B); never waits for #1 to down_read(A) So, deadlock isn't possible here. If lockdep reports something else it should be fixed (and you'd be right to omit lockdep until this is done). Regards, Jarek P. PS: Peter Zijlstra added to CC _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
