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=-2.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 B007AC433E0 for ; Mon, 8 Feb 2021 06:39:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7114A64DF2 for ; Mon, 8 Feb 2021 06:39:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229692AbhBHGjr (ORCPT ); Mon, 8 Feb 2021 01:39:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54614 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229683AbhBHGj2 (ORCPT ); Mon, 8 Feb 2021 01:39:28 -0500 Received: from mail-pl1-x635.google.com (mail-pl1-x635.google.com [IPv6:2607:f8b0:4864:20::635]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id ED4FBC06174A for ; Sun, 7 Feb 2021 22:38:46 -0800 (PST) Received: by mail-pl1-x635.google.com with SMTP id e9so7288737plh.3 for ; Sun, 07 Feb 2021 22:38:46 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=4x8g4SVmzJqTPf9smsHfaxo7jwJgCQqSwNYqpSpJ50c=; b=mZEX5AWKp0Bs0Tl+wUUbEqa6uC+YU6nhEhCsg8sTxDOElkxovZgqW6UEIqB4dmHrmX T+lQZIKDX5IttxIILew7ZNl0UCT/dHUsBs4bIW/IRNUclAlTY4hHd9p/wLKLoAoC0RIA TCOJjvehd1kNQ6GcpV47/FSXXtSzPu6OyZ9Non76zUR18XHC4Xx99s9e4iEJKCt14lhq VSZukK8Tzmqq9Ku1i3n87Rxcm9v3CfxgzT9LrM0HFcQUhj6OdTp6q5UQ64sWpOhB6MLC 8Moqv3ksOr67m7fT+0DDjpSonIMQtd2uuLIlGkWc7o992D9vYBloySGMdWC5+dsEM6oQ DeHQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=4x8g4SVmzJqTPf9smsHfaxo7jwJgCQqSwNYqpSpJ50c=; b=Yr2N2LXIHug1Ha3uejL54ssEMfTUnOpH6uVqc3RPNDfvcsJI2ga5MPXdny7poqcSvI 9sWpwfsPc+RGwZsriJrbxiJ4WNHEDNJMrPyxP0eXhWFqh4lrVRLJGFuq2+jrjTBXRtz2 mNIQeldrn8gOSOj6asD/9hglwUoxfEuXeNtziw9jAk6Npu2x+qmh0rg4DYo5qa5wVrNm YqmJujTMwDJkQn7s7reXgHqDvWT7cFjYd5Y3k4q6vVTbuhYNYJewnSCBqTKjY5RUA1i3 xGFecm8QjtFSsRKyJ6r53Lgys45tE5PyK/vcGSLEVuPQ5yjtow7hhQezIcwBRSLeYzQ0 HJfQ== X-Gm-Message-State: AOAM533Ss2XdRuFHZISXwcHN91GK5hOGG4cCIlcXB7Icx4gXFrrDBtDS Rd71/eCnlH2dDoSOm1j9hMI= X-Google-Smtp-Source: ABdhPJzUJkIesoDmlFS7A/+M6+YlgXTEW0V/v8e22tcWOuoMo+P5jb4/qP/Ho2Y0fdvRaalWwf7g0g== X-Received: by 2002:a17:902:e812:b029:de:5af2:3d09 with SMTP id u18-20020a170902e812b02900de5af23d09mr15211144plg.33.1612766326448; Sun, 07 Feb 2021 22:38:46 -0800 (PST) Received: from localhost ([2409:10:2e40:5100:6e29:95ff:fe2d:8f34]) by smtp.gmail.com with ESMTPSA id d185sm8340857pfd.110.2021.02.07.22.38.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 07 Feb 2021 22:38:45 -0800 (PST) Date: Mon, 8 Feb 2021 15:38:44 +0900 From: Sergey Senozhatsky To: Muchun Song Cc: pmladek@suse.com, sergey.senozhatsky@gmail.com, rostedt@goodmis.org, john.ogness@linutronix.de, akpm@linux-foundation.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] printk: fix deadlock when kernel panic Message-ID: References: <20210206054124.6743-1-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210206054124.6743-1-songmuchun@bytedance.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On (21/02/06 13:41), Muchun Song wrote: > We found a deadlock bug on our server when the kernel panic. It can be > described in the following diagram. > > CPU0: CPU1: > panic rcu_dump_cpu_stacks > kdump_nmi_shootdown_cpus nmi_trigger_cpumask_backtrace > register_nmi_handler(crash_nmi_callback) printk_safe_flush > __printk_safe_flush > raw_spin_lock_irqsave(&read_lock) > // send NMI to other processors > apic_send_IPI_allbutself(NMI_VECTOR) > // NMI interrupt, dead loop > crash_nmi_callback At what point does this decrement num_online_cpus()? Any chance that panic CPU can apic_send_IPI_allbutself() and printk_safe_flush_on_panic() before num_online_cpus() becomes 1? > printk_safe_flush_on_panic > printk_safe_flush > __printk_safe_flush > // deadlock > raw_spin_lock_irqsave(&read_lock) -ss