From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752117AbbEDJ1Z (ORCPT ); Mon, 4 May 2015 05:27:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55256 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751046AbbEDJ1S (ORCPT ); Mon, 4 May 2015 05:27:18 -0400 Message-ID: <55473B47.6080600@redhat.com> Date: Mon, 04 May 2015 11:26:31 +0200 From: Paolo Bonzini User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0 MIME-Version: 1.0 To: Ingo Molnar , Andy Lutomirski CC: Rik van Riel , "linux-kernel@vger.kernel.org" , X86 ML , williams@redhat.com, Andrew Lutomirski , fweisbec@redhat.com, Peter Zijlstra , Heiko Carstens , Thomas Gleixner , Ingo Molnar , "Paul E. McKenney" , Linus Torvalds Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry References: <554399D1.6010405@redhat.com> <20150501155912.GA451@gmail.com> <20150501162109.GA1091@gmail.com> <5543A94B.3020108@redhat.com> <20150501163431.GB1327@gmail.com> <5543C05E.9040209@redhat.com> <20150501184025.GA2114@gmail.com> <5543CFE5.1030509@redhat.com> <20150502052733.GA9983@gmail.com> In-Reply-To: <20150502052733.GA9983@gmail.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/05/2015 07:27, Ingo Molnar wrote: > > - synchronize_rcu() avoids having to send an IPI by taking a > peak at rq->curr's pt_regs::flag, and if: > > - the flag is 0 then it has observed a quiescent state. > > - the flag is 1, then it would set TIF_NOHZ and wait for a > completion from a TIF_NOHZ callback. Isn't this racy? synchronize_rcu CPU nohz CPU --------------------------------------------------------- set flag = 0 read flag = 0 return to userspace set TIF_NOHZ and there's no guarantee that TIF_NOHZ is ever processed by the nohz CPU. Paolo