Re: INFO: possible circular locking dependency at cleanup_workqueue_thread

[Oleg Nesterov <oleg@redhat.com>]

On 05/18, Peter Zijlstra wrote:
>
> On Mon, 2009-05-18 at 21:47 +0200, Oleg Nesterov wrote:
> >
> > 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
> >
> >
> > 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.

Yes, yes, I see. And, as I said, I can't explain what I mean.

I mean... The output above looks as if we take L1 and Z in wrong order.
But Z has nothing to do with this deadlock, it can't depend on any lock
from the correctness pov. Except yes, we have it in L1->L2->Z->L1 cycle.

Oleg.