From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-6.0 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id A29E67D04D for ; Mon, 8 Apr 2019 17:52:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727510AbfDHRwS (ORCPT ); Mon, 8 Apr 2019 13:52:18 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:50430 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726849AbfDHRwS (ORCPT ); Mon, 8 Apr 2019 13:52:18 -0400 Received: from pps.filterd (m0098394.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x38Hnmsc020622 for ; Mon, 8 Apr 2019 13:52:17 -0400 Received: from e12.ny.us.ibm.com (e12.ny.us.ibm.com [129.33.205.202]) by mx0a-001b2d01.pphosted.com with ESMTP id 2rraj89ck0-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Mon, 08 Apr 2019 13:52:16 -0400 Received: from localhost by e12.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 8 Apr 2019 18:52:15 +0100 Received: from b01cxnp23034.gho.pok.ibm.com (9.57.198.29) by e12.ny.us.ibm.com (146.89.104.199) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 8 Apr 2019 18:52:11 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x38HqAST30802012 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 8 Apr 2019 17:52:10 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 422A8B2064; Mon, 8 Apr 2019 17:52:10 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 11BEBB205F; Mon, 8 Apr 2019 17:52:10 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.188]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 8 Apr 2019 17:52:10 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 4E66916C2940; Mon, 8 Apr 2019 10:52:13 -0700 (PDT) Date: Mon, 8 Apr 2019 10:52:13 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: linux-kernel@vger.kernel.org, oleg@redhat.com, jannh@google.com, Jonathan Corbet , Josh Triplett , Lai Jiangshan , linux-doc@vger.kernel.org, Mathieu Desnoyers , Steven Rostedt Subject: Re: [PATCH v2] doc/rcuref: Document real world examples in kernel Reply-To: paulmck@linux.ibm.com References: <20190329140555.118463-1-joel@joelfernandes.org> <20190404201039.GL14111@linux.ibm.com> <20190406021705.GA6615@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190406021705.GA6615@localhost> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19040817-0060-0000-0000-0000032A1979 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010891; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000284; SDB=6.01186207; UDB=6.00621244; IPR=6.00966964; MB=3.00026346; MTD=3.00000008; XFM=3.00000015; UTC=2019-04-08 17:52:14 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19040817-0061-0000-0000-000048E0EB4C Message-Id: <20190408175213.GT14111@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-04-08_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=2 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=8.0.1-1810050000 definitions=main-1904080142 Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Sat, Apr 06, 2019 at 02:17:05AM +0000, Joel Fernandes wrote: > On Thu, Apr 04, 2019 at 01:10:39PM -0700, Paul E. McKenney wrote: > > On Fri, Mar 29, 2019 at 10:05:55AM -0400, Joel Fernandes (Google) wrote: > > > Document similar real world examples in the kernel corresponding to the > > > second and third code snippets. Also correct an issue in > > > release_referenced() in the code snippet example. > > > > > > Cc: oleg@redhat.com > > > Cc: jannh@google.com > > > Signed-off-by: Joel Fernandes (Google) > > > > Good catch, thank you! > > > > As usual, I could not resist doing a bit of wordsmithing. Please let me > > know if I messed anything up in the version shown below. > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > commit adcd92c0ab303b57b28a3cd097bd9ece824c14f6 > > Author: Joel Fernandes (Google) > > Date: Fri Mar 29 10:05:55 2019 -0400 > > > > doc/rcuref: Document real world examples in kernel > > > > Document similar real world examples in the kernel corresponding to the > > second and third code snippets. Also correct an issue in > > release_referenced() in the code snippet example. > > > > Cc: oleg@redhat.com > > Cc: jannh@google.com > > Signed-off-by: Joel Fernandes (Google) > > [ paulmck: Do a bit of wordsmithing. ] > > Signed-off-by: Paul E. McKenney > > > > diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt > > index 613033ff2b9b..c0bab7fb57e7 100644 > > --- a/Documentation/RCU/rcuref.txt > > +++ b/Documentation/RCU/rcuref.txt > > @@ -12,6 +12,7 @@ please read on. > > Reference counting on elements of lists which are protected by traditional > > reader/writer spinlocks or semaphores are straightforward: > > > > +CODE LISTING A: > > 1. 2. > > add() search_and_reference() > > { { > > @@ -28,7 +29,8 @@ add() search_and_reference() > > release_referenced() delete() > > { { > > ... write_lock(&list_lock); > > - atomic_dec(&el->rc, relfunc) ... > > + if(atomic_dec_and_test(&el->rc)) ... > > + kfree(el); > > ... remove_element > > } write_unlock(&list_lock); > > ... > > @@ -44,6 +46,7 @@ search_and_reference() could potentially hold reference to an element which > > has already been deleted from the list/array. Use atomic_inc_not_zero() > > in this scenario as follows: > > > > +CODE LISTING B: > > 1. 2. > > add() search_and_reference() > > { { > > @@ -79,6 +82,7 @@ search_and_reference() code path. In such cases, the > > atomic_dec_and_test() may be moved from delete() to el_free() > > as follows: > > > > +CODE LISTING C: > > 1. 2. > > add() search_and_reference() > > { { > > @@ -114,6 +118,16 @@ element can therefore safely be freed. This in turn guarantees that if > > any reader finds the element, that reader may safely acquire a reference > > without checking the value of the reference counter. > > > > +A clear advantage of the RCU-based pattern in listing C over the one > > +in listing B is that any call to search_and_reference() that locates > > +a given object will succeed in obtaining a reference to that object, > > +even given a concurrent invocation of delete() for that same object. > > This part sounds good to me. > > > +Similarly, a call to delete() is not delayed even if there are an > > +arbitrarily large number of calls to search_and_reference() searching > > +for the same object that delete() was invoked on. Instead, all that is > > +delayed is the eventual invocation of kfree(), which is usually not a > > +problem on modern computer systems, even the small ones. > > + > > small nit: > This part is common to both listing B and C right? The delete() is never > delayed due to the search_and_reference in either case, and the kfree is what > is delayed. My patch was highlighting the difference between the 2 > listings, but this text says what is common between both listings. > > As such I am Ok with the changes you made, and thanks for this document in > the first place. Good point! How about the following patch to be merged into the current patch? Thanx, Paul ------------------------------------------------------------------------ diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt index c0bab7fb57e7..5e6429d66c24 100644 --- a/Documentation/RCU/rcuref.txt +++ b/Documentation/RCU/rcuref.txt @@ -122,11 +122,12 @@ A clear advantage of the RCU-based pattern in listing C over the one in listing B is that any call to search_and_reference() that locates a given object will succeed in obtaining a reference to that object, even given a concurrent invocation of delete() for that same object. -Similarly, a call to delete() is not delayed even if there are an -arbitrarily large number of calls to search_and_reference() searching -for the same object that delete() was invoked on. Instead, all that is -delayed is the eventual invocation of kfree(), which is usually not a -problem on modern computer systems, even the small ones. +Similarly, a clear advantage of both listings B and C over listing A is +that a call to delete() is not delayed even if there are an arbitrarily +large number of calls to search_and_reference() searching for the same +object that delete() was invoked on. Instead, all that is delayed is +the eventual invocation of kfree(), which is usually not a problem on +modern computer systems, even the small ones. In cases where delete() can sleep, synchronize_rcu() can be called from delete(), so that el_free() can be subsumed into delete as follows: