From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752500AbaBBVBa (ORCPT ); Sun, 2 Feb 2014 16:01:30 -0500 Received: from g1t0027.austin.hp.com ([15.216.28.34]:30436 "EHLO g1t0027.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752435AbaBBVB3 (ORCPT ); Sun, 2 Feb 2014 16:01:29 -0500 Message-ID: <1391374883.3164.8.camel@j-VirtualBox> Subject: Re: [RFC][PATCH v2 5/5] mutex: Give spinners a chance to spin_on_owner if need_resched() triggered while queued From: Jason Low To: Peter Zijlstra Cc: Ingo Molnar , Paul McKenney , Waiman Long , Linus Torvalds , Thomas Gleixner , Linux Kernel Mailing List , Rik van Riel , Andrew Morton , Davidlohr Bueso , "H. Peter Anvin" , Andi Kleen , "Chandramouleeswaran, Aswin" , "Norton, Scott J" , chegu_vinod@hp.com Date: Sun, 02 Feb 2014 13:01:23 -0800 In-Reply-To: <20140131200825.GS5002@laptop.programming.kicks-ass.net> References: <1390936396-3962-1-git-send-email-jason.low2@hp.com> <1390936396-3962-6-git-send-email-jason.low2@hp.com> <20140128210753.GJ11314@laptop.programming.kicks-ass.net> <1390949495.2807.52.camel@j-VirtualBox> <20140129115142.GE9636@twins.programming.kicks-ass.net> <1391138977.6284.82.camel@j-VirtualBox> <20140131140941.GF4941@twins.programming.kicks-ass.net> <20140131200825.GS5002@laptop.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2014-01-31 at 21:08 +0100, Peter Zijlstra wrote: > On Fri, Jan 31, 2014 at 12:01:37PM -0800, Jason Low wrote: > > Currently still getting soft lockups with the updated version. > > Bugger.. ok clearly I need to think harder still. I'm fairly sure this > cancelation can work though, just seems tricky to get right :-) Ok, I believe I have found a race condition between m_spin_lock() and m_spin_unlock(). In m_spin_unlock(), we do "next = ACCESS_ONCE(node->next)". Then, if next is not NULL, we proceed to set next->locked to 1. A thread in m_spin_lock() in the unqueue path could execute "next = cmpxchg(&prev->next, node, NULL)" after the thread in m_spin_unlock() accesses its node->next and finds that it is not NULL. Then, the thread in m_spin_lock() could check !node->locked before the thread in m_spin_unlock() sets next->locked to 1. The following addition change was able to solve the initial lockups that were occurring when running fserver on a 2 socket box. --- diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 9eb4dbe..e71a84a 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -513,8 +513,13 @@ static void m_spin_unlock(struct m_spinlock **lock) return; next = ACCESS_ONCE(node->next); - if (unlikely(next)) - break; + + if (unlikely(next)) { + next = cmpxchg(&node->next, next, NULL); + + if (next) + break; + } arch_mutex_cpu_relax(); }