From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965024Ab2JWUhA (ORCPT ); Tue, 23 Oct 2012 16:37:00 -0400 Received: from e5.ny.us.ibm.com ([32.97.182.145]:42218 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933836Ab2JWUg4 (ORCPT ); Tue, 23 Oct 2012 16:36:56 -0400 Date: Tue, 23 Oct 2012 13:29:02 -0700 From: "Paul E. McKenney" To: Oleg Nesterov Cc: Mikulas Patocka , 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 Message-ID: <20121023202902.GJ2585@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20121017224430.GC2518@linux.vnet.ibm.com> <20121018162409.GA28504@redhat.com> <20121018163833.GK2518@linux.vnet.ibm.com> <20121018175747.GA30691@redhat.com> <20121019192838.GM2518@linux.vnet.ibm.com> <20121023165912.GA18712@redhat.com> <20121023180558.GF2585@linux.vnet.ibm.com> <20121023184123.GB24055@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121023184123.GB24055@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12102320-5930-0000-0000-00000D5F98AD Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > But this patch needs more? Or I misunderstood. There is no RCU unlock > in percpu_up_read(). > > IOW. Suppose the code does > > percpu_down_read(); > x = PROTECTED_BY_THIS_RW_SEM; > percpu_up_read(); > > Withoit mb() the load above can be reordered with this_cpu_dec() in > percpu_up_read(). > > However, we do not care if we can guarantee that the next > percpu_down_write() can not return (iow, the next "write" section can > not start) until this load is complete. > > And I _think_ that another synchronize_sched() in percpu_down_write() > added by this patch should work. > > But, "since the end of its last RCU read-side critical section" > does not look enough. > > Or I misundersood you/Mikulas/both ? I clearly need to look more carefully at Mikulas's code... Thanx, Paul