From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752349AbcAFLwX (ORCPT ); Wed, 6 Jan 2016 06:52:23 -0500 Received: from mx1.redhat.com ([209.132.183.28]:57543 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752084AbcAFLwU (ORCPT ); Wed, 6 Jan 2016 06:52:20 -0500 Date: Wed, 6 Jan 2016 13:52:17 +0200 From: "Michael S. Tsirkin" To: Peter Zijlstra Cc: Rich Felker , linux-kernel@vger.kernel.org, linux-sh@vger.kernel.org Subject: Re: [PATCH v2 31/32] sh: support a 2-byte smp_store_mb Message-ID: <20160106134301-mutt-send-email-mst@redhat.com> References: <1451572003-2440-1-git-send-email-mst@redhat.com> <1451572003-2440-32-git-send-email-mst@redhat.com> <20160105232735.GC238@brightrain.aerifal.cx> <20160106131321-mutt-send-email-mst@redhat.com> <20160106114023.GU6344@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160106114023.GU6344@twins.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 06, 2016 at 12:40:23PM +0100, Peter Zijlstra wrote: > On Wed, Jan 06, 2016 at 01:19:44PM +0200, Michael S. Tsirkin wrote: > > On Tue, Jan 05, 2016 at 06:27:35PM -0500, Rich Felker wrote: > > > On Thu, Dec 31, 2015 at 09:09:47PM +0200, Michael S. Tsirkin wrote: > > > > At the moment, xchg on sh only supports 4 and 1 byte values, so using it > > > > from smp_store_mb means attempts to store a 2 byte value using this > > > > macro fail. > > > > > > > > And happens to be exactly what virtio drivers want to do. > > > > > > > > Check size and fall back to a slower, but safe, WRITE_ONCE+smp_mb. > > > > > > Can you please do this for size 1 as well (i.e. all sizes != 4)? If > > > you check the source, the code for size-1 xchg in sh cmpxchg-llsc.h is > > > completely wrong and operates on a 32-bit object at the address passed > > > to it. This code is presently unused anyway and I plan to submit a > > > patch to remove the size 1 case. > > > > > > Rich > > > > Ouch. And PeterZ says I should write a 2-byte xchg in asm instead, > > and Fedora can't even build a full kernel for this arch at the moment :( > > Does the kernel.org hosted cross compiler work? I'll test, thanks for the hint. > > Peter, what do you think? How about I leave this patch as is for now? > > No, and I object to removing the single byte implementation too. Either > remove the full arch or fix xchg() to conform. xchg() should work on all > native word sizes, for SH that would be 1,2 and 4 bytes. Rick, maybe you could explain how is current 1 byte xchg on llsc wrong? It does use 4 byte accesses but IIUC that is all that exists on this architecture. -- MST