From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753115AbZAEEaZ (ORCPT ); Sun, 4 Jan 2009 23:30:25 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752131AbZAEEaN (ORCPT ); Sun, 4 Jan 2009 23:30:13 -0500 Received: from e6.ny.us.ibm.com ([32.97.182.146]:34520 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752114AbZAEEaL (ORCPT ); Sun, 4 Jan 2009 23:30:11 -0500 Date: Sun, 4 Jan 2009 20:30:06 -0800 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@elte.hu, andi@firstfloor.org, snakebyte@gmx.de, akpm@linux-foundation.org, dvhltc@us.ibm.com, niv@us.ibm.com, manfred@colorfullife.com, laijs@cn.fujitsu.com Subject: [PATCH] Fix rcutree grace-period-latency bug on small systems Message-ID: <20090105043006.GA22049@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Kudos to Andi Kleen for finding a grace-period-latency problem! The problem was that the special-case code for small machines never updated the ->signaled field to indicate that grace-period initialization had completed, which prevented force_quiescent_state() from ever expediting grace periods. This problem resulted in grace periods extending for more than 20 seconds. Not subtle. I introduced this bug during my inspection process when I fixed a race between grace-period initialization and force_quiescent_state() execution. The following patch properly updates the ->signaled field for the "small"-system case (no more than 32 CPUs for 32-bit kernels and no more than 64 CPUs for 64-bit kernels). I believe that this patch should be included for 2.6.29. Located-by: Andi Kleen Tested-by: Andi Kleen Signed-off-by: Paul E. McKenney --- rcutree.c | 1 + 1 file changed, 1 insertion(+) diff --git a/kernel/rcutree.c b/kernel/rcutree.c index a342b03..88d921c 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -572,6 +572,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags) /* Special-case the common single-level case. */ if (NUM_RCU_NODES == 1) { rnp->qsmask = rnp->qsmaskinit; + rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state OK. */ spin_unlock_irqrestore(&rnp->lock, flags); return; }