Re: [RFC][PATCH 3/7] lockdep: re-annotate scheduler runqueues

David Miller
2008-08-05T08:36:28Z

From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Mon, 04 Aug 2008 15:03:20 +0200

> Instead of using a per-rq lock class, use the regular nesting operations.
>
> However, take extra care with double_lock_balance() as it can release the
> already held rq->lock (and therefore change its nesting class).
>
> So what can happen is:
>
> spin_lock(rq->lock); // this rq subclass 0
>
> double_lock_balance(rq, other_rq);
> // release rq
> // acquire other_rq->lock subclass 0
> // acquire rq->lock subclass 1
>
> spin_unlock(other_rq->lock);
>
> leaving you with rq->lock in subclass 1
>
> So a subsequent double_lock_balance() call can try to nest a subclass 1
> lock while already holding a subclass 1 lock.
>
> Fix this by introducing double_unlock_balance() which releases the other
> rq's lock, but also re-sets the subclass for this rq's lock to 0.
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

Acked-by: David S. Miller <davem@davemloft.net>

I also tested this on 64-cpu and 128-cpu systems.