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 4507EC43142 for ; Thu, 28 Jun 2018 13:07:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DEA0D273CA for ; Thu, 28 Jun 2018 13:07:11 +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="wTXlkJ60" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DEA0D273CA 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 S935126AbeF1NHK (ORCPT ); Thu, 28 Jun 2018 09:07:10 -0400 Received: from merlin.infradead.org ([205.233.59.134]:35714 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933090AbeF1NHI (ORCPT ); Thu, 28 Jun 2018 09:07:08 -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=wFi6agvxReR/NogVb81FqyUet+GZUKeUFAGRyzXcbso=; b=wTXlkJ60KtcGcvabrN/Xrlpb1 x7VhuEkznJLq5Nmr8GUW8obsK/CvitSpzqhze1pyZvQR8dlVrtEinvihAftzBaHOi1s7xz/C6ACwv BS0WOCF4aS/NO9MIoTusTOhJxjZmOWR4m3s/m7d3se8mw9ta5Kj5GEyGwbsRhW7lbUileA471FM8U ZhQiuPfbBFE62zNqp/zfCLQqHhXKjb4Dqc/dbplTBF+CKu71qcjC/WZLBQCWw1J/bAeBn0mNG4y4T b38Kt6Gl7d7dwv/C0nFG3UpwQBEKAsWrfBqAm5oubPmBqEZIqzz0xWw1PUzWFCA4CuBkbEJqEdIB4 h22vv5Y3Q==; 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 1fYWdO-0006Aq-Br; Thu, 28 Jun 2018 13:06:50 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 562BB20289CE6; Thu, 28 Jun 2018 15:06:46 +0200 (CEST) Date: Thu, 28 Jun 2018 15:06:46 +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: <20180628130646.GH2494@hirez.programming.kicks-ass.net> References: <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> <20180628082653.GX2494@hirez.programming.kicks-ass.net> <20180628123833.GJ3593@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180628123833.GJ3593@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 Thu, Jun 28, 2018 at 05:38:33AM -0700, Paul E. McKenney wrote: > Please let me try again. > > The approach you are suggesting, clever though it is, disables a check https://lkml.kernel.org/r/20180627094633.GG2512@hirez.programming.kicks-ass.net Is the one we're talking about, right? That does not disable any actual check afaict. It simply does not do a wakeup when ran on an offline CPU. And ensures we do an unconditional wakeup soon after from a still running CPU. > of a type that has proved to be an important diagnostic in the past. > It is only reasonable to assume that this check would be important > and helpful in the future, but only if that check remains in the code. I am confused.. > Yes, agreed, given the current structure of the code, this particular > instance of the check would not matter, but experience indicates that > RCU code restructuring is not at all uncommon, with the current effort > being but one case in point. Once more confused... > So, unless I am missing something, the only possible benefit of disabling > this check is getting rid of an acquisition of an uncontended lock in > a code path that is miles (sorry, kilometers) away from any fastpath. > So, again, yes, it is clever. If it sped up a fastpath, I might be > sorely tempted to take it. But the alternative is straightforward and > isn't anywhere near a fastpath. So, though I do very much appreciate > the cleverness and creativity, I am not seeing your change to be a > good tradeoff from a long-term maintainability viewpoint. I think you mean guarantee/invariant instead of check. But I see it no different than any other missed rcu_gp_kthread_wake(). You can similarly fail to make the call while restructuring.