Re: perf related lockdep bug

[Paul E. McKenney]

On Wed, Nov 04, 2015 at 03:34:19PM +0100, Peter Zijlstra wrote:
> 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@xxxxxxxxxxxxx>

Thank you, applied as shown below.

							Thanx, Paul

------------------------------------------------------------------------

commit 05faf451f1239a28fcd63bf4b66c0db57d7b13f9
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date:   Wed Nov 4 08:22:05 2015 -0800

    rcu: Move wakeup out from under rnp->lock
    
    This patch removes a potential deadlock hazard by moving the
    wake_up_process() in rcu_spawn_gp_kthread() out from under rnp->lock.
    
    Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index caf3651fa5c9..183445959d00 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4323,8 +4323,8 @@ static int __init rcu_spawn_gp_kthread(void)
 			sp.sched_priority = kthread_prio;
 			sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
 		}
-		wake_up_process(t);
 		raw_spin_unlock_irqrestore(&rnp->lock, flags);
+		wake_up_process(t);
 	}
 	rcu_spawn_nocb_kthreads();
 	rcu_spawn_boost_kthreads();

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