From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932259AbbJ1AK4 (ORCPT ); Tue, 27 Oct 2015 20:10:56 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:52233 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754729AbbJ1AKv (ORCPT ); Tue, 27 Oct 2015 20:10:51 -0400 Date: Tue, 27 Oct 2015 23:37:44 +0100 From: Peter Zijlstra To: Linus Torvalds Cc: Davidlohr Bueso , Ingo Molnar , Thomas Gleixner , "Paul E. McKenney" , Linux Kernel Mailing List , the arch/x86 maintainers , Davidlohr Bueso Subject: Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb() Message-ID: <20151027223744.GB11242@worktop.amr.corp.intel.com> References: <1445975631-17047-1-git-send-email-dave@stgolabs.net> <1445975631-17047-4-git-send-email-dave@stgolabs.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 28, 2015 at 06:33:56AM +0900, Linus Torvalds wrote: > On Wed, Oct 28, 2015 at 4:53 AM, Davidlohr Bueso wrote: > > > > Note that this might affect callers that could/would rely on the > > atomicity semantics, but there are no guarantees of that for > > smp_store_mb() mentioned anywhere, plus most archs use this anyway. > > Thus we continue to be consistent with the memory-barriers.txt file, > > and more importantly, maintain the semantics of the smp_ nature. > > So with this patch, the whole thing becomes pointless, I feel. (Ok, so > it may have been pointless before too, but at least before this patch > it generated special code, now it doesn't). So why carry it along at > all? So I suppose this boils down to if: XCHG ends up being cheaper than MOV+FENCE. PeterA, any idea?