From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jamie Lokier Subject: Re: barriers vs. reads Date: Tue, 22 Jun 2004 12:28:02 +0100 Sender: linux-fsdevel-owner@vger.kernel.org Message-ID: <20040622112802.GA21456@mail.shareable.org> References: <20040622005302.A1325@almesberger.net> <20040622073919.GV12881@suse.de> <20040622045004.C1325@almesberger.net> <20040622075531.GX12881@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Werner Almesberger , linux-fsdevel@vger.kernel.org Return-path: Received: from mail.shareable.org ([81.29.64.88]:9897 "EHLO mail.shareable.org") by vger.kernel.org with ESMTP id S262418AbUFVL2R (ORCPT ); Tue, 22 Jun 2004 07:28:17 -0400 To: Jens Axboe Content-Disposition: inline In-Reply-To: <20040622075531.GX12881@suse.de> List-Id: linux-fsdevel.vger.kernel.org Jens Axboe wrote: > > But do we have cases where reads must not cross write barriers ? > > To me, it's the expected behaviour. If you issue a barrier write, a read > issued later should not be able to fetch old data. Two things: 1. A read _which doesn't overlap writes before the barrier_ should be ok before the barrier with no visible change. So, look at the block numbers and permit reordering if there's no overlap. This reordering is semantically invisible. 2. Other than O_DIRECT, can the I/O subsystem issue reads that overlap writes in flight? Surely that never occurs? If it never occurs, then reads can be safely moved before write barriers without looking at block numbers. -- Jamie