Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763609AbXJQJRP (ORCPT ); Wed, 17 Oct 2007 05:17:15 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753472AbXJQJQ6 (ORCPT ); Wed, 17 Oct 2007 05:16:58 -0400 Received: from brick.kernel.dk ([87.55.233.238]:20066 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751830AbXJQJQ5 (ORCPT ); Wed, 17 Oct 2007 05:16:57 -0400 Date: Wed, 17 Oct 2007 11:16:29 +0200 From: Jens Axboe To: David Miller Cc: fujita.tomonori@lab.ntt.co.jp, linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org Subject: Re: [PATCH] SPARC64: fix iommu sg chaining Message-ID: <20071017091629.GK5043@kernel.dk> References: <20071017.013325.74747630.davem@davemloft.net> <20071017.014211.41637735.davem@davemloft.net> <20071017084528.GI5043@kernel.dk> <20071017.021305.74746838.davem@davemloft.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20071017.021305.74746838.davem@davemloft.net> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1932 Lines: 56 On Wed, Oct 17 2007, David Miller wrote: > From: Jens Axboe > Date: Wed, 17 Oct 2007 10:45:28 +0200 > > > Righto, it's invalid to call sg_next() on the last entry! > > Unfortunately, that's what the sparc64 code wanted to do, this > transformation in the sparc64 sg chaining patch is not equilavent: > > - struct scatterlist *sg_end = sg + nelems; > + struct scatterlist *sg_end = sg_last(sg, nelems); > ... > - while (sg < sg_end && > + while (sg != sg_end && Auch indeed. That'd probably be better as a do { ... } while (sg != sg_end); > No, that's not what the code was doing. The while loop > has to process the last entry in the list, > > We really needed "sg_end" to be "one past the last element", > rather than "the last element". > > Since you say that sg_next() on the last entry is illegal, > and that's what this code would have done to try and reach > loop termination (it doesn't actually derefrence that > "end plus one" scatterlist entry) I'll try to code this up > some other way. > > Besides, sg_last() is so absurdly expensive, it has to walk the entire > chain in the chaining case. So better to implement this without it. It is, sg_last() should really not be used a lot since it'll leaf through the entire sg list. People should either keep count of the number of entries so that they know when they are dealing with the last valid entry. Or use the for_each_sg() loop helper, if possible. Drivers are usually very simple, the iommu code does more sg tricks and thus is more complex to audit. > I would suggest that other sg_last() uses be audited for the same bug. Agree. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/