Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764542AbYBWVEW (ORCPT ); Sat, 23 Feb 2008 16:04:22 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1761703AbYBWVEA (ORCPT ); Sat, 23 Feb 2008 16:04:00 -0500 Received: from x346.tv-sign.ru ([89.108.83.215]:38046 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758201AbYBWVD7 (ORCPT ); Sat, 23 Feb 2008 16:03:59 -0500 Date: Sun, 24 Feb 2008 00:03:29 +0300 From: Oleg Nesterov To: Linus Torvalds Cc: Dmitry Adamushko , Andrew Morton , Linux Kernel Mailing List , a.p.zijlstra@chello.nl, apw@shadowen.org, Ingo Molnar , nickpiggin@yahoo.com.au, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, Steven Rostedt , linux-arch@vger.kernel.org Subject: [PATCH, 3rd resend] documentation: atomic_add_unless() doesn't imply mb() on failure Message-ID: <20080223210329.GA221@tv-sign.ru> References: <200802230733.m1N7XnMu018253@imap1.linux-foundation.org> <20080223162705.GA7686@tv-sign.ru> <20080223182258.GA19946@tv-sign.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1831 Lines: 44 (sorry for being offtpoic, but while experts are here...) A "typical" implementation of atomic_add_unless() can return 0 immediately after the first atomic_read() (before doing cmpxchg). In that case it doesn't provide any barrier semantics. See include/asm-ia64/atomic.h as an example. We should either change the implementation, or fix the docs. Signed-off-by: Oleg Nesterov Acked-by: Nick Piggin Documentation/atomic_ops.txt | 3 ++- Documentation/memory-barriers.txt | 2 +- 2 files changed, 3 insertions(+), 2 deletions(-) --- t/Documentation/atomic_ops.txt~doc_aau 2008-01-14 23:43:11.000000000 +0300 +++ t/Documentation/atomic_ops.txt 2008-02-23 23:53:12.000000000 +0300 @@ -186,7 +186,8 @@ If the atomic value v is not equal to u, returns non zero. If v is equal to u then it returns zero. This is done as an atomic operation. -atomic_add_unless requires explicit memory barriers around the operation. +atomic_add_unless requires explicit memory barriers around the operation +unless it fails (returns 0). atomic_inc_not_zero, equivalent to atomic_add_unless(v, 1, 0) --- t/Documentation/memory-barriers.txt~doc_aau 2008-01-14 23:43:11.000000000 +0300 +++ t/Documentation/memory-barriers.txt 2008-02-23 23:53:12.000000000 +0300 @@ -1493,7 +1493,7 @@ explicit lock operations, described late atomic_dec_and_test(); atomic_sub_and_test(); atomic_add_negative(); - atomic_add_unless(); + atomic_add_unless(); /* when succeeds (returns 1) */ test_and_set_bit(); test_and_clear_bit(); test_and_change_bit(); -- 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/