From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933589AbXGQOtw (ORCPT ); Tue, 17 Jul 2007 10:49:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753584AbXGQOtp (ORCPT ); Tue, 17 Jul 2007 10:49:45 -0400 Received: from mail.screens.ru ([213.234.233.54]:34557 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753287AbXGQOtp (ORCPT ); Tue, 17 Jul 2007 10:49:45 -0400 Date: Tue, 17 Jul 2007 18:51:18 +0400 From: Oleg Nesterov To: Andrew Morton Cc: Chris Snook , Ingo Molnar , linux-kernel@vger.kernel.org Subject: [PATCH, RESEND] sys_time: use barrier() instead of smp_rmb() Message-ID: <20070717145118.GA286@tv-sign.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org >>From the recent 4e44f3497d41db4c3b9051c61410dee8ae4fb49c commit, > asmlinkage long sys_time(time_t __user * tloc) > { > - time_t i; > - struct timeval tv; > + /* > + * We read xtime.tv_sec atomically - it's updated > + * atomically by update_wall_time(), so no need to > + * even read-lock the xtime seqlock: > + */ > + time_t i = xtime.tv_sec; > > - do_gettimeofday(&tv); > - i = tv.tv_sec; > + smp_rmb(); /* sys_time() results are coherent */ The is no reason to use rmb(), we don't do any LOADs which could be reordered. What we need is a simple compiler barrier, to prevent the reading of the "volatile" xtime.tv_sec twice. Signed-off-by: Oleg Nesterov --- t/kernel/time.c~ 2007-06-26 16:28:59.000000000 +0400 +++ t/kernel/time.c 2007-06-26 16:32:09.000000000 +0400 @@ -64,7 +64,7 @@ asmlinkage long sys_time(time_t __user * */ time_t i = xtime.tv_sec; - smp_rmb(); /* sys_time() results are coherent */ + barrier(); /* sys_time() results are coherent */ if (tloc) { if (put_user(i, tloc))