From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Snook Subject: Re: [PATCH] make atomic_t volatile on all architectures Date: Thu, 09 Aug 2007 03:31:10 -0400 Message-ID: <46BAC2BE.1090106@redhat.com> References: <20070808230733.GA17270@shell.boston.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: akpm@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: Linus Torvalds Return-path: Received: from mx1.redhat.com ([66.187.233.31]:35706 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1764889AbXHIHiQ (ORCPT ); Thu, 9 Aug 2007 03:38:16 -0400 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Linus Torvalds wrote: > > On Wed, 8 Aug 2007, Chris Snook wrote: >> 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. > > I'd be *much* happier with "atomic_read()" doing the "volatile" instead. > > The fact is, volatile on data structures is a bug. It's a wart in the C > language. It shouldn't be used. > > Volatile accesses in *code* can be ok, and if we have "atomic_read()" > expand to a "*(volatile int *)&(x)->value", then I'd be ok with that. > > But marking data structures volatile just makes the compiler screw up > totally, and makes code for initialization sequences etc much worse. > > Linus Fair enough. Casting to (volatile int *) will give us the behavior people expect when using atomic_t without needing to use inefficient barriers. While we have the hood up, should we convert all the atomic_t's to non-volatile and put volatile casts in all the atomic_reads? I don't know enough about the various arches to say with confidence that those changes alone will preserve existing behavior. We might need some arch-specific tweaking of the atomic operations. -- Chris