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

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Message-Id: <20080805.013556.194909428.davem@davemloft.net>
Date:	Tue, 05 Aug 2008 01:35:56 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	a.p.zijlstra@...llo.nl
Cc:	torvalds@...ux-foundation.org, jeremy@...p.org, hugh@...itas.com,
	mingo@...e.hu, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, davej@...hat.com
Subject: Re: [RFC][PATCH 3/7] lockdep: re-annotate scheduler runqueues

From: Peter Zijlstra <a.p.zijlstra@...llo.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@...llo.nl>

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

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