From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S940241AbeE1Qf5 (ORCPT ); Mon, 28 May 2018 12:35:57 -0400 Received: from mail.kernel.org ([198.145.29.99]:56468 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934669AbeE1KIZ (ORCPT ); Mon, 28 May 2018 06:08:25 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Andrea Parri , Peter Zijlstra , "Paul E. McKenney" , Alan Stern , Ivan Kokshaysky , Linus Torvalds , Matt Turner , Richard Henderson , Thomas Gleixner , Will Deacon , linux-alpha@vger.kernel.org, Ingo Molnar , Sasha Levin Subject: [PATCH 3.18 063/185] locking/xchg/alpha: Add unconditional memory barrier to cmpxchg() Date: Mon, 28 May 2018 12:01:44 +0200 Message-Id: <20180528100056.229667403@linuxfoundation.org> X-Mailer: git-send-email 2.17.0 In-Reply-To: <20180528100050.700971285@linuxfoundation.org> References: <20180528100050.700971285@linuxfoundation.org> User-Agent: quilt/0.65 X-stable: review MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Andrea Parri [ Upstream commit cb13b424e986aed68d74cbaec3449ea23c50e167 ] Continuing along with the fight against smp_read_barrier_depends() [1] (or rather, against its improper use), add an unconditional barrier to cmpxchg. This guarantees that dependency ordering is preserved when a dependency is headed by an unsuccessful cmpxchg. As it turns out, the change could enable further simplification of LKMM as proposed in [2]. [1] https://marc.info/?l=linux-kernel&m=150884953419377&w=2 https://marc.info/?l=linux-kernel&m=150884946319353&w=2 https://marc.info/?l=linux-kernel&m=151215810824468&w=2 https://marc.info/?l=linux-kernel&m=151215816324484&w=2 [2] https://marc.info/?l=linux-kernel&m=151881978314872&w=2 Signed-off-by: Andrea Parri Acked-by: Peter Zijlstra Acked-by: Paul E. McKenney Cc: Alan Stern Cc: Ivan Kokshaysky Cc: Linus Torvalds Cc: Matt Turner Cc: Richard Henderson Cc: Thomas Gleixner Cc: Will Deacon Cc: linux-alpha@vger.kernel.org Link: http://lkml.kernel.org/r/1519152356-4804-1-git-send-email-parri.andrea@gmail.com Signed-off-by: Ingo Molnar Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman --- arch/alpha/include/asm/xchg.h | 15 +++++++-------- 1 file changed, 7 insertions(+), 8 deletions(-) --- a/arch/alpha/include/asm/xchg.h +++ b/arch/alpha/include/asm/xchg.h @@ -127,10 +127,9 @@ ____xchg(, volatile void *ptr, unsigned * store NEW in MEM. Return the initial value in MEM. Success is * indicated by comparing RETURN with OLD. * - * The memory barrier should be placed in SMP only when we actually - * make the change. If we don't change anything (so if the returned - * prev is equal to old) then we aren't acquiring anything new and - * we don't need any memory barrier as far I can tell. + * The memory barrier is placed in SMP unconditionally, in order to + * guarantee that dependency ordering is preserved when a dependency + * is headed by an unsuccessful operation. */ static inline unsigned long @@ -149,8 +148,8 @@ ____cmpxchg(_u8, volatile char *m, unsig " or %1,%2,%2\n" " stq_c %2,0(%4)\n" " beq %2,3f\n" - __ASM__MB "2:\n" + __ASM__MB ".subsection 2\n" "3: br 1b\n" ".previous" @@ -176,8 +175,8 @@ ____cmpxchg(_u16, volatile short *m, uns " or %1,%2,%2\n" " stq_c %2,0(%4)\n" " beq %2,3f\n" - __ASM__MB "2:\n" + __ASM__MB ".subsection 2\n" "3: br 1b\n" ".previous" @@ -199,8 +198,8 @@ ____cmpxchg(_u32, volatile int *m, int o " mov %4,%1\n" " stl_c %1,%2\n" " beq %1,3f\n" - __ASM__MB "2:\n" + __ASM__MB ".subsection 2\n" "3: br 1b\n" ".previous" @@ -222,8 +221,8 @@ ____cmpxchg(_u64, volatile long *m, unsi " mov %4,%1\n" " stq_c %1,%2\n" " beq %1,3f\n" - __ASM__MB "2:\n" + __ASM__MB ".subsection 2\n" "3: br 1b\n" ".previous"