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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 55349C2D0E7 for ; Wed, 25 Mar 2020 18:58:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2756520777 for ; Wed, 25 Mar 2020 18:58:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585162700; bh=o2BxTxrj2k/0+cVDWwgkhZMSJ2JxXuAjc4rM4LKbWQI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=KqwO/PbOwg4GP+ufF8dBo6igGPcXpITgyW10XCQoXcfrUaifM92yeOczrn0WgPgE1 yBBSoIoHURdlfZBKSsaD5p444fu/GNrd+98NxmM1CLnFKg4JLHsFk1mV5iPlykBv4x vDzizapCq7cuxo62Zl9AneMp3CloLpRlrlwRPxtU= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727725AbgCYS6R (ORCPT ); Wed, 25 Mar 2020 14:58:17 -0400 Received: from mail.kernel.org ([198.145.29.99]:56356 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727027AbgCYS6R (ORCPT ); Wed, 25 Mar 2020 14:58:17 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id BE5F52074D; Wed, 25 Mar 2020 18:58:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585162695; bh=o2BxTxrj2k/0+cVDWwgkhZMSJ2JxXuAjc4rM4LKbWQI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=cieVYahw7ltmho4E7Q/hOVLLfXwjAMsqgxZvokkGkzO+orFVOyE4NVoY5pWcXrudb Br0S73Avfih96H0pJ1oOJRQqF6GD4PNS7Zv1Ht9hw5jh/ouGNkfERP+ZQWXErAoOJq Rvcgq4ETgIy9GR4NyWsd3gRF0YXfuIacIrB8b0Ys= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 8E8BA3520BDC; Wed, 25 Mar 2020 11:58:15 -0700 (PDT) Date: Wed, 25 Mar 2020 11:58:15 -0700 From: "Paul E. McKenney" To: Cong Wang Cc: Thomas Gleixner , syzbot , David Miller , Jamal Hadi Salim , Jiri Pirko , Jakub Kicinski , LKML , Linux Kernel Network Developers , syzkaller-bugs Subject: Re: WARNING: ODEBUG bug in tcindex_destroy_work (3) Message-ID: <20200325185815.GW19865@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <000000000000742e9e05a10170bc@google.com> <87a74arown.fsf@nanos.tec.linutronix.de> <87ftdypyec.fsf@nanos.tec.linutronix.de> <875zeuftwm.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Wed, Mar 25, 2020 at 11:36:16AM -0700, Cong Wang wrote: > On Mon, Mar 23, 2020 at 6:01 PM Thomas Gleixner wrote: > > > > Cong Wang writes: > > > On Mon, Mar 23, 2020 at 2:14 PM Thomas Gleixner wrote: > > >> > We use an ordered workqueue for tc filters, so these two > > >> > works are executed in the same order as they are queued. > > >> > > >> The workqueue is ordered, but look how the work is queued on the work > > >> queue: > > >> > > >> tcf_queue_work() > > >> queue_rcu_work() > > >> call_rcu(&rwork->rcu, rcu_work_rcufn); > > >> > > >> So after the grace period elapses rcu_work_rcufn() queues it in the > > >> actual work queue. > > >> > > >> Now tcindex_destroy() is invoked via tcf_proto_destroy() which can be > > >> invoked from preemtible context. Now assume the following: > > >> > > >> CPU0 > > >> tcf_queue_work() > > >> tcf_queue_work(&r->rwork, tcindex_destroy_rexts_work); > > >> > > >> -> Migration > > >> > > >> CPU1 > > >> tcf_queue_work(&p->rwork, tcindex_destroy_work); > > >> > > >> So your RCU callbacks can be placed on different CPUs which obviously > > >> has no ordering guarantee at all. See also: > > > > > > Good catch! > > > > > > I thought about this when I added this ordered workqueue, but it > > > seems I misinterpret max_active, so despite we have max_active==1, > > > more than 1 work could still be queued on different CPU's here. > > > > The workqueue is not the problem. it works perfectly fine. The way how > > the work gets queued is the issue. > > Well, a RCU work is also a work, so the ordered workqueue should > apply to RCU works too, from users' perspective. Users should not > need to learn queue_rcu_work() is actually a call_rcu() which does > not guarantee the ordering for an ordered workqueue. And the workqueues might well guarantee the ordering in cases where the pair of RCU callbacks are invoked in a known order. But that workqueues ordering guarantee does not extend upstream to RCU, nor do I know of a reasonable way to make this happen within the confines of RCU. If you have ideas, please do not keep them a secret, but please also understand that call_rcu() must meet some pretty severe performance and scalability constraints. I suppose that queue_rcu_work() could track outstanding call_rcu() invocations, and (one way or another) defer the second queue_rcu_work() if a first one is still pending from the current task, but that might not make the common-case user of queue_rcu_work() all that happy. But perhaps there is a way to restrict these semantics to ordered workqueues. In that case, one could imagine the second and subsequent too-quick call to queue_rcu_work() using the rcu_head structure's ->next field to queue these too-quick callbacks, and then having rcu_work_rcufn() check for queued too-quick callbacks, queuing the first one. But I must defer to Tejun on this one. And one additional caution... This would meter out ordered queue_rcu_work() requests at a rate of no faster than one per RCU grace period. The queue might build up, resulting in long delays. Are you sure that your use case can live with this? > > > I don't know how to fix this properly, I think essentially RCU work > > > should be guaranteed the same ordering with regular work. But this > > > seems impossible unless RCU offers some API to achieve that. > > > > I don't think that's possible w/o putting constraints on the flexibility > > of RCU (Paul of course might disagree). > > > > I assume that the filters which hang of tcindex_data::perfect and > > tcindex_data:p must be freed before tcindex_data, right? > > > > Refcounting of tcindex_data should do the trick. I.e. any element which > > you add to a tcindex_data instance takes a refcount and when that is > > destroyed then the rcu/work callback drops a reference which once it > > reaches 0 triggers tcindex_data to be freed. > > Yeah, but the problem is more than just tcindex filter, we have many > places make the same assumption of ordering. But don't you also have a situation where there might be a large group of queue_rcu_work() invocations whose order doesn't matter, followed by a single queue_rcu_work() invocation that must be ordered after the earlier group? If so, ordering -all- of these invocations might be overkill. Or did I misread your code? Thanx, Paul