From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Jesper Juhl" Subject: Re: [PATCH] make atomic_t volatile on all architectures Date: Thu, 9 Aug 2007 01:18:22 +0200 Message-ID: <9a8748490708081618p43cdcdfdn5de6f292247def5b@mail.gmail.com> References: <20070808230733.GA17270@shell.boston.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: akpm@linux-foundation.org, torvalds@linux-foundation.org, ak@suse.de, heiko.carstens@de.ibm.com, davem@davemloft.net, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au, wjiang@resilience.com, cfriesen@nortel.com, zlynx@acm.org To: "Chris Snook" Return-path: Received: from el-out-1112.google.com ([209.85.162.176]:23299 "EHLO el-out-1112.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936379AbXHHXSY (ORCPT ); Wed, 8 Aug 2007 19:18:24 -0400 Received: by el-out-1112.google.com with SMTP id j27so86630elf for ; Wed, 08 Aug 2007 16:18:23 -0700 (PDT) In-Reply-To: <20070808230733.GA17270@shell.boston.redhat.com> Content-Disposition: inline Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 09/08/2007, Chris Snook wrote: > From: Chris Snook > > Some architectures currently do not declare the contents of an atomic_t to be > volatile. This causes confusion since atomic_read() might not actually read > anything if an optimizing compiler re-uses a value stored in a register, which > can break code that loops until something external changes the value of an > atomic_t. Avoiding such bugs requires using barrier(), which causes re-loads > of all registers used in the loop, thus hurting performance instead of helping > it, particularly on architectures where it's unnecessary. Since we generally > want to re-read the contents of an atomic variable on every access anyway, > let's standardize the behavior across all architectures and avoid the > performance and correctness problems of requiring the use of barrier() in > loops that expect atomic_t variables to change externally. This is relevant > even on non-smp architectures, since drivers may use atomic operations in > interrupt handlers. > > Signed-off-by: Chris Snook > Hmm, I thought we were trying to move away from volatile since it is very weakly defined and towards explicit barriers and locks... Points to --> Documentation/volatile-considered-harmful.txt -- Jesper Juhl Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html Plain text mails only, please http://www.expita.com/nomime.html