From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner+w=401wt.eu-S938794AbXHIJys@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S938794AbXHIJys (ORCPT <rfc822;w@1wt.eu>);
	Thu, 9 Aug 2007 05:54:48 -0400
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754963AbXHIJyl
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Thu, 9 Aug 2007 05:54:41 -0400
Received: from ns.suse.de ([195.135.220.2]:36138 "EHLO mx1.suse.de"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1762800AbXHIJyk (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Thu, 9 Aug 2007 05:54:40 -0400
From: Andi Kleen <ak@suse.de>
Organization: SUSE Linux Products GmbH, Nuernberg, GF: Markus Rex, HRB 16746 (AG Nuernberg)
To: Nick Piggin <npiggin@suse.de>
Subject: Re: [patch 2/2] x86_64: ticket lock spinlock
Date: Thu, 9 Aug 2007 11:54:34 +0200
User-Agent: KMail/1.9.6
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Ingo Molnar <mingo@elte.hu>, linux-arch@vger.kernel.org,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <20070808042234.GE11018@wotan.suse.de> <200708081226.55651.ak@suse.de> <20070809014254.GB12539@wotan.suse.de>
In-Reply-To: <20070809014254.GB12539@wotan.suse.de>
MIME-Version: 1.0
Content-Type: text/plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
Message-Id: <200708091154.34581.ak@suse.de>
Sender: linux-kernel-owner@vger.kernel.org
X-Mailing-List: linux-kernel@vger.kernel.org

On Thursday 09 August 2007 03:42:54 Nick Piggin wrote:
> On Wed, Aug 08, 2007 at 12:26:55PM +0200, Andi Kleen wrote:
> > 
> > >   *
> > >   * (the type definitions are in asm/spinlock_types.h)
> > >   */
> > >  
> > > +#if (NR_CPUS > 256)
> > > +#error spinlock supports a maximum of 256 CPUs
> > > +#endif
> > > +
> > >  static inline int __raw_spin_is_locked(raw_spinlock_t *lock)
> > >  {
> > > -	return *(volatile signed int *)(&(lock)->slock) <= 0;
> > > +	int tmp = *(volatile signed int *)(&(lock)->slock);
> > 
> > Why is slock not volatile signed int in the first place? 
> 
> Don't know really. Why does spin_is_locked need it to be volatile?

I suppose in case a caller doesn't have a memory barrier
(they should in theory, but might not). Without any barrier
or volatile gcc might optimize it away.

The other accesses in spinlocks hopefully all have barriers.

Ok anyways the patches look good.

-Andi