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,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 9BB67CA9ED0 for ; Sun, 3 Nov 2019 05:01:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 71805215EA for ; Sun, 3 Nov 2019 05:01:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726719AbfKCFB0 (ORCPT ); Sun, 3 Nov 2019 01:01:26 -0400 Received: from out30-132.freemail.mail.aliyun.com ([115.124.30.132]:58281 "EHLO out30-132.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726408AbfKCFB0 (ORCPT ); Sun, 3 Nov 2019 01:01:26 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R101e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04420;MF=laijs@linux.alibaba.com;NM=1;PH=DS;RN=9;SR=0;TI=SMTPD_---0Th1RgFd_1572757281; Received: from 192.168.2.229(mailfrom:laijs@linux.alibaba.com fp:SMTPD_---0Th1RgFd_1572757281) by smtp.aliyun-inc.com(127.0.0.1); Sun, 03 Nov 2019 13:01:22 +0800 Subject: Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs() To: Boqun Feng Cc: linux-kernel@vger.kernel.org, "Paul E. McKenney" , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , rcu@vger.kernel.org References: <20191102124559.1135-1-laijs@linux.alibaba.com> <20191102124559.1135-3-laijs@linux.alibaba.com> <20191103020150.GA23770@tardis> From: Lai Jiangshan Message-ID: <7489f817-adaf-275b-b19d-18ad248b071f@linux.alibaba.com> Date: Sun, 3 Nov 2019 13:01:21 +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: <20191103020150.GA23770@tardis> 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 2019/11/3 10:01 上午, Boqun Feng wrote: > Hi Jiangshan, > > > I haven't checked the correctness of this patch carefully, but.. > > > On Sat, Nov 02, 2019 at 12:45:54PM +0000, Lai Jiangshan wrote: >> Don't need to set ->rcu_read_lock_nesting negative, irq-protected >> rcu_preempt_deferred_qs_irqrestore() doesn't expect >> ->rcu_read_lock_nesting to be negative to work, it even >> doesn't access to ->rcu_read_lock_nesting any more. > > rcu_preempt_deferred_qs_irqrestore() will report RCU qs, and may > eventually call swake_up() or its friends to wake up, say, the gp > kthread, and the wake up functions could go into the scheduler code > path which might have RCU read-side critical section in it, IOW, > accessing ->rcu_read_lock_nesting. Sure, thank you for pointing it out. I should rewrite the changelog in next round. Like this: rcu: cleanup rcu_preempt_deferred_qs() IRQ-protected rcu_preempt_deferred_qs_irqrestore() itself doesn't expect ->rcu_read_lock_nesting to be negative to work. There might be RCU read-side critical section in it (from wakeup() or so), 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once) will ensure that ->rcu_read_unlock_special is zero and these RCU read-side critical sections will not call rcu_read_unlock_special(). Thanks Lai === PS: Were 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once) not applied earlier, it will be protected by previous patch (patch1) in this series "rcu: use preempt_count to test whether scheduler locks is held" when rcu_read_unlock_special() is called. > > Again, haven't checked closely, but this argument in the commit log > seems untrue. > > Regards, > Boqun > >> >> It is true that NMI over rcu_preempt_deferred_qs_irqrestore() >> may access to ->rcu_read_lock_nesting, but it is still safe >> since rcu_read_unlock_special() can protect itself from NMI. >> >> Signed-off-by: Lai Jiangshan >> --- >> kernel/rcu/tree_plugin.h | 5 ----- >> 1 file changed, 5 deletions(-) >> >> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h >> index aba5896d67e3..2fab8be2061f 100644 >> --- a/kernel/rcu/tree_plugin.h >> +++ b/kernel/rcu/tree_plugin.h >> @@ -552,16 +552,11 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t) >> static void rcu_preempt_deferred_qs(struct task_struct *t) >> { >> unsigned long flags; >> - bool couldrecurse = t->rcu_read_lock_nesting >= 0; >> >> if (!rcu_preempt_need_deferred_qs(t)) >> return; >> - if (couldrecurse) >> - t->rcu_read_lock_nesting -= RCU_NEST_BIAS; >> local_irq_save(flags); >> rcu_preempt_deferred_qs_irqrestore(t, flags); >> - if (couldrecurse) >> - t->rcu_read_lock_nesting += RCU_NEST_BIAS; >> } >> >> /* >> -- >> 2.20.1 >>