From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kyle Moffett Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures Date: Mon, 10 Sep 2007 15:59:06 -0400 Message-ID: References: <18115.52863.638655.658466@cargo.ozlabs.ibm.com> <200709101516.03234.vda.linux@googlemail.com> <200709101746.34513.vda.linux@googlemail.com> Mime-Version: 1.0 (Apple Message framework v752.2) Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed Content-Transfer-Encoding: 7bit Cc: Linus Torvalds , Arjan van de Ven , Nick Piggin , Satyam Sharma , Herbert Xu , Paul Mackerras , Christoph Lameter , Chris Snook , Ilpo Jarvinen , "Paul E. McKenney" , Stefan Richter , Linux Kernel Mailing List , linux-arch@vger.kernel.org, Netdev , Andrew Morton , ak@suse.de, heiko.carstens@de.ibm.com, David Miller , 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, segher@kernel.crashing To: Denys Vlasenko Return-path: Received: from smtpoutm.mac.com ([17.148.16.73]:55877 "EHLO smtpoutm.mac.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754063AbXIJUCh (ORCPT ); Mon, 10 Sep 2007 16:02:37 -0400 In-Reply-To: <200709101746.34513.vda.linux@googlemail.com> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Sep 10, 2007, at 12:46:33, Denys Vlasenko wrote: > My point is that people are confused as to what atomic_read() > exactly means, and this is bad. Same for cpu_relax(). First one > says "read", and second one doesn't say "barrier". Q&A: Q: When is it OK to use atomic_read()? A: You are asking the question, so never. Q: But I need to check the value of the atomic at this point in time... A: Your code is buggy if it needs to do that on an atomic_t for anything other than debugging or optimization. Use either atomic_*_return() or a lock and some normal integers. Q: "So why can't the atomic_read DTRT magically?" A: Because "the right thing" depends on the situation and is usually best done with something other than atomic_t. If somebody can post some non-buggy code which is correctly using atomic_read() *and* depends on the compiler generating extra nonsensical loads due to "volatile" then the issue *might* be reconsidered. This also includes samples of code which uses atomic_read() and needs memory barriers (so that we can fix the buggy code, not so we can change atomic_read()). So far the only code samples anybody has posted are buggy regardless of whether or not the value and/or accessors are flagged "volatile" or not. And hey, maybe the volatile ops *should* be implemented in inline ASM for future- proof-ness, but that's a separate issue. Cheers, Kyle Moffett