Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754981Ab3DVTt3 (ORCPT ); Mon, 22 Apr 2013 15:49:29 -0400 Received: from merlin.infradead.org ([205.233.59.134]:44905 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754940Ab3DVTt1 (ORCPT ); Mon, 22 Apr 2013 15:49:27 -0400 Message-ID: <1366660147.6454.6.camel@laptop> Subject: Re: Preemptable Ticket Spinlock From: Peter Zijlstra To: Rik van Riel Cc: Jiannan Ouyang , LKML , Raghavendra K T , Avi Kivity , Gleb Natapov , Ingo Molnar , Marcelo Tosatti , Srikar , "H. Peter Anvin" , "Nikunj A. Dadhania" , KVM , Thomas Gleixner , Chegu Vinod , "Andrew M. Theurer" , Srivatsa Vaddagiri , Andrew Jones , Karen Noel Date: Mon, 22 Apr 2013 21:49:07 +0200 In-Reply-To: <51753289.70406@redhat.com> References: <51745650.9050204@redhat.com> <1366631460.4443.3.camel@laptop> <51753289.70406@redhat.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.6.2-0ubuntu0.1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1671 Lines: 42 On Mon, 2013-04-22 at 08:52 -0400, Rik van Riel wrote: > On 04/22/2013 07:51 AM, Peter Zijlstra wrote: > > On Sun, 2013-04-21 at 17:12 -0400, Rik van Riel wrote: > >> > >> If we always incremented the ticket number by 2 (instead of 1), then > >> we could use the lower bit of the ticket number as the spinlock. > > > > ISTR that paravirt ticket locks already do that and use the lsb to > > indicate the unlock needs to perform wakeups. > > > > Also, since all of this is virt nonsense, shouldn't it live in the > > paravirt ticket lock code and leave the native code as is? > > Sure, but that is still no reason not to have the virt > implementation be as fast as possible, and share the same > data type as the non-virt implementation. It has to share the same data-type.. > Also, is it guaranteed that the native spin_lock code has > not been called yet before we switch over to the paravirt > functions? > > If the native spin_lock code has been called already at > that time, the native code would still need to be modified > to increment the ticket number by 2, so we end up with a > compatible value in each spin lock's .tickets field, and > prevent a deadlock after we switch over to the paravirt > variant. I thought the stuff already made it upstream, but apparently not; the lastest posting I'm aware of is here: https://lkml.org/lkml/2012/5/2/105 That stuff changes the normal ticket increment as well.. -- 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/