Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S938820AbXHIOdf (ORCPT ); Thu, 9 Aug 2007 10:33:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S940466AbXHIOdA (ORCPT ); Thu, 9 Aug 2007 10:33:00 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:36521 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S939883AbXHIOc6 (ORCPT ); Thu, 9 Aug 2007 10:32:58 -0400 Date: Thu, 9 Aug 2007 07:32:55 -0700 From: "Paul E. McKenney" To: Chris Snook Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, torvalds@linux-foundation.org, netdev@vger.kernel.org, akpm@linux-foundation.org, ak@suse.de, heiko.carstens@de.ibm.com, davem@davemloft.net, schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au, wjiang@resilience.com, cfriesen@nortel.com, zlynx@acm.org, rpjday@mindspring.com, jesper.juhl@gmail.com Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha Message-ID: <20070809143255.GA8424@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20070809132442.GA13042@shell.boston.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070809132442.GA13042@shell.boston.redhat.com> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2512 Lines: 63 On Thu, Aug 09, 2007 at 09:24:42AM -0400, Chris Snook wrote: > From: Chris Snook > > Purify volatile use for atomic[64]_t on alpha. Why not the same access-once semantics for atomic_set() as for atomic_read()? As this patch stands, it might introduce architecture-specific compiler-induced bugs due to the fact that atomic_set() used to imply volatile behavior but no longer does. See below for one approach to fixing this. > Signed-off-by: Chris Snook > > --- linux-2.6.23-rc2-orig/include/asm-alpha/atomic.h 2007-07-08 19:32:17.000000000 -0400 > +++ linux-2.6.23-rc2/include/asm-alpha/atomic.h 2007-08-09 09:19:00.000000000 -0400 > @@ -14,18 +14,22 @@ > > > /* > - * Counter is volatile to make sure gcc doesn't try to be clever > - * and move things around on us. We need to use _exactly_ the address > - * the user gave us, not some alias that contains the same information. > + * Make sure gcc doesn't try to be clever and move things around > + * on us. We need to use _exactly_ the address the user gave us, > + * not some alias that contains the same information. > */ > -typedef struct { volatile int counter; } atomic_t; > -typedef struct { volatile long counter; } atomic64_t; > +typedef struct { int counter; } atomic_t; > +typedef struct { long counter; } atomic64_t; > > #define ATOMIC_INIT(i) ( (atomic_t) { (i) } ) > #define ATOMIC64_INIT(i) ( (atomic64_t) { (i) } ) > > -#define atomic_read(v) ((v)->counter + 0) > -#define atomic64_read(v) ((v)->counter + 0) > +/* > + * Casting to volatile here minimizes the need for barriers, > + * without having to declare the type itself as volatile. > + */ > +#define atomic_read(v) (*(volatile int *)&(v)->counter + 0) > +#define atomic64_read(v) (*(volatile long *)&(v)->counter + 0) > > #define atomic_set(v,i) ((v)->counter = (i)) > #define atomic64_set(v,i) ((v)->counter = (i)) How about: #define atomic_set(v,i) ((*(volatile int *)&(v)->counter) = (i)) #define atomic64_set(v,i) ((*(volatile long *)&(v)->counter) = (i)) Thanx, Paul > - > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/