From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751430AbdJENRH (ORCPT ); Thu, 5 Oct 2017 09:17:07 -0400 Received: from mail.kernel.org ([198.145.29.99]:58666 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751280AbdJENRG (ORCPT ); Thu, 5 Oct 2017 09:17:06 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B3D8821905 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=goodmis.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=rostedt@goodmis.org Date: Thu, 5 Oct 2017 09:17:03 -0400 From: Steven Rostedt To: "Paul E. McKenney" 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, peterz@infradead.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com Subject: Re: [PATCH tip/core/rcu 1/9] rcu: Provide GP ordering in face of migrations and delays Message-ID: <20171005091703.2ac1c12c@gandalf.local.home> In-Reply-To: <1507152575-11055-1-git-send-email-paulmck@linux.vnet.ibm.com> References: <20171004212915.GA10089@linux.vnet.ibm.com> <1507152575-11055-1-git-send-email-paulmck@linux.vnet.ibm.com> X-Mailer: Claws Mail 3.14.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 4 Oct 2017 14:29:27 -0700 "Paul E. McKenney" wrote: > Consider the following admittedly improbable sequence of events: > > o RCU is initially idle. > > o Task A on CPU 0 executes rcu_read_lock(). A starts rcu_read_lock() critical section. > > o Task B on CPU 1 executes synchronize_rcu(), which must > wait on Task A: B waits for A. > > o Task B registers the callback, which starts a new > grace period, awakening the grace-period kthread > on CPU 3, which immediately starts a new grace period. [ isn't B blocked (off rq)? How does it migrate? ] > > o Task B migrates to CPU 2, which provides a quiescent > state for both CPUs 1 and 2. > > o Both CPUs 1 and 2 take scheduling-clock interrupts, > and both invoke RCU_SOFTIRQ, both thus learning of the > new grace period. > > o Task B is delayed, perhaps by vCPU preemption on CPU 2. > > o CPUs 2 and 3 pass through quiescent states, which are reported > to core RCU. > > o Task B is resumed just long enough to be migrated to CPU 3, > and then is once again delayed. > > o Task A executes rcu_read_unlock(), exiting its RCU read-side > critical section. A calls rcu_read_unlock() ending the critical section > > o CPU 0 passes through a quiescent sate, which is reported to > core RCU. Only CPU 1 continues to block the grace period. > > o CPU 1 passes through a quiescent state, which is reported to > core RCU. This ends the grace period, and CPU 1 therefore > invokes its callbacks, one of which awakens Task B via > complete(). > > o Task B resumes (still on CPU 3) and starts executing > wait_for_completion(), which sees that the completion has > already completed, and thus does not block. It returns from > the synchronize_rcu() without any ordering against the > end of Task A's RCU read-side critical section. B runs > > It can therefore mess up Task A's RCU read-side critical section, > in theory, anyway. I don't see how B ran during A's critical section. -- Steve > > However, if CPU hotplug ever gets rid of stop_machine(), there will be > more straightforward ways for this sort of thing to happen, so this > commit adds a memory barrier in order to enforce the needed ordering. >