linux-kernel - Re: rcu: BUG: spinlock recursion on CPU#3, trinity-child19/5970

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Date:	Mon, 2 Jul 2012 04:35:41 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Sasha Levin <levinsasha928@...il.com>,
	Dave Jones <davej@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: rcu: BUG: spinlock recursion on CPU#3, trinity-child19/5970

On Mon, Jul 02, 2012 at 12:32:19PM +0200, Peter Zijlstra wrote:
> On Fri, 2012-06-29 at 23:40 +0200, Sasha Levin wrote:
> > 
> > The bad news are that what you saw was a lockdep enabled run
> > (CONFIG_PROVE_RCU is enabled, and lockdep was enabled). There were no
> > lockdep warnings at any point while reproducing it. 
> 
> rcu_switch_from() -> rcu_preempt_note_context_switch() is called _after_
> the lockdep context switch annotation. Therefore lockdep things the prev
> task isn't holding any locks anymore.

In other words, moving rcu_preempt_note_context_switch() deeper into
the scheduler is totally bogus, and I need to move it back out into the
original rcu_note_context_switch(), reverting 616c310e (Move PREEMPT_RCU
preemption to switch_to() invocation), correct?

Sigh.  "It seemed like a good idea at the time."

							Thanx, Paul

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