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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=no 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 8C078CA9ECB for ; Fri, 1 Nov 2019 02:29:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6C1EB2067D for ; Fri, 1 Nov 2019 02:29:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728243AbfKAC3q (ORCPT ); Thu, 31 Oct 2019 22:29:46 -0400 Received: from out30-42.freemail.mail.aliyun.com ([115.124.30.42]:46423 "EHLO out30-42.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728218AbfKAC3p (ORCPT ); Thu, 31 Oct 2019 22:29:45 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R201e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01f04391;MF=laijs@linux.alibaba.com;NM=1;PH=DS;RN=9;SR=0;TI=SMTPD_---0Tgp7hUD_1572575381; Received: from C02XQCBJJG5H.local(mailfrom:laijs@linux.alibaba.com fp:SMTPD_---0Tgp7hUD_1572575381) by smtp.aliyun-inc.com(127.0.0.1); Fri, 01 Nov 2019 10:29:42 +0800 Subject: Re: [PATCH 02/11] rcu: fix bug when rcu_exp_handler() in nested interrupt To: Boqun Feng , "Paul E. McKenney" 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> <20191031100806.1326-3-laijs@linux.alibaba.com> <20191031134731.GP20975@paulmck-ThinkPad-P72> <20191031143119.GA15954@paulmck-ThinkPad-P72> <6b621228-4cab-6e2c-9912-cddc56ad6775@linux.alibaba.com> <20191031185258.GX20975@paulmck-ThinkPad-P72> <20191101001948.GA182@boqun-laptop.fareast.corp.microsoft.com> From: Lai Jiangshan Message-ID: <155e3e05-e0dc-26a7-c940-f86a819ffb2e@linux.alibaba.com> Date: Fri, 1 Nov 2019 10:29:41 +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: <20191101001948.GA182@boqun-laptop.fareast.corp.microsoft.com> 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 8:19 上午, Boqun Feng wrote: > On Thu, Oct 31, 2019 at 11:52:58AM -0700, Paul E. McKenney wrote: >> On Thu, Oct 31, 2019 at 11:14:23PM +0800, Lai Jiangshan wrote: >>> >>> >>> On 2019/10/31 10:31 下午, Paul E. McKenney wrote: >>>> On Thu, Oct 31, 2019 at 06:47:31AM -0700, Paul E. McKenney wrote: >>>>> On Thu, Oct 31, 2019 at 10:07:57AM +0000, Lai Jiangshan wrote: >>>>>> These is a possible bug (although which I can't triger yet) >>>>>> since 2015 8203d6d0ee78 >>>>>> (rcu: Use single-stage IPI algorithm for RCU expedited grace period) >>>>>> >>>>>> rcu_read_unlock() >>>>>> ->rcu_read_lock_nesting = -RCU_NEST_BIAS; >>>>>> interrupt(); // before or after rcu_read_unlock_special() >>>>>> rcu_read_lock() >>>>>> fetch some rcu protected pointers >>>>>> // exp GP starts in other cpu. >>>>>> some works >>>>>> NESTED interrupt for rcu_exp_handler(); >>>> >>>> Also, which platforms support nested interrupts? Last I knew, this was >>>> prohibited. >>>> >>>>>> report exp qs! BUG! >>>>> >>>>> Why would a quiescent state for the expedited grace period be reported >>>>> here? This CPU is still in an RCU read-side critical section, isn't it? >>>> >>>> And I now see what you were getting at here. Yes, the current code >>>> assumes that interrupt-disabled regions, like hardware interrupt >>>> handlers, cannot be interrupted. But if interrupt-disabled regions such >>>> as hardware interrupt handlers can be interrupted (as opposed to being >>>> NMIed), wouldn't that break a whole lot of stuff all over the place in >>>> the kernel? So that sounds like an arch bug to me. >>> >>> I don't know when I started always assuming hardware interrupt >>> handler can be nested by (other) interrupt. I can't find any >>> documents say Linux don't allow nested interrupt handler. >>> Google search suggests the opposite. > > FWIW, there is a LWN article talking about we disallow interrupt nesting > in *most* cases: > > https://lwn.net/Articles/380931/ Much thanks for the information! > > , that's unless a interrupt handler explicitly calls > local_irq_enable_in_hardirq(), it remains irq disabled, which means no > nesting interrupt allowed. > Even so the problem here will be fixed by patch7/8. > >> >> The results I am seeing look to be talking about threaded interrupt >> handlers, which indeed can be interrupted by hardware interrupts. As can >> softirq handlers. But these are not examples of a hardware interrupt >> handler being interrupted by another hardware interrupt. For that to >> work reasonably, something like a system priority level is required, >> as in the old DYNIX/ptx kernel, or, going even farther back, DEC's RT-11. >> >>> grep -rIni nested Documentation/memory-barriers.txt Documentation/x86/ >>> It still have some words about nested interrupt handler. >> >> Some hardware does not differentiate between interrupts and exceptions, >> for example, an illegal-instruction trap within an interrupt handler >> might look in some ways like a nested interrupt. >> >>> The whole patchset doesn't depend on this patch, and actually >>> it is reverted later in the patchset. Dropping this patch >>> can be an option for next round. >> >> Sounds like a plan! >> >> Thanx, Paul >> > [...] >