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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 40C41C4167B for ; Tue, 31 Oct 2023 18:12:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1376631AbjJaSMH (ORCPT ); Tue, 31 Oct 2023 14:12:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37518 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230455AbjJaSMG (ORCPT ); Tue, 31 Oct 2023 14:12:06 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B441DA2; Tue, 31 Oct 2023 11:12:02 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 43F2FC433C8; Tue, 31 Oct 2023 18:12:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1698775922; bh=9Jf3KU3ADv1NIFoN1APkqO365A/HCOvTo+JR2JcocMY=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=m1gSkGf+mN9nCYslCyx8aeuQNeOTNjEp13rhkxSfnXOxiAlTZpNI+17No1M4HTmSr 1VDU1rskOdb2pb/n57E2y4O9KHsS4gLtIjifqOj3cPkRgVljsY0MRgQM0MQmRkClF1 4ohlthrUN+MFeDQH9ZBmwxBzPdsuCaku+dM1CeY4Bk5/wZpsxpFTKWYu5SJdC+2mf5 UP9F/ua0/3jY80iNpM7ImM7kHKoqg2Nwe7yJKJmAf68TAvXcYjavDdSaKw+ykncFLy hEZjc+Klg0lZoDBhTLyzXCHM5g/wyWBPgoudZp0T0AlEAIbgj5SBbMmG7w7dN5E7wO sEPfk7YalbQtQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id C9039CE0B77; Tue, 31 Oct 2023 11:12:01 -0700 (PDT) Date: Tue, 31 Oct 2023 11:12:01 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Frederic Weisbecker , LKML , Boqun Feng , Joel Fernandes , Josh Triplett , Mathieu Desnoyers , Neeraj Upadhyay , Steven Rostedt , Uladzislau Rezki , rcu , Zqiang , "Liam R . Howlett" , matz@suse.de, ubizjak@gmail.com Subject: Re: [PATCH 2/4] rcu/tasks: Handle new PF_IDLE semantics Message-ID: Reply-To: paulmck@kernel.org References: <20231027144050.110601-3-frederic@kernel.org> <20231027192026.GG26550@noisy.programming.kicks-ass.net> <2a0d52a5-5c28-498a-8df7-789f020e36ed@paulmck-laptop> <20231027224628.GI26550@noisy.programming.kicks-ass.net> <200c57ce-90a7-418b-9527-602dbf64231f@paulmck-laptop> <20231030082138.GJ26550@noisy.programming.kicks-ass.net> <622438a5-4d20-4bc9-86b9-f3de55ca6cda@paulmck-laptop> <20231031095202.GC35651@noisy.programming.kicks-ass.net> <58c82a9d-f796-4585-b392-401b8b9dbc2e@paulmck-laptop> <20231031152033.GC15024@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231031152033.GC15024@noisy.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 31, 2023 at 04:20:33PM +0100, Peter Zijlstra wrote: > On Tue, Oct 31, 2023 at 07:24:13AM -0700, Paul E. McKenney wrote: > > > So, at least until GCC catches up to clang's code generation, I take it > > that you don't want WRITE_ONCE() for that ->nvcsw increment. Thoughts on > > ->on_rq? > > I've not done the patch yet, but I suspect those would be fine, those > are straight up stores, hard to get wrong (famous last words). Assuming that the reads are already either marked with READ_ONCE() or are under appropriate locks, my immediate thought would be something like the all-too-lightly tested patch shown below. The ASSERT_EXCLUSIVE_WRITER() causes KCSAN to complain if there is a concurrent store of any kind to the location. Of course, please feel free to ignore. Thoughts? Thanx, Paul ------------------------------------------------------------------------ diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 81885748871d..aeace19ad7f5 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2124,12 +2124,14 @@ void activate_task(struct rq *rq, struct task_struct *p, int flags) enqueue_task(rq, p, flags); - p->on_rq = TASK_ON_RQ_QUEUED; + WRITE_ONCE(p->on_rq, TASK_ON_RQ_QUEUED); + ASSERT_EXCLUSIVE_WRITER(p->on_rq); } void deactivate_task(struct rq *rq, struct task_struct *p, int flags) { - p->on_rq = (flags & DEQUEUE_SLEEP) ? 0 : TASK_ON_RQ_MIGRATING; + WRITE_ONCE(p->on_rq, (flags & DEQUEUE_SLEEP) ? 0 : TASK_ON_RQ_MIGRATING); + ASSERT_EXCLUSIVE_WRITER(p->on_rq); dequeue_task(rq, p, flags); }