From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758780Ab2JXUof (ORCPT ); Wed, 24 Oct 2012 16:44:35 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36592 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755618Ab2JXUod (ORCPT ); Wed, 24 Oct 2012 16:44:33 -0400 Date: Wed, 24 Oct 2012 16:44:14 -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: <20121024203636.GK2465@linux.vnet.ibm.com> Message-ID: References: <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> <20121024203636.GK2465@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 Wed, Oct 24, 2012 at 04:22:17PM -0400, Mikulas Patocka wrote: > > > > > > 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); > > } > > The readers are lightweight enough that you are worried about the overhead > of rcu_read_lock_sched() and rcu_read_unlock_sched()? Really??? > > Thanx, Paul There was no lock in previous kernels, so we should make it as simple as possible. Disabling and reenabling preemption is probably not a big deal, but if don't have to do it, why do it? Mikulas