Re: INFO: possible circular locking dependency atcleanup_workqueue_thread

[Peter Zijlstra]

On Mon, 2009-05-18 at 22:16 +0200, Oleg Nesterov wrote:
> 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.

AB-BC-CA deadlock

Thread 1		Thread 2		Thread 3

L(L1)
			L(L2)
						L(Z)
L(L2)
			L(Z)
						L(L1)

And you're saying, we can't have that deadlock because we don't have the
3 separate functions?

That is, there is no concurrency on Z because its always taken under L2?

For those situations we have the spin_lock_nest_lock(X, y) annotation,
where we say, there cannot be any concurrency on x element of X, because
all such locks are always taken under y.

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