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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 8 Apr 2019 19:29:09 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x38IT81R36110428 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 8 Apr 2019 18:29:08 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E9612B2064; Mon, 8 Apr 2019 18:29:07 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BBE9FB207A; Mon, 8 Apr 2019 18:29:07 +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 18:29:07 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 0FF7316C0EB6; Mon, 8 Apr 2019 11:29:11 -0700 (PDT) Date: Mon, 8 Apr 2019 11:29:11 -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> <20190408175213.GT14111@linux.ibm.com> <20190408180859.GA133872@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190408180859.GA133872@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19040818-0052-0000-0000-000003A937AA X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010891; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000284; SDB=6.01186219; UDB=6.00621251; IPR=6.00966976; MB=3.00026346; MTD=3.00000008; XFM=3.00000015; UTC=2019-04-08 18:29:11 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19040818-0053-0000-0000-0000606D2D2A Message-Id: <20190408182911.GV14111@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-1904080146 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 08, 2019 at 02:08:59PM -0400, Joel Fernandes wrote: > On Mon, Apr 08, 2019 at 10:52:13AM -0700, Paul E. McKenney wrote: > > 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: > > This one looks better to me, thanks a lot! > > Reviewed-by: Joel Fernandes (Google) Thank you! (Though this one gets merged into your original patch.) Thanx, Paul