From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 942DEC43441 for ; Sun, 11 Nov 2018 19:56:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 628A620871 for ; Sun, 11 Nov 2018 19:56:57 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 628A620871 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.ibm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728753AbeKLFqW (ORCPT ); Mon, 12 Nov 2018 00:46:22 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:54324 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726633AbeKLFqU (ORCPT ); Mon, 12 Nov 2018 00:46:20 -0500 Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id wABJs6II055473 for ; Sun, 11 Nov 2018 14:56:52 -0500 Received: from e16.ny.us.ibm.com (e16.ny.us.ibm.com [129.33.205.206]) by mx0b-001b2d01.pphosted.com with ESMTP id 2npdbnt2hp-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Sun, 11 Nov 2018 14:56:52 -0500 Received: from localhost by e16.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Sun, 11 Nov 2018 19:56:51 -0000 Received: from b01cxnp23032.gho.pok.ibm.com (9.57.198.27) by e16.ny.us.ibm.com (146.89.104.203) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Sun, 11 Nov 2018 19:56:47 -0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id wABJukEZ19005492 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Sun, 11 Nov 2018 19:56:46 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id A18FAB2066; Sun, 11 Nov 2018 19:56:46 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7E5B9B2067; Sun, 11 Nov 2018 19:56:46 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.207.24]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sun, 11 Nov 2018 19:56:46 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 3563516C5DA8; Sun, 11 Nov 2018 11:56:47 -0800 (PST) From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, tytso@mit.edu, "Paul E . McKenney" Subject: [PATCH tip/core/rcu 15/20] doc: Make reader aware of rcu_dereference_protected Date: Sun, 11 Nov 2018 11:56:40 -0800 X-Mailer: git-send-email 2.17.1 In-Reply-To: <20181111195619.GA6958@linux.ibm.com> References: <20181111195619.GA6958@linux.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 18111119-0072-0000-0000-000003C6F415 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010028; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000270; SDB=6.01116023; UDB=6.00577118; IPR=6.00896124; MB=3.00024114; MTD=3.00000008; XFM=3.00000015; UTC=2018-11-11 19:56:51 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18111119-0073-0000-0000-00004A134C0C Message-Id: <20181111195645.9205-15-paulmck@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-11-11_13:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=1 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1811110190 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Joel Fernandes (Google)" The whatisRCU.txt document says rcu_dereference() cannot be used outside of rcu_read_lock() protected sections. The commit adds a mention of rcu_dereference_protected(), so that the new reader knows that this API can be used to avoid update-side use of rcu_read_lock() and rcu_read_unlock(). Cc: tytso@mit.edu Suggested-by: tytso@mit.edu Signed-off-by: Joel Fernandes (Google) [ paulmck: Update wording, including further feedback from Joel. ] Signed-off-by: Paul E. McKenney --- Documentation/RCU/whatisRCU.txt | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt index 7c33445fd0e5..4a6854318b17 100644 --- a/Documentation/RCU/whatisRCU.txt +++ b/Documentation/RCU/whatisRCU.txt @@ -266,7 +266,7 @@ rcu_dereference() unnecessary overhead on Alpha CPUs. Note that the value returned by rcu_dereference() is valid - only within the enclosing RCU read-side critical section. + only within the enclosing RCU read-side critical section [1]. For example, the following is -not- legal: rcu_read_lock(); @@ -292,6 +292,19 @@ rcu_dereference() typically used indirectly, via the _rcu list-manipulation primitives, such as list_for_each_entry_rcu(). + [1] The variant rcu_dereference_protected() can be used outside + of an RCU read-side critical section as long as the usage is + protected by locks acquired by the update-side code. This variant + avoids the lockdep warning that would happen when using (for + example) rcu_dereference() without rcu_read_lock() protection. + Using rcu_dereference_protected() also has the advantage + of permitting compiler optimizations that rcu_dereference() + must prohibit. The rcu_dereference_protected() variant takes + a lockdep expression to indicate which locks must be acquired + by the caller. If the indicated protection is not provided, + a lockdep splat is emitted. See RCU/Design/Requirements.html + and the API's code comments for more details and example usage. + The following diagram shows how each API communicates among the reader, updater, and reclaimer. -- 2.17.1