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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 4 Apr 2019 21:10:38 +0100 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 x34KAbms25231442 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 4 Apr 2019 20:10:37 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BB885B2064; Thu, 4 Apr 2019 20:10:37 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8CE4CB205F; Thu, 4 Apr 2019 20:10:37 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.188]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 4 Apr 2019 20:10:37 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 6EDAE16C2A41; Thu, 4 Apr 2019 13:10:39 -0700 (PDT) Date: Thu, 4 Apr 2019 13:10:39 -0700 From: "Paul E. McKenney" To: "Joel Fernandes (Google)" 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190329140555.118463-1-joel@joelfernandes.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19040420-2213-0000-0000-000003718407 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010873; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000284; SDB=6.01184350; UDB=6.00620121; IPR=6.00965103; MB=3.00026298; MTD=3.00000008; XFM=3.00000015; UTC=2019-04-04 20:10:41 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19040420-2214-0000-0000-00005DE7A305 Message-Id: <20190404201039.GL14111@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-04-04_11:,, 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-1904040127 Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org 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. +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. + In cases where delete() can sleep, synchronize_rcu() can be called from delete(), so that el_free() can be subsumed into delete as follows: @@ -130,3 +144,7 @@ delete() kfree(el); ... } + +As additional examples in the kernel, the pattern in listing C is used by +reference counting of struct pid, while the pattern in listing B is used by +struct posix_acl.