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Subject: Re: INFO: possible circular locking dependency at
 cleanup_workqueue_thread
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Johannes Berg <johannes@sipsolutions.net>, Ingo Molnar <mingo@elte.hu>,
       Zdenek Kabelac <zdenek.kabelac@gmail.com>,
       "Rafael J. Wysocki" <rjw@sisk.pl>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Date: Mon, 18 May 2009 22:00:57 +0200
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On Mon, 2009-05-18 at 21:47 +0200, Oleg Nesterov wrote:
> On 05/17, Johannes Berg wrote:
> >
> > I'm not entirely sure yet, but I would think the problem might be a
> > false positive in the workqueue code -- remember this report only
> > triggers because cleanup_workqueue_thread() acquires the fake lock for
> > the workqueue.
> 
> I spent a lot of time, but I can't explain this report too :( Even
> if it is false positive, I don't understand why lockdep complains.
> 
> > Maybe it shouldn't do that from the CPU_POST_DEAD
> > notifier?
> 
> Well, in any case we should understand why we have the problem, before
> changing the code. And CPU_POST_DEAD is not special, why should we treat
> it specially and skip lock_map_acquire(wq->lockdep_map) ?
> 
> 
> But, I am starting to suspect we have some problems with lockdep too.
> OK, I can't explain what I mean... But consider this code:
> 
> 	DEFINE_SPINLOCK(Z);
> 	DEFINE_SPINLOCK(L1);
> 	DEFINE_SPINLOCK(L2);
> 
> 	#define L(l)	spin_lock(&l)
> 	#define U(l)	spin_unlock(&l)
> 
> 	void t1(void)
> 	{
> 		L(L1);
> 		L(L2);
> 
> 		U(L2);
> 		U(L1);
> 	}

(1) L1 -> L2  

> 	void t2(void)
> 	{
> 		L(L2);
> 			L(Z);

(2) L2 -> Z

> 		L(L1);

(3) Z -> L1

> 		U(L1);
> 			U(Z);
> 		U(L2);
> 	}
> 
> 	void tst(void)
> 	{
> 		t1();
> 		t2();
> 	}
> 
> We have the trivial AB-BA deadlock with L1 and L2, but lockdep says:
> 
> 	[ INFO: possible circular locking dependency detected ]
> 	2.6.30-rc6-00043-g22ef37e-dirty #3
> 	-------------------------------------------------------
> 	perl/676 is trying to acquire lock:
> 	 (L1){+.+...}, at: [<ffffffff802522b8>] t2+0x28/0x50
> 
> 	but task is already holding lock:
> 	 (Z){+.+...}, at: [<ffffffff802522ac>] t2+0x1c/0x50
> 
> 	which lock already depends on the new lock.
> 
> 
> 	the existing dependency chain (in reverse order) is:
> 
> 	-> #2 (Z){+.+...}:
> 
> 	-> #1 (L2){+.+...}:
> 
> 	-> #0 (L1){+.+...}:
> 
> 	other info that might help us debug this:
> 
> 	2 locks held by perl/676:
> 	 #0:  (L2){+.+...}, at: [<ffffffff802522a0>] t2+0x10/0x50
> 	 #1:  (Z){+.+...}, at: [<ffffffff802522ac>] t2+0x1c/0x50
> 
> This output looks obviously wrong, Z does not depend on L1 or any
> other lock.

It does, L1 -> L2 -> Z as per 1 and 2
which 3 obviously reverses.