From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f170.google.com (mail-wi0-f170.google.com [209.85.212.170]) by kanga.kvack.org (Postfix) with ESMTP id B6DB76B0074 for ; Thu, 15 Jan 2015 03:58:28 -0500 (EST) Received: by mail-wi0-f170.google.com with SMTP id z2so848225wiv.1 for ; Thu, 15 Jan 2015 00:58:28 -0800 (PST) Received: from e06smtp11.uk.ibm.com (e06smtp11.uk.ibm.com. [195.75.94.107]) by mx.google.com with ESMTPS id x9si1581103wju.70.2015.01.15.00.58.18 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 15 Jan 2015 00:58:18 -0800 (PST) Received: from /spool/local by e06smtp11.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 15 Jan 2015 08:58:16 -0000 Received: from b06cxnps4075.portsmouth.uk.ibm.com (d06relay12.portsmouth.uk.ibm.com [9.149.109.197]) by d06dlp02.portsmouth.uk.ibm.com (Postfix) with ESMTP id D4D2C2190067 for ; Thu, 15 Jan 2015 08:57:41 +0000 (GMT) Received: from d06av11.portsmouth.uk.ibm.com (d06av11.portsmouth.uk.ibm.com [9.149.37.252]) by b06cxnps4075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id t0F8wEhP55246896 for ; Thu, 15 Jan 2015 08:58:14 GMT Received: from d06av11.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av11.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id t0F8wDIJ012552 for ; Thu, 15 Jan 2015 01:58:14 -0700 From: Christian Borntraeger Subject: [PATCH 6/8] kernel: tighten rules for ACCESS ONCE Date: Thu, 15 Jan 2015 09:58:32 +0100 Message-Id: <1421312314-72330-7-git-send-email-borntraeger@de.ibm.com> In-Reply-To: <1421312314-72330-1-git-send-email-borntraeger@de.ibm.com> References: <1421312314-72330-1-git-send-email-borntraeger@de.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: linux-kernel@vger.kernel.org Cc: linux-arch@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, xen-devel@lists.xenproject.org, linux-mm@kvack.org, Christian Borntraeger Now that all non-scalar users of ACCESS_ONCE have been converted to READ_ONCE or ASSIGN once, lets tighten ACCESS_ONCE to only work on scalar types. This variant was proposed by Alexei Starovoitov. Signed-off-by: Christian Borntraeger Reviewed-by: Paul E. McKenney --- include/linux/compiler.h | 21 ++++++++++++++++----- 1 file changed, 16 insertions(+), 5 deletions(-) diff --git a/include/linux/compiler.h b/include/linux/compiler.h index a1c81f8..5e186bf 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -447,12 +447,23 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int * to make the compiler aware of ordering is to put the two invocations of * ACCESS_ONCE() in different C statements. * - * This macro does absolutely -nothing- to prevent the CPU from reordering, - * merging, or refetching absolutely anything at any time. Its main intended - * use is to mediate communication between process-level code and irq/NMI - * handlers, all running on the same CPU. + * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE + * on a union member will work as long as the size of the member matches the + * size of the union and the size is smaller than word size. + * + * The major use cases of ACCESS_ONCE used to be (1) Mediating communication + * between process-level code and irq/NMI handlers, all running on the same CPU, + * and (2) Ensuring that the compiler does not fold, spindle, or otherwise + * mutilate accesses that either do not require ordering or that interact + * with an explicit memory barrier or atomic instruction that provides the + * required ordering. + * + * If possible use READ_ONCE/ASSIGN_ONCE instead. */ -#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) +#define __ACCESS_ONCE(x) ({ \ + __maybe_unused typeof(x) __var = 0; \ + (volatile typeof(x) *)&(x); }) +#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x)) /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */ #ifdef CONFIG_KPROBES -- 1.9.3 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org