Re: perf related lockdep bug

[Peter Zijlstra]

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.

---
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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