From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933187AbbFIMXn (ORCPT ); Tue, 9 Jun 2015 08:23:43 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:45925 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753910AbbFIMXe (ORCPT ); Tue, 9 Jun 2015 08:23:34 -0400 Date: Tue, 9 Jun 2015 14:23:27 +0200 From: Peter Zijlstra To: Vineet Gupta Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, arnd@arndb.de, arc-linux-dev@synopsys.com Subject: Re: [PATCH 17/28] ARC: add compiler barrier to LLSC based cmpxchg Message-ID: <20150609122327.GW3644@twins.programming.kicks-ass.net> References: <1433850508-26317-1-git-send-email-vgupta@synopsys.com> <1433850508-26317-18-git-send-email-vgupta@synopsys.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1433850508-26317-18-git-send-email-vgupta@synopsys.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 Tue, Jun 09, 2015 at 05:18:17PM +0530, Vineet Gupta wrote: > When auditing cmpxchg call sites, Chuck noted that gcc was optimizing > away some of the desired LDs. > > | do { > | new = old = *ipi_data_ptr; > | new |= 1U << msg; > | } while (cmpxchg(ipi_data_ptr, old, new) != old); > > was generating to below > > | 8015cef8: ld r2,[r4,0] <-- First LD > | 8015cefc: bset r1,r2,r1 > | > | 8015cf00: llock r3,[r4] <-- atomic op > | 8015cf04: brne r3,r2,8015cf10 > | 8015cf08: scond r1,[r4] > | 8015cf0c: bnz 8015cf00 > | > | 8015cf10: brne r3,r2,8015cf00 <-- Branch doesn't go to orig LD > > Although this was fixed by adding a ACCESS_ONCE in this call site, it > seems safer (for now at least) to add compiler barrier to LLSC based > cmpxchg This is required even. cmpxchg() should include a full memory barrier _before_ and _after_ the op. Both imply a compiler barrier. Acked-by: Peter Zijlstra (Intel)