Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932959AbbFIL5d (ORCPT ); Tue, 9 Jun 2015 07:57:33 -0400 Received: from us01smtprelay-2.synopsys.com ([198.182.60.111]:33116 "EHLO smtprelay.synopsys.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753853AbbFILu3 (ORCPT ); Tue, 9 Jun 2015 07:50:29 -0400 From: Vineet Gupta To: , CC: , , Vineet Gupta , "Peter Zijlstra (Intel)" Subject: [PATCH 17/28] ARC: add compiler barrier to LLSC based cmpxchg Date: Tue, 9 Jun 2015 17:18:17 +0530 Message-ID: <1433850508-26317-18-git-send-email-vgupta@synopsys.com> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1433850508-26317-1-git-send-email-vgupta@synopsys.com> References: <1433850508-26317-1-git-send-email-vgupta@synopsys.com> MIME-Version: 1.0 Content-Type: text/plain X-Originating-IP: [10.12.197.3] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1962 Lines: 59 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 Reported-by: Chuck Jordan Cc: Peter Zijlstra (Intel) Signed-off-by: Vineet Gupta --- arch/arc/include/asm/cmpxchg.h | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h index 03cd6894855d..90de5c528da2 100644 --- a/arch/arc/include/asm/cmpxchg.h +++ b/arch/arc/include/asm/cmpxchg.h @@ -25,10 +25,11 @@ __cmpxchg(volatile void *ptr, unsigned long expected, unsigned long new) " scond %3, [%1] \n" " bnz 1b \n" "2: \n" - : "=&r"(prev) - : "r"(ptr), "ir"(expected), - "r"(new) /* can't be "ir". scond can't take limm for "b" */ - : "cc"); + : "=&r"(prev) /* Early clobber, to prevent reg reuse */ + : "r"(ptr), /* Not "m": llock only supports reg direct addr mode */ + "ir"(expected), + "r"(new) /* can't be "ir". scond can't take LIMM for "b" */ + : "cc", "memory"); /* so that gcc knows memory is being written here */ return prev; } -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/