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=-18.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT 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 AAA78C43603 for ; Fri, 5 Mar 2021 12:43:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 845D665049 for ; Fri, 5 Mar 2021 12:43:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232412AbhCEMn3 (ORCPT ); Fri, 5 Mar 2021 07:43:29 -0500 Received: from mail.kernel.org ([198.145.29.99]:59222 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233937AbhCEMml (ORCPT ); Fri, 5 Mar 2021 07:42:41 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 7B7EC6501E; Fri, 5 Mar 2021 12:42:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1614948161; bh=DlwxevAwl1h31rQTJECm1wGE30vmIYH41fbcchGjIdY=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=AlP4TYIgptktxgnYmrUkGw3V2S4N0lR5SI+E4H0AOYXJfF9denRC5SuV93rJluyEr AiYBFxKAp5qQ0moY/Q3rJltZieMo7lKCR/jLPZay/yaBpe9VBhhYt0Rc7ujPSs6MDp AAfoAHROAydEEzLCfgDL/LPyak1+RWcIFjD2FORg= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Muchun Song , Petr Mladek , Sergey Senozhatsky Subject: [PATCH 4.9 13/41] printk: fix deadlock when kernel panic Date: Fri, 5 Mar 2021 13:22:20 +0100 Message-Id: <20210305120851.935800288@linuxfoundation.org> X-Mailer: git-send-email 2.30.1 In-Reply-To: <20210305120851.255002428@linuxfoundation.org> References: <20210305120851.255002428@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Muchun Song commit 8a8109f303e25a27f92c1d8edd67d7cbbc60a4eb upstream. printk_safe_flush_on_panic() caused the following deadlock on our server: 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 printk_safe_flush_on_panic printk_safe_flush __printk_safe_flush // deadlock raw_spin_lock_irqsave(&read_lock) DEADLOCK: read_lock is taken on CPU1 and will never get released. It happens when panic() stops a CPU by NMI while it has been in the middle of printk_safe_flush(). Handle the lock the same way as logbuf_lock. The printk_safe buffers are flushed only when both locks can be safely taken. It can avoid the deadlock _in this particular case_ at expense of losing contents of printk_safe buffers. Note: It would actually be safe to re-init the locks when all CPUs were stopped by NMI. But it would require passing this information from arch-specific code. It is not worth the complexity. Especially because logbuf_lock and printk_safe buffers have been obsoleted by the lockless ring buffer. Fixes: cf9b1106c81c ("printk/nmi: flush NMI messages on the system panic") Signed-off-by: Muchun Song Reviewed-by: Petr Mladek Cc: Acked-by: Sergey Senozhatsky Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20210210034823.64867-1-songmuchun@bytedance.com Signed-off-by: Greg Kroah-Hartman --- kernel/printk/nmi.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) --- a/kernel/printk/nmi.c +++ b/kernel/printk/nmi.c @@ -52,6 +52,8 @@ struct nmi_seq_buf { }; static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq); +static DEFINE_RAW_SPINLOCK(nmi_read_lock); + /* * Safe printk() for NMI context. It uses a per-CPU buffer to * store the message. NMIs are not nested, so there is always only @@ -134,8 +136,6 @@ static void printk_nmi_flush_seq_line(st */ static void __printk_nmi_flush(struct irq_work *work) { - static raw_spinlock_t read_lock = - __RAW_SPIN_LOCK_INITIALIZER(read_lock); struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work); unsigned long flags; size_t len, size; @@ -148,7 +148,7 @@ static void __printk_nmi_flush(struct ir * different CPUs. This is especially important when printing * a backtrace. */ - raw_spin_lock_irqsave(&read_lock, flags); + raw_spin_lock_irqsave(&nmi_read_lock, flags); i = 0; more: @@ -197,7 +197,7 @@ more: goto more; out: - raw_spin_unlock_irqrestore(&read_lock, flags); + raw_spin_unlock_irqrestore(&nmi_read_lock, flags); } /** @@ -239,6 +239,14 @@ void printk_nmi_flush_on_panic(void) raw_spin_lock_init(&logbuf_lock); } + if (in_nmi() && raw_spin_is_locked(&nmi_read_lock)) { + if (num_online_cpus() > 1) + return; + + debug_locks_off(); + raw_spin_lock_init(&nmi_read_lock); + } + printk_nmi_flush(); }