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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham 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 C6649C432C3 for ; Sun, 17 Nov 2019 23:15:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9225A20730 for ; Sun, 17 Nov 2019 23:15:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1574032541; bh=5LLOZx13gw+mpOTgUjg5Y1D3Ju4jWNlY0E33O2DSs/8=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=uMe1BfzwjAdg95ygw8AJuHUlrKj0XIlQC4qgOOwM0Vk7mWGXOlC9fWz6iAJOu2QtA BULdmT/3b1yKz43F1lZbc2Y7hrxb4VXWnsfhAxnU5O6GrWkrJLQ5GrBHQO5RkDnt0b wYoQ4tP++B6jiV6c4c2fxr/9OJMCws96CxSn2mxQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726322AbfKQXPl (ORCPT ); Sun, 17 Nov 2019 18:15:41 -0500 Received: from mail.kernel.org ([198.145.29.99]:55902 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726314AbfKQXPl (ORCPT ); Sun, 17 Nov 2019 18:15:41 -0500 Received: from paulmck-ThinkPad-P72.home (199-192-87-166.static.wiline.com [199.192.87.166]) (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 9AD9620727; Sun, 17 Nov 2019 23:15:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1574032539; bh=5LLOZx13gw+mpOTgUjg5Y1D3Ju4jWNlY0E33O2DSs/8=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=h/wGXScgvRUk3b8oBjiphMgJuvqsRAdGUkdLlgeIU1W87g7+Ee0gx6kmne3WmdH/K fLwc9uoalybPtht/cmnva8e3+M93aLwfiQrqxxkCd8bMtNRKGhIFPMGok0giEqsfDN +XkIIWUA6mrQq3U1z3QIGedkkd8i2sYFscyUvmB8= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id B535A35229EF; Sun, 17 Nov 2019 13:53:39 -0800 (PST) Date: Sun, 17 Nov 2019 13:53:39 -0800 From: "Paul E. McKenney" To: Lai Jiangshan Cc: linux-kernel@vger.kernel.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , rcu@vger.kernel.org Subject: Re: [PATCH 08/11] rcu: don't use negative ->rcu_read_lock_nesting Message-ID: <20191117215339.GD2889@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20191031100806.1326-1-laijs@linux.alibaba.com> <20191031100806.1326-9-laijs@linux.alibaba.com> <20191101123323.GC17910@paulmck-ThinkPad-P72> <3437ee3f-2807-16eb-5e9b-77189fa31cdf@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <3437ee3f-2807-16eb-5e9b-77189fa31cdf@linux.alibaba.com> 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 Sat, Nov 16, 2019 at 09:04:56PM +0800, Lai Jiangshan wrote: > On 2019/11/1 8:33 下午, Paul E. McKenney wrote: > > On Thu, Oct 31, 2019 at 10:08:03AM +0000, Lai Jiangshan wrote: > > > Negative ->rcu_read_lock_nesting was introduced to prevent > > > scheduler deadlock which was just prevented by deferred qs. > > > So negative ->rcu_read_lock_nesting is useless now and > > > rcu_read_unlock() can be simplified. > > > > > > And negative ->rcu_read_lock_nesting is bug-prone, > > > it is good to kill it. > > > > > > Signed-off-by: Lai Jiangshan > > > --- > > > kernel/rcu/tree_exp.h | 30 ++---------------------------- > > > kernel/rcu/tree_plugin.h | 21 +++++---------------- > > > 2 files changed, 7 insertions(+), 44 deletions(-) > > > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > > index c0d06bce35ea..9dcbd2734620 100644 > > > --- a/kernel/rcu/tree_exp.h > > > +++ b/kernel/rcu/tree_exp.h > > > @@ -621,11 +621,11 @@ static void rcu_exp_handler(void *unused) > > > * report the quiescent state, otherwise defer. > > > */ > > > if (!t->rcu_read_lock_nesting) { > > > + rdp->exp_deferred_qs = true; > > > if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) || > > > rcu_dynticks_curr_cpu_in_eqs()) { > > > - rcu_report_exp_rdp(rdp); > > > + rcu_preempt_deferred_qs(t); > > > } else { > > > - rdp->exp_deferred_qs = true; > > > set_tsk_need_resched(t); > > > set_preempt_need_resched(); > > > } > > > @@ -646,32 +646,6 @@ static void rcu_exp_handler(void *unused) > > > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true); > > > return; > > > } > > > - > > > - /* > > > - * The final and least likely case is where the interrupted > > > - * code was just about to or just finished exiting the RCU-preempt > > > - * read-side critical section, and no, we can't tell which. > > > - * So either way, set ->deferred_qs to flag later code that > > > - * a quiescent state is required. > > > - * > > > - * If the CPU is fully enabled (or if some buggy RCU-preempt > > > - * read-side critical section is being used from idle), just > > > - * invoke rcu_preempt_deferred_qs() to immediately report the > > > - * quiescent state. We cannot use rcu_read_unlock_special() > > > - * because we are in an interrupt handler, which will cause that > > > - * function to take an early exit without doing anything. > > > - * > > > - * Otherwise, force a context switch after the CPU enables everything. > > > - */ > > > - rdp->exp_deferred_qs = true; > > > - if (rcu_preempt_need_deferred_qs(t) && > > > - (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) || > > > - WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs()))) { > > > - rcu_preempt_deferred_qs(t); > > > - } else { > > > - set_tsk_need_resched(t); > > > - set_preempt_need_resched(); > > > - } > > > } > > > /* PREEMPTION=y, so no PREEMPTION=n expedited grace period to clean up after. */ > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > > index dbded2b8c792..c62631c79463 100644 > > > --- a/kernel/rcu/tree_plugin.h > > > +++ b/kernel/rcu/tree_plugin.h > > > @@ -344,8 +344,6 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp) > > > } > > > /* Bias and limit values for ->rcu_read_lock_nesting. */ > > > -#define RCU_NEST_BIAS INT_MAX > > > -#define RCU_NEST_NMAX (-INT_MAX / 2) > > > #define RCU_NEST_PMAX (INT_MAX / 2) > > > /* > > > @@ -373,21 +371,15 @@ void __rcu_read_unlock(void) > > > { > > > struct task_struct *t = current; > > > - if (t->rcu_read_lock_nesting != 1) { > > > - --t->rcu_read_lock_nesting; > > > - } else { > > > + if (--t->rcu_read_lock_nesting == 0) { > > > barrier(); /* critical section before exit code. */ > > > - t->rcu_read_lock_nesting = -RCU_NEST_BIAS; > > > - barrier(); /* assign before ->rcu_read_unlock_special load */ > > > > But if we take an interrupt here, and the interrupt handler contains > > an RCU read-side critical section, don't we end up in the same hole > > that resulted in this article when the corresponding rcu_read_unlock() > > executes? https://lwn.net/Articles/453002/ > > Hello, Paul > > I'm replying the email of V1, which is relying on deferred_qs changes > in [PATCH 07/11] (V1). > ([PATCH 04/11](V1) relies on it too as you pointed out) > > I hope I can answer the question wrt https://lwn.net/Articles/453002/ > maybe partially. > > With the help of deferred_qs mechanism and the special.b.deferred_qs > bit, I HOPED rcu_read_unlock_special() can find if itself is > risking in scheduler locks via special.b.deferred_qs bit. > > --t->rcu_read_lock_nesting; > //outmost rcu c.s, rcu_read_lock_nesting is 0. but special is not zero > INTERRUPT > // the fallowing code will normally be in_interrupt() > // or NOT in_interrupt() when wakeup_softirqd() in invoke_softirq() > // or NOT in_interrupt() when preempt_shedule_irq() > // or other cases I missed. > scheduler_lock() > rcu_read_lock() > rcu_read_unlock() > // special has been set but with no special.b.deferred_qs > rcu_read_unlock_special() > raise_softirq_irqoff() > wake_up() when !in_interrupt() // dead lock > > preempt_shedule_irq() is guaranteed to clear rcu_read_unlock_special > when rcu_read_lock_nesting = 0 before calling into scheduler locks. > > But, at least, what caused my hope to be failed was the case > wakeup_softirqd() in invoke_softirq() (which was once protected by > softirq in about 2 years between ec433f0c5152 and facd8b80c67a). > I don't think it is hard to fix it if we keep using > special.b.deferred_qs as this V1 series. It is quite possible that special.b.deferred_qs might be useful for debugging. But it should now be possible to take care of the nohz_full issue for expedited grace periods, which might in turn allow rcu_read_unlock_special() to avoid acquiring scheduler locks. This could avoid the need for negative ->rcu_read_lock_nesting, in turn allowing your simplified _rcu_read_unlock(). Would you like to do the expedited grace-period modifications, or would you rather that I do so? Thanx, Paul