From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.1 required=3.0 tests=DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,T_DKIM_INVALID, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 226E5C43142 for ; Tue, 26 Jun 2018 17:51:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D318A26E5D for ; Tue, 26 Jun 2018 17:51:41 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="XXVcvfqD" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D318A26E5D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933973AbeFZRvk (ORCPT ); Tue, 26 Jun 2018 13:51:40 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:40422 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752597AbeFZRvg (ORCPT ); Tue, 26 Jun 2018 13:51:36 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=Qv71m6jFU7lsZ2rKFZvtBOPZBhBWmX95qPyFmYuRgf0=; b=XXVcvfqDCjSac0dJE9jpjmHrl /o0/lgGj15Do2of/k1ReaAIJ4R4rrkj1ptXuWneYm3IJjaBAqRtFVCIh7Ymv8MC0vfmvYbT3kTI8C rL/aOR6LYHdSpS0M3Uw/zQtzjtlrjGDegdQ84EivthjI7SwoOrhh5mjhRXz7+WJWa9k0IDHuVy6lm VYOWV/RvqaZPG3gOtoFWSXaSso6TNoCaVG9n7/b/x2BrsIaJ+S9v8JuEyJ+3elDEwq0kQvWAYF29Q +Ai5xoyIqcSAHTAF4vnEFeejmm39BYtgyTEw1RWdrMcNVKekIkPhHtxH/49oIjJ16Xk8NXMlIeH0Y YGGmRuduQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1fXs7d-0000qd-TW; Tue, 26 Jun 2018 17:51:22 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 0606C2029F1DB; Tue, 26 Jun 2018 19:51:20 +0200 (CEST) Date: Tue, 26 Jun 2018 19:51:19 +0200 From: Peter Zijlstra 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, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH tip/core/rcu 13/22] rcu: Fix grace-period hangs due to race with CPU offline Message-ID: <20180626175119.GL2494@hirez.programming.kicks-ass.net> References: <20180626002052.GA24146@linux.vnet.ibm.com> <20180626171048.2181-13-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180626171048.2181-13-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 26, 2018 at 10:10:39AM -0700, Paul E. McKenney wrote: > Without special fail-safe quiescent-state-propagation checks, grace-period > hangs can result from the following scenario: > > 1. CPU 1 goes offline. > > 2. Because CPU 1 is the only CPU in the system blocking the current > grace period, as soon as rcu_cleanup_dying_idle_cpu()'s call to > rcu_report_qs_rnp() returns. > > 3. At this point, the leaf rcu_node structure's ->lock is no longer > held: rcu_report_qs_rnp() has released it, as it must in order > to awaken the RCU grace-period kthread. > > 4. At this point, that same leaf rcu_node structure's ->qsmaskinitnext > field still records CPU 1 as being online. This is absolutely > necessary because the scheduler uses RCU, and ->qsmaskinitnext Can you expand a bit on this, where does the scheduler care about the online state of the CPU that's about to call into arch_cpu_idle_dead()? > contains RCU's idea as to which CPUs are online. Therefore, > invoking rcu_report_qs_rnp() after clearing CPU 1's bit from > ->qsmaskinitnext would result in a lockdep-RCU splat due to > RCU being used from an offline CPU. > > 5. RCU's grace-period kthread awakens, sees that the old grace period > has completed and that a new one is needed. It therefore starts > a new grace period, but because CPU 1's leaf rcu_node structure's > ->qsmaskinitnext field still shows CPU 1 as being online, this new > grace period is initialized to wait for a quiescent state from the > now-offline CPU 1. If we're past cpuhp_report_idle_cpu() -> rcu_report_dead(), then cpu_offline() is true. Is that not sufficient state to avoid this? > 6. Without the fail-safe force-quiescent-state checks, there would > be no quiescent state from the now-offline CPU 1, which would > eventually result in RCU CPU stall warnings and memory exhaustion.