From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755847AbdJJAW5 (ORCPT ); Mon, 9 Oct 2017 20:22:57 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:43244 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755457AbdJJAWx (ORCPT ); Mon, 9 Oct 2017 20:22:53 -0400 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, torvalds@linux-foundation.org, mark.rutland@arm.com, dhowells@redhat.com, linux-arch@vger.kernel.org, peterz@infradead.org, will.deacon@arm.com, "Paul E. McKenney" Subject: [PATCH RFC tip/core/rcu 01/15] doc: READ_ONCE() now implies smp_barrier_depends() Date: Mon, 9 Oct 2017 17:22:35 -0700 X-Mailer: git-send-email 2.5.2 In-Reply-To: <20171010001951.GA6476@linux.vnet.ibm.com> References: <20171010001951.GA6476@linux.vnet.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 17101000-0048-0000-0000-000001F32E20 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007870; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000235; SDB=6.00928858; UDB=6.00467489; IPR=6.00709129; BA=6.00005631; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00017467; XFM=3.00000015; UTC=2017-10-10 00:22:52 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17101000-0049-0000-0000-000042D1C8FC Message-Id: <1507594969-8347-1-git-send-email-paulmck@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-10-09_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=1 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1707230000 definitions=main-1710100003 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This commit updates an example in memory-barriers.txt to account for the fact that READ_ONCE() now implies smp_barrier_depends(). Signed-off-by: Paul E. McKenney --- Documentation/memory-barriers.txt | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index b759a60624fd..b592bb265e5c 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -227,17 +227,18 @@ There are some minimal guarantees that may be expected of a CPU: (*) On any given CPU, dependent memory accesses will be issued in order, with respect to itself. This means that for: - Q = READ_ONCE(P); smp_read_barrier_depends(); D = READ_ONCE(*Q); + Q = READ_ONCE(P); D = READ_ONCE(*Q); the CPU will issue the following memory operations: Q = LOAD P, D = LOAD *Q and always in that order. On most systems, smp_read_barrier_depends() - does nothing, but it is required for DEC Alpha. The READ_ONCE() - is required to prevent compiler mischief. Please note that you - should normally use something like rcu_dereference() instead of - open-coding smp_read_barrier_depends(). + does nothing, but it is required for DEC Alpha, and is supplied by + READ_ONCE(). The READ_ONCE() is also required to prevent compiler + mischief. Please note that you should normally use something + like READ_ONCE() or rcu_dereference() instead of open-coding + smp_read_barrier_depends(). (*) Overlapping loads and stores within a particular CPU will appear to be ordered within that CPU. This means that for: -- 2.5.2