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 F3DCBC43142 for ; Thu, 28 Jun 2018 08:27:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9911F24623 for ; Thu, 28 Jun 2018 08:27:29 +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="mLSEmGTz" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9911F24623 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 S934754AbeF1I10 (ORCPT ); Thu, 28 Jun 2018 04:27:26 -0400 Received: from merlin.infradead.org ([205.233.59.134]:52104 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933589AbeF1I1Y (ORCPT ); Thu, 28 Jun 2018 04:27:24 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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=Fiy2xpryowEaFh5QWeXWRpND0F3gPngbzBs3bNsoAF8=; b=mLSEmGTzssBlZckNhb0FqGx5b 6birtN4bK6YgQQ/qrWLrb1oiOioWOA9gUs40Bcpqqh5lLgxKnb25xzzExR8xmfKXFGtbAOA8uMRJe zzilLFUyUPg9XF7GSgy9+uUqsYjpLmIG/EmweOrShO8H0IBQaSGHcBUCgU0+WrhGdBTLwHgL1sUPx lFLAreufcbA6goaqr3Wc25BaR8clUumkJQNVefDGHooXaSnO57J1+SOBTfNWzRgYmQhqLDnL+5Kjk agXyiRHVhLHPe2MirmUYXeZxdHMLFKhWL5SQdWFGWkhBTDdpHb2p+swVpIcLrWPLhI0pHYd755aiJ V3r9CFRsQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1fYSGV-0002Yi-NA; Thu, 28 Jun 2018 08:26:55 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 100A120297645; Thu, 28 Jun 2018 10:26:53 +0200 (CEST) Date: Thu, 28 Jun 2018 10:26:53 +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: <20180628082653.GX2494@hirez.programming.kicks-ass.net> References: <20180626175119.GL2494@hirez.programming.kicks-ass.net> <20180626182950.GH3593@linux.vnet.ibm.com> <20180626202615.GA32162@linux.vnet.ibm.com> <20180626203225.GT2494@hirez.programming.kicks-ass.net> <20180626234004.GQ3593@linux.vnet.ibm.com> <20180627091106.GB7184@worktop.programming.kicks-ass.net> <20180627094633.GG2512@hirez.programming.kicks-ass.net> <20180627155721.GZ3593@linux.vnet.ibm.com> <20180627175134.GV2494@hirez.programming.kicks-ass.net> <20180628051334.GG3593@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180628051334.GG3593@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 Wed, Jun 27, 2018 at 10:13:34PM -0700, Paul E. McKenney wrote: > On Wed, Jun 27, 2018 at 07:51:34PM +0200, Peter Zijlstra wrote: > > On Wed, Jun 27, 2018 at 08:57:21AM -0700, Paul E. McKenney wrote: > > > > Another variant, which simply skips the wakeup whever ran on an offline > > > > CPU, relying on the wakeup from rcutree_migrate_callbacks() right after > > > > the CPU really is dead. > > > > > > Cute! ;-) > > > > > > And a much smaller change. > > > > > > However, this means that if someone indirectly and erroneously causes > > > rcu_report_qs_rsp() to be invoked from an offline CPU, the result is an > > > intermittent and difficult-to-debug grace-period hang. A lockdep splat > > > whose stack trace directly implicates the culprit is much better. > > > > How so? We do an unconditional wakeup right after finding the offline > > cpu dead. There is only very limited code between offline being true and > > the CPU reporting in dead. > > I am thinking more generally than this particular patch. People > sometimes invoke things from places they shouldn't, for example, the > situation leading to your patch that allows use of RCU much earlier in > the CPU-online process. It is nicer to get a splat in those situations > than a silent hang. The rcu_rnp_online_cpus() thing would catch that, right? The public RCU API isn't that big, and should already complain afaict.