From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751620AbcADOFS (ORCPT ); Mon, 4 Jan 2016 09:05:18 -0500 Received: from casper.infradead.org ([85.118.1.10]:54078 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750972AbcADOFP (ORCPT ); Mon, 4 Jan 2016 09:05:15 -0500 Date: Mon, 4 Jan 2016 15:05:09 +0100 From: Peter Zijlstra To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Arnd Bergmann , linux-arch@vger.kernel.org, Andrew Cooper , virtualization@lists.linux-foundation.org, Stefano Stabellini , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , David Miller , linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-metag@vger.kernel.org, linux-mips@linux-mips.org, x86@kernel.org, user-mode-linux-devel@lists.sourceforge.net, adi-buildroot-devel@lists.sourceforge.net, linux-sh@vger.kernel.org, linux-xtensa@linux-xtensa.org, xen-devel@lists.xenproject.org, Ingo Molnar Subject: Re: [PATCH v2 31/32] sh: support a 2-byte smp_store_mb Message-ID: <20160104140509.GB6344@twins.programming.kicks-ass.net> References: <1451572003-2440-1-git-send-email-mst@redhat.com> <1451572003-2440-32-git-send-email-mst@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1451572003-2440-32-git-send-email-mst@redhat.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > Signed-off-by: Michael S. Tsirkin > --- > arch/sh/include/asm/barrier.h | 10 +++++++++- > 1 file changed, 9 insertions(+), 1 deletion(-) > > diff --git a/arch/sh/include/asm/barrier.h b/arch/sh/include/asm/barrier.h > index f887c64..0cc5735 100644 > --- a/arch/sh/include/asm/barrier.h > +++ b/arch/sh/include/asm/barrier.h > @@ -32,7 +32,15 @@ > #define ctrl_barrier() __asm__ __volatile__ ("nop;nop;nop;nop;nop;nop;nop;nop") > #endif > > -#define __smp_store_mb(var, value) do { (void)xchg(&var, value); } while (0) > +#define __smp_store_mb(var, value) do { \ > + if (sizeof(var) != 4 && sizeof(var) != 1) { \ > + WRITE_ONCE(var, value); \ > + __smp_mb(); \ > + } else { \ > + (void)xchg(&var, value); \ > + } \ > +} while (0) So SH is an orphaned arch, which is also why I did not comment on using xchg() for the UP smp_store_mb() thing. But I really think we should try fixing the xchg() implementation instead of this duct-tape.