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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, UNPARSEABLE_RELAY,URIBL_BLOCKED,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 D1149C432C3 for ; Sat, 16 Nov 2019 13:05:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AB6C320729 for ; Sat, 16 Nov 2019 13:05:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727471AbfKPNFD (ORCPT ); Sat, 16 Nov 2019 08:05:03 -0500 Received: from out30-42.freemail.mail.aliyun.com ([115.124.30.42]:42316 "EHLO out30-42.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727437AbfKPNFD (ORCPT ); Sat, 16 Nov 2019 08:05:03 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R111e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04407;MF=laijs@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0TiEXByO_1573909496; Received: from C02XQCBJJG5H.local(mailfrom:laijs@linux.alibaba.com fp:SMTPD_---0TiEXByO_1573909496) by smtp.aliyun-inc.com(127.0.0.1); Sat, 16 Nov 2019 21:04:57 +0800 From: Lai Jiangshan Subject: Re: [PATCH 08/11] rcu: don't use negative ->rcu_read_lock_nesting To: paulmck@kernel.org Cc: linux-kernel@vger.kernel.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , rcu@vger.kernel.org References: <20191031100806.1326-1-laijs@linux.alibaba.com> <20191031100806.1326-9-laijs@linux.alibaba.com> <20191101123323.GC17910@paulmck-ThinkPad-P72> Message-ID: <3437ee3f-2807-16eb-5e9b-77189fa31cdf@linux.alibaba.com> Date: Sat, 16 Nov 2019 21:04:56 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20191101123323.GC17910@paulmck-ThinkPad-P72> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org 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. Thanks Lai