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 31BC8CA9ECF for ; Fri, 1 Nov 2019 12:33:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F09B0217D9 for ; Fri, 1 Nov 2019 12:33:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1572611605; bh=655oP4EcLo1bTQpP1velcNHS7UmLoENn3nPbnIRZ0WE=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=lGsLZwq5BEIUFW99hao3Cbf00bnOo0e/4uVibd3Wj4N6u76GVxSmL0QBtaQvzST8Q yhDuoJmK6+OJs1W/6ucZHApo25HKYbXHk0Bjnkf47q9XaGEaYHGPUHhmdI5BeDbhKq 1kOKaek4gCPvLKCz3SjY8dvOcL/FaqFSORypCKcU= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726805AbfKAMdY (ORCPT ); Fri, 1 Nov 2019 08:33:24 -0400 Received: from mail.kernel.org ([198.145.29.99]:44300 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726229AbfKAMdY (ORCPT ); Fri, 1 Nov 2019 08:33:24 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 A67462067D; Fri, 1 Nov 2019 12:33:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1572611603; bh=655oP4EcLo1bTQpP1velcNHS7UmLoENn3nPbnIRZ0WE=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=sP38mqAoM8SXz9neSyycjsVBSuXKFmNVooL2T/T3dR12CLTAnzRiNply+FsxwSTrQ uWa5alKKp40v8CyC97lCKRqKpBotwWM6gWEDjSadbi3gKYZXGhbeF+NhdBYIIP0CMd zoBba/XCkFkUKaCX6tqDJq0zivuK5V6EB7NMgwTc= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 77B413522AF9; Fri, 1 Nov 2019 05:33:23 -0700 (PDT) Date: Fri, 1 Nov 2019 05:33:23 -0700 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: <20191101123323.GC17910@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20191031100806.1326-1-laijs@linux.alibaba.com> <20191031100806.1326-9-laijs@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191031100806.1326-9-laijs@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 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/ Thanx, Paul > if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s))) > rcu_read_unlock_special(t); > - barrier(); /* ->rcu_read_unlock_special load before assign */ > - t->rcu_read_lock_nesting = 0; > } > if (IS_ENABLED(CONFIG_PROVE_LOCKING)) { > int rrln = t->rcu_read_lock_nesting; > > - WARN_ON_ONCE(rrln < 0 && rrln > RCU_NEST_NMAX); > + WARN_ON_ONCE(rrln < 0); > } > } > EXPORT_SYMBOL_GPL(__rcu_read_unlock); > @@ -535,12 +527,9 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags) > */ > static bool rcu_preempt_need_deferred_qs(struct task_struct *t) > { > - return (__this_cpu_read(rcu_data.exp_deferred_qs) && > - (!t->rcu_read_lock_nesting || > - t->rcu_read_lock_nesting == -RCU_NEST_BIAS)) > - || > - (READ_ONCE(t->rcu_read_unlock_special.s) && > - t->rcu_read_lock_nesting <= 0); > + return (__this_cpu_read(rcu_data.exp_deferred_qs) || > + READ_ONCE(t->rcu_read_unlock_special.s)) && > + !t->rcu_read_lock_nesting; > } > > /* > -- > 2.20.1 >