From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757151Ab2BBScG (ORCPT ); Thu, 2 Feb 2012 13:32:06 -0500 Received: from e9.ny.us.ibm.com ([32.97.182.139]:50685 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753015Ab2BBScD (ORCPT ); Thu, 2 Feb 2012 13:32:03 -0500 Date: Thu, 2 Feb 2012 10:31:22 -0800 From: "Paul E. McKenney" To: Josh Triplett Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, patches@linaro.org, "Paul E. McKenney" Subject: Re: [PATCH RFC tip/core/rcu 38/41] rcu: Rework detection of use of RCU by offline CPUs Message-ID: <20120202183122.GC2518@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20120201194131.GA10028@linux.vnet.ibm.com> <1328125319-5205-1-git-send-email-paulmck@linux.vnet.ibm.com> <1328125319-5205-38-git-send-email-paulmck@linux.vnet.ibm.com> <20120202061106.GU29058@leaf> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120202061106.GU29058@leaf> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12020218-7182-0000-0000-000000A7BA35 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 01, 2012 at 10:11:06PM -0800, Josh Triplett wrote: > On Wed, Feb 01, 2012 at 11:41:56AM -0800, Paul E. McKenney wrote: > > From: "Paul E. McKenney" > > > > Because newly offlined CPUs continue executing after completing the > > CPU_DYING notifiers, they legitimately enter the scheduler and use > > RCU while appearing to be offline. This calls for a more sophisticated > > approach as follows: > > > > 1. RCU marks the CPU online during the CPU_UP_PREPARE phase. > > > > 2. RCU marks the CPU offline during the CPU_DEAD phase. > > > > 3. Diagnostics regarding use of read-side RCU by offline CPUs use > > RCU's accounting rather than the cpu_online_map. (Note that > > __call_rcu() still uses cpu_online_map to detect illegal > > invocations within CPU_DYING notifiers.) > > > > 4. Offline CPUs are prevented from hanging the system by > > force_quiescent_state(), which pays attention to cpu_online_map. > > Some additional work (in a later commit) will be needed to > > guarantee that force_quiescent_state() waits a full jiffy before > > assuming that a CPU is offline, for example, when called from > > idle entry. > > > > This approach avoids the false positives encountered when attempting to > > use more exact classification of CPU online/offline state. > > Doesn't this fix need to happen *before* the earlier patches in this > series that add splats for RCU usage while offline? Otherwise, > bisection can hit those splats. If someone actually does hit the splats during a real bisection, I will buy you the beverage of your choice. ;-) Thanx, Paul