From owner-freebsd-alpha Fri Dec 15 13:20:37 2000 From owner-freebsd-alpha@FreeBSD.ORG Fri Dec 15 13:20:36 2000 Return-Path: Delivered-To: freebsd-alpha@freebsd.org Received: from pike.osd.bsdi.com (pike.osd.bsdi.com [204.216.28.222]) by hub.freebsd.org (Postfix) with ESMTP id 6846C37B6B1 for ; Fri, 15 Dec 2000 13:19:31 -0800 (PST) Received: from laptop.baldwin.cx (john@jhb-laptop.osd.bsdi.com [204.216.28.241]) by pike.osd.bsdi.com (8.11.1/8.9.3) with ESMTP id eBFLJ7E28513; Fri, 15 Dec 2000 13:19:07 -0800 (PST) (envelope-from jhb@FreeBSD.org) Message-ID: X-Mailer: XFMail 1.4.0 on FreeBSD X-Priority: 3 (Normal) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8bit MIME-Version: 1.0 In-Reply-To: <20001215211930.D62048@cicely5.cicely.de> Date: Fri, 15 Dec 2000 13:19:25 -0800 (PST) From: John Baldwin To: Bernd Walter Subject: Re: mb and wmb in atomic_ Cc: freebsd-alpha@FreeBSD.org Sender: owner-freebsd-alpha@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.org On 15-Dec-00 Bernd Walter wrote: > On Fri, Dec 15, 2000 at 11:35:47AM -0800, John Baldwin wrote: >> >> On 15-Dec-00 Bernd Walter wrote: >> > Why are the mb and wmb operations needed in the atomic_ functions? >> > If I understood it correctly the locked operations are in synced >> > with others CPUs and there is no memory operation beside the variable >> > itself. >> >> They should probably only be used with the 'acq' and 'rel' variants. Hmm, >> btw, >> it looks like I have the order of the barriers in the 'acq' and 'rel' >> variants >> wrong. The barriers should be on the inside, not the outside. Anyone >> disagree? > > Depending on atomic(9) I agree. > I don't asume the acq variant realy needs one as the locked operation > should be finished at once at least out of the CPUs thus no post operation > has the chance to make anything before. > Do we have the situation anywhere that the bus reorders memory access? > Do we care about this? Can the CPU perform out-of-order execution though? And out-of-order memory accesses as a result? That is what memory barriers really protect you against. -- John Baldwin -- http://www.FreeBSD.org/~jhb/ PGP Key: http://www.baldwin.cx/~john/pgpkey.asc "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-alpha" in the body of the message