Re: perf related lockdep bug

[Paul E. McKenney]

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?

								Thanx, Paul

> ---
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index f07343b54fe5..a9c57b386258 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4333,8 +4333,8 @@ 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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