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 D1726C433EF for ; Tue, 19 Jul 2022 14:42:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239632AbiGSOmA (ORCPT ); Tue, 19 Jul 2022 10:42:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58886 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239645AbiGSOlo (ORCPT ); Tue, 19 Jul 2022 10:41:44 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DA08A40BF6; Tue, 19 Jul 2022 07:38:32 -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 dfw.source.kernel.org (Postfix) with ESMTPS id 3701B617DA; Tue, 19 Jul 2022 14:38:32 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4D190C341CB; Tue, 19 Jul 2022 14:38:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1658241511; bh=pGf1YmN+o/VpKXgM03koQq4c5xvoYJDJMg48G91Fs5A=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=mauTgrih6Ao+I8qPQF/CnqucCGeA9rOuIh7GuSnMETgQ7UqsGeI921C1ASGNR6K3W nMQuQvS+07b+gyeVTVbiqVoUwnKBeLi0iJsslde8RQSEI3GXLtmUgNU8lqQBoJduM4 cvulXe7PXF/kcfqk6jZ65IWz+ODz5BolKky64zY2nPWtuzSTRgDxyROF0Y00BSB8XP OdeZZqRpfKYJ06sW0ZhzOktJlKzH9fBNYV0pvQXa1CLgZSpu1vmKKT1UaqypmefqUw 7ONoJona0VYF/igaLoHNCOT9Z13QasZTLtwsmMSg1wRMgqHlmrPMTgp+IM6GO2FSYP /lGCr3cJX9nYA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id DFC8C5C0813; Tue, 19 Jul 2022 07:38:30 -0700 (PDT) Date: Tue, 19 Jul 2022 07:38:30 -0700 From: "Paul E. McKenney" To: "Zhang, Qiang1" 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: <20220719143830.GM1790663@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220718001610.263700-1-qiang1.zhang@intel.com> <20220718152132.GE1790663@paulmck-ThinkPad-P17-Gen-1> <20220719002604.GK1790663@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 19, 2022 at 04:34:58AM +0000, Zhang, Qiang1 wrote: > On Mon, Jul 18, 2022 at 11:54:53PM +0000, Zhang, Qiang1 wrote: > > 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. > > > > if I understand correctly, you mean that CPU0 enters an RCU Tasks > > Trace read-side critical section in scheduling-clock interrupt context. > > > >Exactly, as might happen if one of the functions in the scheduling-clock interrupt hander were traced/instrumented. > > > > >o CPU 1 starts a new RCU Tasks Trace grace period. > > > > The grace period kthread will scan running tasks on each CPU, The > > tasks currently running on CPU0 will be recorded in the holdout list. > > > >Yes, very good. > > > > >o CPU 0 reaches the newly added rcu_note_voluntary_context_switch(). > > > > In this time, if CPU0 still in RCU Tasks Trace read-side critical > > section, the tasks which running on CPU0 will insert CPU0 blocked > > list. when this tasks exit RCU Tasks Trace read-side critical section, this task will remove from CPU0 block list. > > > > Did I understand the scenario described above correctly? > > > >Looks like it to me. > > > >Could you please resend the patch with this explained in the commit log? Possibly for the benefit of your future self. ;-) > > > > Hi Paul, > > I have resent v3 again, but maybe still need your wording 😊. I do see both copies, thank you. And yes, I am deferring the wordsmithing to a time when I am awake. ;-) Thanx, Paul > Thanks > Zqiang > > > Thanx, Paul > > > > Thanks > > Zqiang > > > > > > > > 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 > > >