On Mon, 3 Dec 2007, Jarek Poplawski wrote:
> > 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).
I think the reasoning goes the way Arjan described. Suppose in between
#1 and #2 there is thread #3 trying to do down_write(A) and waiting for
#1. Then thread #2 doesn't have to wait for #1 directly, but it would
have to wait for #3.
In my case the simplest answer appears to be the replace the rwsem
with something slightly more complicated (a mutex plus a boolean flag
-- the loss of concurrency won't matter much since it isn't on a hot
path).
Thanks for the comment.
Alan Stern
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