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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 A9932C433DF for ; Mon, 15 Jun 2020 16:41:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 821822067B for ; Mon, 15 Jun 2020 16:41:06 +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="GXykYu6g" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730991AbgFOQlF (ORCPT ); Mon, 15 Jun 2020 12:41:05 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58432 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730622AbgFOQlE (ORCPT ); Mon, 15 Jun 2020 12:41:04 -0400 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:e::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BE7A0C061A0E for ; Mon, 15 Jun 2020 09:41:04 -0700 (PDT) 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; bh=7LuY4oP/Ae5Vicv3QaUp0hzRom8eAXJcz6uKt/S03vU=; b=GXykYu6ggGOh3TZnYvrW2zjpMx 1swQBHT81LnTympTTOMoIEfSxYE3MhVzQ+ahw3BTqpZiGU1X/oLPEu0U40rVsyXzyJ8n9SkgP/r4M 6HPysHro5LML2h/5+TIjxeMyznjUx1T5O4RbIDAK5Qj64MMptWjtTKxayVjlqYds+IvcRm7lxcYiK dL5vR3NgQenKk0/LZ14IWwpDzQq1SS/2jkMExdnw39H56pSziyBupAkBlwYAut6ZYUnMxfcqgigwH TPmV3ODMQgheZvfummSHBRFm65K/QiRGdZbw8sWywEjhSVJ56y/XqvheyPZK0emoaX7WBcdfhLE0d kS+YLQxg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1jksAE-0004Ss-EV; Mon, 15 Jun 2020 16:40:50 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id A9A613003E1; Mon, 15 Jun 2020 18:40:48 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 82D96203B8160; Mon, 15 Jun 2020 18:40:48 +0200 (CEST) Date: Mon, 15 Jun 2020 18:40:48 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: mingo@kernel.org, tglx@linutronix.de, linux-kernel@vger.kernel.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, frederic@kernel.org Subject: Re: [PATCH 0/6] sched: TTWU, IPI, and assorted stuff Message-ID: <20200615164048.GC2531@hirez.programming.kicks-ass.net> References: <20200615125654.678940605@infradead.org> <20200615162330.GF2723@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200615162330.GF2723@paulmck-ThinkPad-P72> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 15, 2020 at 09:23:30AM -0700, Paul E. McKenney wrote: > On Mon, Jun 15, 2020 at 02:56:54PM +0200, Peter Zijlstra wrote: > > Hi, > > > > So Paul reported rcutorture hitting a NULL dereference, and patch #1 fixes it. > > > > Now, patch #1 is obviously correct, but I can't explain how exactly it leads to > > the observed NULL pointer dereference. The NULL pointer deref happens in > > find_matching_se()'s last while() loop when is_same_group() fails even though > > both parents are NULL. > > My bisection of yet another bug sometimes hits the scheduler NULL pointer > dereference on older commits. I will try out patch #2. Thanks! I've got 16*TREE03 running since this morning, so far so nothing :/ (FWIW that's 16/9 times overcommit, idle time fluctuates around 10%). > Whether this is reassuring or depressing, I have no idea. :-/ Worrysome at least, I don't trust stuff I can't explain. > > The only explanation I have for that is that we just did an activate_task() > > while: 'task_cpu(p) != cpu_of(rq)', because then 'p->se.cfs_rq' doesn't match. > > However, I can't see how the lack of #1 would lead to that. Never-the-less, > > patch #2 adds assertions to warn us of this case. > > > > Patch #3 is a trivial rename that ought to eradicate some confusion. > > > > The last 3 patches is what I ended up with for cleaning up the whole > > smp_call_function/irq_work/ttwu thing more. > > Would it be possible to allow a target CPU # on those instances of > __call_single_data? This is extremely helpful for debugging lost > smp_call_function*() calls. target or source ? Either would be possible, perhaps even both. We have a spare u32 in __call_single_node. Something like the below on top of 1-4. If we want to keep this, we should probably stick it under some CONFIG_DBUG thing or other. --- a/include/linux/smp_types.h +++ b/include/linux/smp_types.h @@ -61,6 +61,7 @@ struct __call_single_node { unsigned int u_flags; atomic_t a_flags; }; + u16 src, dst; }; #endif /* __LINUX_SMP_TYPES_H */ --- a/kernel/smp.c +++ b/kernel/smp.c @@ -135,8 +135,14 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(cal void __smp_call_single_queue(int cpu, struct llist_node *node) { + struct __call_single_node *n = + container_of(node, struct __call_single_node, llist); + WARN_ON_ONCE(cpu == smp_processor_id()); + n->src = smp_processor_id(); + n->dst = cpu; + /* * The list addition should be visible before sending the IPI * handler locks the list to pull the entry off it because of