From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752952Ab1GYVKU (ORCPT ); Mon, 25 Jul 2011 17:10:20 -0400 Received: from terminus.zytor.com ([198.137.202.10]:43766 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751585Ab1GYVKT (ORCPT ); Mon, 25 Jul 2011 17:10:19 -0400 Message-ID: <4E2DDBAA.60200@zytor.com> Date: Mon, 25 Jul 2011 14:10:02 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:5.0) Gecko/20110707 Thunderbird/5.0 MIME-Version: 1.0 To: Cyrill Gorcunov CC: Ian Campbell , linux-kernel@vger.kernel.org, Pekka Enberg , Andi Kleen , Ingo Molnar Subject: Re: [PATCH 0/3] minor cleanups to EFLAGS initialisation in ret_from_fork References: <1311587883.27940.20.camel@cthulhu.hellion.org.uk> <20110725101902.GP4362@sun> <20110725182049.GD27137@sun> In-Reply-To: <20110725182049.GD27137@sun> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/25/2011 11:20 AM, Cyrill Gorcunov wrote: > On Mon, Jul 25, 2011 at 02:19:02PM +0400, Cyrill Gorcunov wrote: >> On Mon, Jul 25, 2011 at 10:58:03AM +0100, Ian Campbell wrote: >>> The following series removes the use of a global kernel_eflags variable >>> from the x86_64 ret_from_fork path and (very slightly) merges the 32 and >>> 64 bit version of that code path. >>> >>> kernel_eflags could be made a __read_mostly but actually there is no >>> reason to prefer the value at cpu_init() time to a compile time constant >>> value for the initial eflags after a fork. >>> >>> Ian. >>> >> >> Thanks, Ian! I think noone against this simplification, Peter, Andi? >> >> Cyrill > > Ian, I've missed in first place that you've opened IRQs window _before_ > schedule_tail() call, ie it's not 1:1 code mapping as it was before. > > Note kernel_eflags has IF clear and what we have: the ret_from_fork on > x86-64 happens _only_ inside context_switch call, ie > > schedule (sched.c) > ... > raw_spin_lock_irq > ... > context_switch > switch_to > "jnz ret_from_fork\n\t" > pushq_cfi kernel_eflags(%rip) > popfq_cfi # reset kernel eflags > > ---> irqs are still disabled > > call schedule_tail # rdi: 'prev' task parameter > finish_lock_switch > raw_spin_unlock_irq > > I bet raw_spin_lock_irq at the beginning of the schedule() is set > for a reason and such change is not safe. Though I may be missing > something again... > This definitely doesn't look "obviously safe" to me. However, does anyone see a problem with unconditionally leaving IF disabled even on 32 bits (I haven't traced all the paths yet), i.e. doing the *opposite* of Ian's patch #2? -hpa