From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner+willy=40w.ods.org-S1750987AbWGDJU5@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1750987AbWGDJU5 (ORCPT <rfc822;willy@w.ods.org>);
	Tue, 4 Jul 2006 05:20:57 -0400
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751242AbWGDJU4
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Tue, 4 Jul 2006 05:20:56 -0400
Received: from aun.it.uu.se ([130.238.12.36]:54230 "EHLO aun.it.uu.se")
	by vger.kernel.org with ESMTP id S1751245AbWGDJU4 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Tue, 4 Jul 2006 05:20:56 -0400
Date: Tue, 4 Jul 2006 11:05:22 +0200 (MEST)
Message-Id: <200607040905.k6495M2l028360@harpo.it.uu.se>
From: Mikael Pettersson <mikpe@it.uu.se>
To: arjan@infradead.org, eranian@hpl.hp.com
Subject: Re: [PATCH 1/2] x86-64 TIF flags for debug regs and io bitmap in ctxsw
Cc: ak@suse.de, linux-kernel@vger.kernel.org
Sender: linux-kernel-owner@vger.kernel.org
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, 4 Jul 2006 01:14:13 -0700, Stephane Eranian wrote:
>On Tue, Jul 04, 2006 at 09:51:49AM +0200, Arjan van de Ven wrote:
>> > -		}
>> > -	}
>> > +	if (unlikely((task_thread_info(next_p)->flags & _TIF_WORK_CTXSW))
>> > +	    || test_tsk_thread_flag(prev_p, TIF_IO_BITMAP))
>> > +		__switch_to_xtra(prev_p, next_p, tss);
>> 
>> well isn't this replacing an if() (which isn't cheap but also not
>> expensive, due to unlikely()) with an atomic operation (which *is*
>> expensive) ?
>> 
>Andi is right. I double checked the test_tsk_thread_flag() and it does not
>use atomic ops.

The test_tsk_thread_flag() does not, but what about all the
other places in the patch where currently unsychronised loads
or stores of ->io_bitmap_ptr or ->debugreg7 get replaced or
extended with locked-on-SMP {set,clear}_{tsk_,}thread_flag()
operations?

They should all just be plain C bitops (&, |, ^, etc).