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=-5.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 4D7F2C43461 for ; Wed, 9 Sep 2020 15:16:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0A54D22240 for ; Wed, 9 Sep 2020 15:16:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1599664592; bh=oAgyl5YCzOi2Oys2dADhKhmSZ1VDNwqV2xCYDMZdcWw=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=y4e9EdEyZikcvInBuAk2Q3u/f/YZvmpIfDRUFpPoPcbxq3JwC/qdcv6x7suJ7wjz+ WgBQLkFkkj+kfmOFlQ5ysoznsfJpEQa4XllRHbSS0TbBannRcV8X9YQzHryyWmNMVm 3KnNdlTFjmvW9Qaet7p8CRlU886LUhRgTVT7mUEE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728631AbgIIPQ3 (ORCPT ); Wed, 9 Sep 2020 11:16:29 -0400 Received: from mail.kernel.org ([198.145.29.99]:36526 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729135AbgIILao (ORCPT ); Wed, 9 Sep 2020 07:30:44 -0400 Received: from paulmck-ThinkPad-P72.home (unknown [50.45.173.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E1F5820897; Wed, 9 Sep 2020 11:22:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1599650561; bh=oAgyl5YCzOi2Oys2dADhKhmSZ1VDNwqV2xCYDMZdcWw=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=VGlHUA84CYJaxv0ABm+a7LxuNh29gVlf7PMfczNBCR98EuKwBIr0Bd91ClwrEdbao ZJmTM2fuC1vKlva5mCMB7bzzkTO9BoXDAgwzncyqAvGsXx1PyTsiLJdMRE2XiO4/rq m3UHhFdkjCrWGoIUrbT2sCHDNVlTZYQL/eUnmEZA= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id AB27F3523091; Wed, 9 Sep 2020 04:22:41 -0700 (PDT) Date: Wed, 9 Sep 2020 04:22:41 -0700 From: "Paul E. McKenney" To: "Zhang, Qiang" Cc: Joel Fernandes , Uladzislau Rezki , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , rcu , LKML Subject: Re: RCU: Question rcu_preempt_blocked_readers_cgp in rcu_gp_fqs_loop func Message-ID: <20200909112241.GE29330@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200818220245.GO27891@paulmck-ThinkPad-P72> <20200819135654.GB3875610@google.com> <20200819152159.GX27891@paulmck-ThinkPad-P72> <20200819155808.GA8817@pc636> <20200820223957.GB120898@google.com> <20200821153328.GH2855@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Wed, Sep 09, 2020 at 07:03:39AM +0000, Zhang, Qiang wrote: > > When config preempt RCU, and then there are multiple levels node, the current task is preempted in rcu read critical region. > the current task be add to "rnp->blkd_tasks" link list, and the "rnp->gp_tasks" may be assigned a value . these rnp is leaf node in RCU tree. > > But in "rcu_gp_fqs_loop" func, we check blocked readers in root node. > > static void rcu_gp_fqs_loop(void) > { > ..... > struct rcu_node *rnp = rcu_get_root(); > ..... > if (!READ_ONCE(rnp->qsmask) && > !rcu_preempt_blocked_readers_cgp(rnp)) ------> rnp is root node > break; > .... > } > > the root node's blkd_tasks never add task, the "rnp->gp_tasks" is never be assigned value, this check is invailed. > Should we check leaf nodes like this There are two cases: 1. There is only a single rcu_node structure, which is both root and leaf. In this case, the current check is required: Both ->qsmask and the ->blkd_tasks list must be checked. Your rcu_preempt_blocked_readers() would work in this case, but the current code is a bit faster because it does not need to acquire the ->lock nor does it need the loop overhead. 2. There are multiple levels. In this case, as you say, the root rcu_node structure's ->blkd_tasks list will always be empty. But also in this case, the root rcu_node structure's ->qsmask cannot be zero until all the leaf rcu_node structures' ->qsmask fields are zero and their ->blkd_tasks lists no longer have tasks blocking the current grace period. This means that your rcu_preempt_blocked_readers() function would never return true in this case. So the current code is fine. Are you seeing failures on mainline kernels? If so, what is the failure mode? Thanx, Paul > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1846,6 +1846,25 @@ static bool rcu_gp_init(void) > return true; > } > > +static bool rcu_preempt_blocked_readers(void) > +{ > + struct rcu_node *rnp; > + unsigned long flags; > + bool ret = false; > + > + rcu_for_each_leaf_node(rnp) { > + raw_spin_lock_irqsave_rcu_node(rnp, flags); > + if (rcu_preempt_blocked_readers_cgp(rnp)) { > + ret = true; > + raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > + break; > + } > + raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > + } > + > + return ret; > +} > + > /* > * Helper function for swait_event_idle_exclusive() wakeup at force-quiescent-state > * time. > @@ -1864,7 +1883,7 @@ static bool rcu_gp_fqs_check_wake(int *gfp) > return true; > > // The current grace period has completed. > - if (!READ_ONCE(rnp->qsmask) && !rcu_preempt_blocked_readers_cgp(rnp)) > + if (!READ_ONCE(rnp->qsmask) && !rcu_preempt_blocked_readers()) > return true; > > return false; > @@ -1927,7 +1946,7 @@ static void rcu_gp_fqs_loop(void) > /* Locking provides needed memory barriers. */ > /* If grace period done, leave loop. */ > if (!READ_ONCE(rnp->qsmask) && > - !rcu_preempt_blocked_readers_cgp(rnp)) > + !rcu_preempt_blocked_readers()) > break; > /* If time for quiescent-state forcing, do it. */ > if (!time_after(rcu_state.jiffies_force_qs, jiffies) || > -- > > > thanks > Qiang