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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 26 Jun 2018 15:29:41 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22036.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w5QJTedg6750592 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Tue, 26 Jun 2018 19:29:40 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 45FA6B2065; Tue, 26 Jun 2018 15:29:34 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 209A2B2066; Tue, 26 Jun 2018 15:29:34 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Tue, 26 Jun 2018 15:29:34 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 4038216CA0A3; Tue, 26 Jun 2018 12:31:46 -0700 (PDT) Date: Tue, 26 Jun 2018 12:31:46 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH tip/core/rcu 16/27] rcu: Add comment documenting how rcu_seq_snap works Reply-To: paulmck@linux.vnet.ibm.com References: <20180626003448.GA26209@linux.vnet.ibm.com> <20180626003513.27812-16-paulmck@linux.vnet.ibm.com> <20180626171454.GI2494@hirez.programming.kicks-ass.net> <20180626180855.GE3593@linux.vnet.ibm.com> <20180626192113.GM2494@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180626192113.GM2494@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18062619-0064-0000-0000-00000320A42E X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009259; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01052715; UDB=6.00539697; IPR=6.00830639; MB=3.00021867; MTD=3.00000008; XFM=3.00000015; UTC=2018-06-26 19:29:45 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18062619-0065-0000-0000-000039B9B14A Message-Id: <20180626193146.GK3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-06-26_09:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 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=8.0.1-1806210000 definitions=main-1806260215 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 26, 2018 at 09:21:13PM +0200, Peter Zijlstra wrote: > On Tue, Jun 26, 2018 at 11:08:55AM -0700, Paul E. McKenney wrote: > > On Tue, Jun 26, 2018 at 07:14:54PM +0200, Peter Zijlstra wrote: > > > On Mon, Jun 25, 2018 at 05:35:02PM -0700, Paul E. McKenney wrote: > > > > From: "Joel Fernandes (Google)" > > > > > > > > rcu_seq_snap may be tricky to decipher. Lets document how it works with > > > > an example to make it easier. > > > > > > Since you had me looking at them functions; why isn't rcu_seq_snap() > > > using smp_load_acquire() and rcu_seq_end() using smp_store_release() ? > > > Their respective comments seem to suggest that would be sufficent. > > > > I do not believe that this would suffice. Would it make sense to refer > > to Documentation/RCU/Design/Memory-Ordering in the comment header? > > No, because I can't read that thing in an editor. > > > Except that this would invite sprinkling this pathname far and wide... > > > > The key point is that these functions are part of the any-to-any > > memory-ordering guarantee that RCU grace periods provide. > > Then the existing comment is misleading and really needs change. Would it be sufficient to add something like "The memory barrier is required to support the many-to-many ordering guaranteed by RCU grace periods"? Thanx, Paul