From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965040AbbKDK2G (ORCPT ); Wed, 4 Nov 2015 05:28:06 -0500 Received: from casper.infradead.org ([85.118.1.10]:45919 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964845AbbKDK2E (ORCPT ); Wed, 4 Nov 2015 05:28:04 -0500 Date: Wed, 4 Nov 2015 11:28:00 +0100 From: Peter Zijlstra To: Dave Jones , Linux Kernel , Ingo Molnar , Stephane Eranian Cc: Paul McKenney , Andi Kleen Subject: Re: perf related lockdep bug Message-ID: <20151104102800.GZ11639@twins.programming.kicks-ass.net> References: <20151104051717.GA6098@codemonkey.org.uk> <20151104102151.GG17308@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151104102151.GG17308@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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();