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
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