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 96926C433EF for ; Mon, 18 Jul 2022 15:21:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235314AbiGRPVh (ORCPT ); Mon, 18 Jul 2022 11:21:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36592 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234749AbiGRPVg (ORCPT ); Mon, 18 Jul 2022 11:21:36 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A02DD24F19; Mon, 18 Jul 2022 08:21:35 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 4B21EB81648; Mon, 18 Jul 2022 15:21:34 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id DFD6FC341C0; Mon, 18 Jul 2022 15:21:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1658157693; bh=I/TBfThf+GVBYyftAkmpjprcJnF2VvnJnQVBOLBfJ10=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=lA/7IRBZr7io4jctntTUqJROC4dYgBh5BxVx4jcNtGnVdM11O4X7CsC450a1re6oe 5HcnxNne/Z+uLZV3TkA4bXPdyZvzldmDBjP5SaIeGKRIqLAsDBCcXrBjU7BQQ4HbBD k0nlRC6D8rDt8MSuda/pZ6bysmnDDc4Tt9AD6S+P/Wx1msxVKQ2LedvfG5O1Xrihsx 9mBwp3+DvAvEHZqy3nTbug44S2RTnz+g5dh3A6DR7Js4C3uk4wI9CQ2MOfN4uPEKdg JSPwj0wSaev+cAkiikzwUekLkb9ySmhtPT7uXl4RXhjfmhlJ/zz47l1fCLJOJMq9k4 CHTehAd3VdZTg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 844E55C0833; Mon, 18 Jul 2022 08:21:32 -0700 (PDT) Date: Mon, 18 Jul 2022 08:21:32 -0700 From: "Paul E. McKenney" To: Zqiang Cc: frederic@kernel.org, quic_neeraju@quicinc.com, joel@joelfernandes.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] rcu-tasks: Make RCU Tasks Trace checking for userspace execution Message-ID: <20220718152132.GE1790663@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220718001610.263700-1-qiang1.zhang@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220718001610.263700-1-qiang1.zhang@intel.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 18, 2022 at 08:16:10AM +0800, Zqiang wrote: > For RCU tasks trace, the userspace execution is also a valid quiescent > state, if the task is in userspace, the ->trc_reader_nesting should be > zero and if the ->trc_reader_special.b.need_qs is not set, set the > tasks ->trc_reader_special.b.need_qs is TRC_NEED_QS_CHECKED, this cause > grace-period kthread remove it from holdout list if it remains here. > > This commit add rcu_tasks_trace_qs() to rcu_flavor_sched_clock_irq() > when the kernel built with no PREEMPT_RCU. > > Signed-off-by: Zqiang The looks plausible to me, but can you tell me how this avoids the following sequence of events? o CPU 0 takes a scheduling-clock interrupt. Just before this point CPU 0 was running in user context, thus as you say should not be in an RCU Tasks quiescent state. o CPU 0 enters an RCU Tasks Trace read-side critical section. o CPU 1 starts a new RCU Tasks Trace grace period. o CPU 0 reaches the newly added rcu_note_voluntary_context_switch(). Except that the quiescent state implied by userspace execution was before the new grace period, and thus does not apply to it. (Yes, I know, if this is a bug in this patch, the bug already exists due to the call in rcu_flavor_sched_clock_irq() for !PREEMPT kernels, but if this change is safe, it should be possible to explain why.) Thanx, Paul > --- > v1->v2: > Fix build error due to undeclared rcu_tasks_trace_qs(), note in no-PREEMPT_RCU > kernel, the RCU Tasks is replaced by RCU, so rcu_note_voluntary_context_switch() > only include rcu_tasks_trace_qs(). > > kernel/rcu/tree_plugin.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index 4152816dd29f..5fb0b2dd24fd 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -976,7 +976,7 @@ static void rcu_flavor_sched_clock_irq(int user) > * neither access nor modify, at least not while the > * corresponding CPU is online. > */ > - > + rcu_note_voluntary_context_switch(current); > rcu_qs(); > } > } > -- > 2.25.1 >