linux-kernel - Re: perf related lockdep bug

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Date:	Wed, 4 Nov 2015 15:34:19 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Dave Jones <davej@...emonkey.org.uk>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Stephane Eranian <eranian@...il.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: perf related lockdep bug

On Wed, Nov 04, 2015 at 06:01:33AM -0800, Paul E. McKenney wrote:
> On Wed, Nov 04, 2015 at 11:28:00AM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 04, 2015 at 11:21:51AM +0100, Peter Zijlstra wrote:
> > 
> > > The problem appears to be due to the new RCU expedited grace period
> > > stuff, with rcu_read_unlock() now randomly trying to acquire locks it
> > > previously didn't.
> > > 
> > > Lemme go look at those rcu bits again..
> > 
> > Paul, I think this is because of:
> > 
> >   8203d6d0ee78 ("rcu: Use single-stage IPI algorithm for RCU expedited grace period")
> > 
> > What happens is that the IPI comes in and tags any random
> > rcu_read_unlock() with the special bit, which then goes on and takes
> > locks.
> > 
> > Now the problem is that we have scheduler activity inside this lock;
> > the one reported lockdep seems easy enough to fix, see below.
> > 
> > I'll got and see if there's more sites than can cause this.
> 
> This one only happens during boot time, but it would be good hygiene
> in any case.  May I have your SOB on this?

Of course,

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
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