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=-11.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 9B770C433E2 for ; Fri, 4 Sep 2020 14:20:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6D3682074A for ; Fri, 4 Sep 2020 14:20:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1599229206; bh=BtytqVGyyDVRZDTEU0QB6ywre+STIRvU9Yz2aZXK7HA=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=o3ogMlHL0QmYczagjrl8s27l4Bm/U9ez4avCxfWv3CXMIoBEGH7DdgOcXzWZ8hUVg kDmw2LmDu6sPoFMdnw9tCs8RUxK5H890MvoOf+GvMt4kT7VY/g0xbyCENGCDgkBacI zv42vlJKuEJnpToDmGugFH1VZZQaZPs067Z7WhN0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730682AbgIDOUD (ORCPT ); Fri, 4 Sep 2020 10:20:03 -0400 Received: from mail.kernel.org ([198.145.29.99]:44352 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730497AbgIDNmy (ORCPT ); Fri, 4 Sep 2020 09:42:54 -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 6A59220C56; Fri, 4 Sep 2020 13:41:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1599226902; bh=BtytqVGyyDVRZDTEU0QB6ywre+STIRvU9Yz2aZXK7HA=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=sfmkLiVVP2qqSOIje7b185U4Yw009wVHgfkdq0kq/k6zAGJsGl2atuNQ3OX8krNzn DwEH68uwfki4NBuH95EUkpySMm8gyzQCk2fEHJFfKEKn9GF6xVd/x8n+LY5vF3sDdA TaTpGmyIOd+dXq7Sg72tsFGJEH2tn8mi4bSbg1hg= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 4C2AB352118D; Fri, 4 Sep 2020 06:41:42 -0700 (PDT) Date: Fri, 4 Sep 2020 06:41:42 -0700 From: "Paul E. McKenney" To: Boqun Feng Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH tip/core/rcu 05/13] rcu: Always set .need_qs from __rcu_read_lock() for strict GPs Message-ID: <20200904134142.GB29330@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200831181101.GA950@paulmck-ThinkPad-P72> <20200831181120.1044-5-paulmck@kernel.org> <20200904040534.GD7922@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200904040534.GD7922@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 04, 2020 at 12:05:34PM +0800, Boqun Feng wrote: > Hi Paul, > > On Mon, Aug 31, 2020 at 11:11:12AM -0700, paulmck@kernel.org wrote: > > From: "Paul E. McKenney" > > > > The ->rcu_read_unlock_special.b.need_qs field in the task_struct > > structure indicates that the RCU core needs a quiscent state from the > > corresponding task. The __rcu_read_unlock() function checks this (via > > an eventual call to rcu_preempt_deferred_qs_irqrestore()), and if set > > reports a quiscent state immediately upon exit from the outermost RCU > > read-side critical section. > > > > Currently, this flag is only set when the scheduling-clock interrupt > > decides that the current RCU grace period is too old, as in about > > one full second too old. But if the kernel has been built with > > CONFIG_RCU_STRICT_GRACE_PERIOD=y, we clearly do not want to wait that > > long. This commit therefore sets the .need_qs field immediately at the > > start of the RCU read-side critical section from within __rcu_read_lock() > > in order to unconditionally enlist help from __rcu_read_unlock(). > > > > So why not make rcu_preempt_deferred_qs_irqrestore() always treat > need_qs is true if CONFIG_RCU_STRICT_GRACE_PERIOD = y? IOW: > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index 982fc5be5269..2a9f31545453 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -449,6 +449,8 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags) > * t->rcu_read_unlock_special cannot change. > */ > special = t->rcu_read_unlock_special; > + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread) > + special.b.need_qs = true; > rdp = this_cpu_ptr(&rcu_data); > if (!special.s && !rdp->exp_deferred_qs) { > local_irq_restore(flags); > > , and in this way, you can save one store for each rcu_read_lock() ;-) Because unless I am missing something subtle, if the .need_qs flag is not set, execution is not guaranteed to reach rcu_preempt_deferred_qs_irqrestore(). Thanx, Paul > Regards, > Boqun > > > But note the additional check for rcu_state.gp_kthread, which prevents > > attempts to awaken RCU's grace-period kthread during early boot before > > there is a scheduler. Leaving off this check results in early boot hangs. > > So early that there is no console output. Thus, this additional check > > fails until such time as RCU's grace-period kthread has been created, > > avoiding these empty-console hangs. > > > > Reported-by Jann Horn > > Signed-off-by: Paul E. McKenney > > --- > > kernel/rcu/tree_plugin.h | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 44cf77d..668bbd2 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -376,6 +376,8 @@ void __rcu_read_lock(void) > > rcu_preempt_read_enter(); > > if (IS_ENABLED(CONFIG_PROVE_LOCKING)) > > WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX); > > + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread) > > + WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true); > > barrier(); /* critical section after entry code. */ > > } > > EXPORT_SYMBOL_GPL(__rcu_read_lock); > > -- > > 2.9.5 > >