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 96976CA9ED3 for ; Tue, 5 Nov 2019 02:09:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 72D8921882 for ; Tue, 5 Nov 2019 02:09:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729428AbfKECJU (ORCPT ); Mon, 4 Nov 2019 21:09:20 -0500 Received: from out30-133.freemail.mail.aliyun.com ([115.124.30.133]:35101 "EHLO out30-133.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728987AbfKECJU (ORCPT ); Mon, 4 Nov 2019 21:09:20 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R271e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04394;MF=laijs@linux.alibaba.com;NM=1;PH=DS;RN=9;SR=0;TI=SMTPD_---0ThE8SMa_1572919755; Received: from C02XQCBJJG5H.local(mailfrom:laijs@linux.alibaba.com fp:SMTPD_---0ThE8SMa_1572919755) by smtp.aliyun-inc.com(127.0.0.1); Tue, 05 Nov 2019 10:09:16 +0800 Subject: Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs() To: paulmck@kernel.org Cc: Boqun Feng , linux-kernel@vger.kernel.org, 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> <7489f817-adaf-275b-b19d-18ad248b071f@linux.alibaba.com> <20191104145539.GY20975@paulmck-ThinkPad-P72> From: Lai Jiangshan Message-ID: Date: Tue, 5 Nov 2019 10:09:15 +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: <20191104145539.GY20975@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 2019/11/4 10:55 下午, Paul E. McKenney wrote: > On Sun, Nov 03, 2019 at 01:01:21PM +0800, Lai Jiangshan wrote: >> >> >> 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. > > This one in -rcu, you mean? > > 5c5d9065e4eb ("rcu: Clear ->rcu_read_unlock_special only once") Yes, but the commit ID is floating in the tree. > > Some adjustment was needed due to my not applying the earlier patches > that assumed nested interrupts. Please let me know if further adjustments > are needed. I don't think the earlier patches are needed. If the possible? nested interrupts described in my previous emails is an issue, the patch "rcu: don't use negative ->rcu_read_lock_nesting" in this series is enough to fixed it. If any adjustments needed for this series, I will just put the adjustments the series. Thanks Lai > > Thanx, Paul > >>> 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 >>>>