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=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 18C63C388F9 for ; Wed, 11 Nov 2020 20:45:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A0D10208B8 for ; Wed, 11 Nov 2020 20:45:09 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="QF9Lp+Dv" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726707AbgKKUpI (ORCPT ); Wed, 11 Nov 2020 15:45:08 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36344 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725900AbgKKUpI (ORCPT ); Wed, 11 Nov 2020 15:45:08 -0500 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D8FA5C0613D1; Wed, 11 Nov 2020 12:45:07 -0800 (PST) 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; bh=CsuP2ZbFV021yD83Asf/WJrsarPfzdmIq9MXLED49SE=; b=QF9Lp+Dv+5PrX67MKMP/B8Ri5v WnPV6mS+OhKhV1U72wkx3T64VjCFcm7V2HI4G7FzyyHU9uInlAIqTjutPsdBYLM3OdRRlLOsXV5/s hfnzgSqwDYUgqnnqY6OiG2pc6eugSLTBBj89ba1fqWiIpkPafPZ5pcwlN1JpwFCFCgsc1hgWNatpd 08yPCU5Lh9qGFj+2DZ1HzeEFzJhC+59QerfgRx2tzvZUvhjF78gAonDyzKShdXP0x7SXlNSnAMxOM istBiplwdwTvAISnRyQr5JA3d8mrkWQfbxNDDxbAdPhzO3k/k8eap/2fPZwCeZSEuZ1O1RZ+aIlvu kJw7TsWw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kcwzF-000766-PU; Wed, 11 Nov 2020 20:45:02 +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 9EED9301E02; Wed, 11 Nov 2020 21:45:00 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 6A0352BCE962B; Wed, 11 Nov 2020 21:45:00 +0100 (CET) Date: Wed, 11 Nov 2020 21:45:00 +0100 From: Peter Zijlstra To: Paul Bolle Cc: linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org, Scott Wood , Valentin Schneider , Daniel Bristot de Oliveira , x86@kernel.org Subject: Re: [tip: sched/core] sched: Fix balance_callback() Message-ID: <20201111204500.GR2628@hirez.programming.kicks-ass.net> References: <20201023102346.203901269@infradead.org> <160508300397.11244.13967684821070428528.tip-bot2@tip-bot2> <6356963f376a0798e8c939f813c2efe05d32c6d7.camel@tiscali.nl> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6356963f376a0798e8c939f813c2efe05d32c6d7.camel@tiscali.nl> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 11, 2020 at 09:30:42PM +0100, Paul Bolle wrote: > tip-bot2 for Peter Zijlstra schreef op wo 11-11-2020 om 08:23 [+0000]: > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > [...] > > +static void do_balance_callbacks(struct rq *rq, struct callback_head *head) > > +{ > > + void (*func)(struct rq *rq); > > + struct callback_head *next; > > + > > + lockdep_assert_held(&rq->lock); > > + > > + while (head) { > > + func = (void (*)(struct rq *))head->func; > > + next = head->next; > > + head->next = NULL; > > + head = next; > > Naive question: is there some subtle C-issue that is evaded here by setting > head->next to NULL prior to copying over it? > > (I know this piece of code only got copied around in this patch and this is > therefor not something that this patch actually introduced.) It's like list_del_init(), it zeros the entry before unlinking it. queue_balance_callback() relies on this.