From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935538Ab2JXUW5 (ORCPT ); Wed, 24 Oct 2012 16:22:57 -0400 Received: from mx1.redhat.com ([209.132.183.28]:11337 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933660Ab2JXUW4 (ORCPT ); Wed, 24 Oct 2012 16:22:56 -0400 Date: Wed, 24 Oct 2012 16:22:17 -0400 (EDT) From: Mikulas Patocka X-X-Sender: mpatocka@file.rdu.redhat.com To: "Paul E. McKenney" cc: Oleg Nesterov , Linus Torvalds , Ingo Molnar , Peter Zijlstra , Srikar Dronamraju , Ananth N Mavinakayanahalli , Anton Arapov , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers In-Reply-To: <20121024162304.GB2465@linux.vnet.ibm.com> Message-ID: References: <20121018175747.GA30691@redhat.com> <20121019192838.GM2518@linux.vnet.ibm.com> <20121023165912.GA18712@redhat.com> <20121023180558.GF2585@linux.vnet.ibm.com> <20121023184123.GB24055@redhat.com> <20121023202902.GJ2585@linux.vnet.ibm.com> <20121023203254.GA3410@linux.vnet.ibm.com> <20121024162304.GB2465@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 24 Oct 2012, Paul E. McKenney wrote: > On Tue, Oct 23, 2012 at 05:39:43PM -0400, Mikulas Patocka wrote: > > > > > > On Tue, 23 Oct 2012, Paul E. McKenney wrote: > > > > > On Tue, Oct 23, 2012 at 01:29:02PM -0700, Paul E. McKenney wrote: > > > > On Tue, Oct 23, 2012 at 08:41:23PM +0200, Oleg Nesterov wrote: > > > > > On 10/23, Paul E. McKenney wrote: > > > > > > > > > > > > * Note that this guarantee implies a further memory-ordering guarantee. > > > > > > * On systems with more than one CPU, when synchronize_sched() returns, > > > > > > * each CPU is guaranteed to have executed a full memory barrier since > > > > > > * the end of its last RCU read-side critical section > > > > > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > > > > > > > > > Ah wait... I misread this comment. > > > > > > > > And I miswrote it. It should say "since the end of its last RCU-sched > > > > read-side critical section." So, for example, RCU-sched need not force > > > > a CPU that is idle, offline, or (eventually) executing in user mode to > > > > execute a memory barrier. Fixed this. > > > > Or you can write "each CPU that is executing a kernel code is guaranteed > > to have executed a full memory barrier". > > Perhaps I could, but it isn't needed, nor is it particularly helpful. > Please see suggestions in preceding email. It is helpful, because if you add this requirement (that already holds for the current implementation), you can drop rcu_read_lock_sched() and rcu_read_unlock_sched() from the following code that you submitted. static inline void percpu_up_read(struct percpu_rw_semaphore *p) { /* * Decrement our count, but protected by RCU-sched so that * the writer can force proper serialization. */ rcu_read_lock_sched(); this_cpu_dec(*p->counters); rcu_read_unlock_sched(); } > > The current implementation fulfills this requirement, you can just add it > > to the specification so that whoever changes the implementation keeps it. > > I will consider doing that if and when someone shows me a situation where > adding that requirement makes things simpler and/or faster. From what I > can see, your example does not do so. > > Thanx, Paul If you do, the above code can be simplified to: { barrier(); this_cpu_dec(*p->counters); } Mikulas