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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 26 Jun 2018 18:25:42 -0400 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 w5QMPfXQ17039748 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Tue, 26 Jun 2018 22:25:42 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 494DBB2064; Tue, 26 Jun 2018 18:25:35 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 214FBB2065; Tue, 26 Jun 2018 18:25:35 -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 18:25:35 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 8CEC216CA20B; Tue, 26 Jun 2018 15:27:47 -0700 (PDT) Date: Tue, 26 Jun 2018 15:27:47 -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> <20180626193146.GK3593@linux.vnet.ibm.com> <20180626201503.GS2494@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180626201503.GS2494@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18062622-0064-0000-0000-00000320B210 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009260; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01052774; UDB=6.00539732; IPR=6.00830698; MB=3.00021870; MTD=3.00000008; XFM=3.00000015; UTC=2018-06-26 22:25:47 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18062622-0065-0000-0000-000039BA0A5D Message-Id: <20180626222747.GP3593@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=995 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1806260245 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 10:15:03PM +0200, Peter Zijlstra wrote: > On Tue, Jun 26, 2018 at 12:31:46PM -0700, Paul E. McKenney wrote: > > > 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"? > > What would lead me to wonder why after the load and not (also) before. The memory barriers on the other sides of these primitives are supplied by the caller. For example, in srcu_gp_end(), there is a set of spin_unlock_irq_rcu_node()/spin_lock_irq_rcu_node() pairs between the rcu_seq_end() of the prior SRCU grace period and the rcu_seq_start() of the next one. However, these things aren't anywhere near a fastpath, so I could add the smp_mb() calls on the other sides for readability, if that would be useful. Thanx, Paul