From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752172Ab1GYSVA (ORCPT <rfc822;w@1wt.eu>);
	Mon, 25 Jul 2011 14:21:00 -0400
Received: from mail-ey0-f171.google.com ([209.85.215.171]:57960 "EHLO
	mail-ey0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752499Ab1GYSUz (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 25 Jul 2011 14:20:55 -0400
Date: Mon, 25 Jul 2011 22:20:50 +0400
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Ian Campbell <Ian.Campbell@citrix.com>, linux-kernel@vger.kernel.org,
        Pekka Enberg <penberg@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Andi Kleen <ak@linux.intel.com>, Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH 0/3] minor cleanups to EFLAGS initialisation in
 ret_from_fork
Message-ID: <20110725182049.GD27137@sun>
References: <1311587883.27940.20.camel@cthulhu.hellion.org.uk>
 <20110725101902.GP4362@sun>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20110725101902.GP4362@sun>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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...

	Cyrill