Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759577AbZAHRxr (ORCPT ); Thu, 8 Jan 2009 12:53:47 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754206AbZAHRxe (ORCPT ); Thu, 8 Jan 2009 12:53:34 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:35067 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753253AbZAHRxc (ORCPT ); Thu, 8 Jan 2009 12:53:32 -0500 Date: Thu, 8 Jan 2009 09:52:24 -0800 (PST) From: Linus Torvalds X-X-Sender: torvalds@localhost.localdomain To: Steven Rostedt cc: Chris Mason , Peter Zijlstra , Ingo Molnar , paulmck@linux.vnet.ibm.com, Gregory Haskins , Matthew Wilcox , Andi Kleen , Andrew Morton , Linux Kernel Mailing List , linux-fsdevel , linux-btrfs , Thomas Gleixner , Nick Piggin , Peter Morreale , Sven Dietrich Subject: Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning In-Reply-To: Message-ID: References: <1231347442.11687.344.camel@twins> <1231365115.11687.361.camel@twins> <1231366716.11687.377.camel@twins> <1231408718.11687.400.camel@twins> <20090108141808.GC11629@elte.hu> <1231426014.11687.456.camel@twins> <1231434515.14304.27.camel@think.oraclecorp.com> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1510 Lines: 39 On Thu, 8 Jan 2009, Steven Rostedt wrote: > > We keep spinning if the owner changes. I think we want to - if you have multiple CPU's and a heavily contended lock that acts as a spinlock, we still _do_ want to keep spinning even if another CPU gets the lock. And I don't even believe that is the bug. I suspect the bug is simpler. I think the "need_resched()" needs to go in the outer loop, or at least happen in the "!owner" case. Because at least with preemption, what can happen otherwise is - process A gets the lock, but gets preempted before it sets lock->owner. End result: count = 0, owner = NULL. - processes B/C goes into the spin loop, filling up all CPU's (assuming dual-core here), and will now both loop forever if they hold the kernel lock (or have some other preemption disabling thing over their down()). And all the while, process A would _happily_ set ->owner, and eventually release the mutex, but it never gets to run to do either of them so. In fact, you might not even need a process C: all you need is for B to be on the same runqueue as A, and having enough load on the other CPU's that A never gets migrated away. So "C" might be in user space. I dunno. There are probably variations on the above. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/