Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S946285AbXHMOHO (ORCPT ); Mon, 13 Aug 2007 10:07:14 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S943912AbXHMLEh (ORCPT ); Mon, 13 Aug 2007 07:04:37 -0400 Received: from mx1.redhat.com ([66.187.233.31]:40963 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S943846AbXHMLEd (ORCPT ); Mon, 13 Aug 2007 07:04:33 -0400 Date: Mon, 13 Aug 2007 07:04:15 -0400 From: Chris Snook To: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Linus Torvalds Cc: akpm@linux-foundation.org, paulmck@linux.vnet.ibm.com, Segher Boessenkool , "Luck, Tony" , Chris Friesen , "Robert P. J. Day" Subject: [PATCH 1/23] document preferred use of volatile with atomic_t Message-ID: <20070813110415.GA24018@shell.boston.redhat.com> References: <46C03885.7000109@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <46C03885.7000109@redhat.com> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1670 Lines: 45 From: Chris Snook Document proper use of volatile for atomic_t operations. Signed-off-by: Chris Snook --- linux-2.6.23-rc3-orig/Documentation/atomic_ops.txt 2007-07-08 19:32:17.000000000 -0400 +++ linux-2.6.23-rc3/Documentation/atomic_ops.txt 2007-08-13 03:36:43.000000000 -0400 @@ -12,13 +12,20 @@ C integer type will fail. Something like the following should suffice: - typedef struct { volatile int counter; } atomic_t; + typedef struct { int counter; } atomic_t; + + Historically, counter has been declared as a volatile int. This +is now discouraged in favor of explicitly casting it as volatile where +volatile behavior is required. Most architectures will only require such +a cast in atomic_read() and atomic_set(), as well as their 64-bit versions +if applicable, since the more complex atomic operations directly or +indirectly use assembly that results in volatile behavior. The first operations to implement for atomic_t's are the initializers and plain reads. #define ATOMIC_INIT(i) { (i) } - #define atomic_set(v, i) ((v)->counter = (i)) + #define atomic_set(v, i) (*(volatile int *)&(v)->counter = (i)) The first macro is used in definitions, such as: @@ -38,7 +45,7 @@ Next, we have: - #define atomic_read(v) ((v)->counter) + #define atomic_read(v) (*(volatile int *)&(v)->counter) which simply reads the current value of the counter. - 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/