From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753634AbaKXNDt (ORCPT ); Mon, 24 Nov 2014 08:03:49 -0500 Received: from e06smtp14.uk.ibm.com ([195.75.94.110]:38721 "EHLO e06smtp14.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753504AbaKXNDl (ORCPT ); Mon, 24 Nov 2014 08:03:41 -0500 From: Christian Borntraeger To: linux-kernel@vger.kernel.org Cc: "linux-arch@vger.kernel.org, linux-mips@linux-mips.org, linux-x86_64@vger.kernel.org, linux-s390"@vger.kernel.org, Paolo Bonzini , paulmck@linux.vnet.ibm.com, mingo@kernel.org, torvalds@linux-foundation.org, Catalin Marinas , Will Deacon , Christian Borntraeger Subject: [PATCH/RFC 4/7] x86: Replace ACCESS_ONCE in gup with a barrier Date: Mon, 24 Nov 2014 14:03:27 +0100 Message-Id: <1416834210-61738-5-git-send-email-borntraeger@de.ibm.com> X-Mailer: git-send-email 1.9.3 In-Reply-To: <1416834210-61738-1-git-send-email-borntraeger@de.ibm.com> References: <1416834210-61738-1-git-send-email-borntraeger@de.ibm.com> X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14112413-0017-0000-0000-000001F464B7 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ACCESS_ONCE does not work reliably on non-scalar types. For example gcc 4.6 and 4.7 might remove the volatile tag for such accesses during the SRA (scalar replacement of aggregates) step (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145) Let's use a barrier instead of ACCESS_ONCE to avoid that issue. Signed-off-by: Christian Borntraeger --- arch/x86/mm/gup.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c index 207d9aef..35991f6 100644 --- a/arch/x86/mm/gup.c +++ b/arch/x86/mm/gup.c @@ -14,8 +14,12 @@ static inline pte_t gup_get_pte(pte_t *ptep) { + pte_t pte; + #ifndef CONFIG_X86_PAE - return ACCESS_ONCE(*ptep); + pte = *ptep; + barrier(); + return pte; #else /* * With get_user_pages_fast, we walk down the pagetables without taking @@ -49,7 +53,6 @@ static inline pte_t gup_get_pte(pte_t *ptep) * very careful -- it does not atomically load the pte or anything that * is likely to be useful for you. */ - pte_t pte; retry: pte.pte_low = ptep->pte_low; -- 1.9.3