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 A640AC433E9 for ; Tue, 2 Mar 2021 06:40:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5D30B614A7 for ; Tue, 2 Mar 2021 06:40:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1573387AbhCBDWE (ORCPT ); Mon, 1 Mar 2021 22:22:04 -0500 Received: from mail.kernel.org ([198.145.29.99]:49320 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243162AbhCAU04 (ORCPT ); Mon, 1 Mar 2021 15:26:56 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 63D3564E5F; Mon, 1 Mar 2021 18:06:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1614622011; bh=clNgsSlW6BodqZSmbhyAxuKEX+FLcS59BXvNc8EeZLU=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=AMHzC8wH2X7IUHEkELZ3LG1Yj1bzElMINknEIeFz3uGiG4BbJ6+XsHQ7Xtq3oyvP3 TcU6MulTlX6WujGtw6NIaCuBjWZf5hDVKSqrhXaKWzEiipW3HckmCf+vmAuBs44Pqi lvPqx4bkZ5emY6U/u9g/wjpuPrUJeF3n9xfYI1eM= 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 5.11 723/775] printk: fix deadlock when kernel panic Date: Mon, 1 Mar 2021 17:14:51 +0100 Message-Id: <20210301161237.067857251@linuxfoundation.org> X-Mailer: git-send-email 2.30.1 In-Reply-To: <20210301161201.679371205@linuxfoundation.org> References: <20210301161201.679371205@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/printk_safe.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) --- a/kernel/printk/printk_safe.c +++ b/kernel/printk/printk_safe.c @@ -45,6 +45,8 @@ struct printk_safe_seq_buf { static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq); static DEFINE_PER_CPU(int, printk_context); +static DEFINE_RAW_SPINLOCK(safe_read_lock); + #ifdef CONFIG_PRINTK_NMI static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq); #endif @@ -180,8 +182,6 @@ static void report_message_lost(struct p */ static void __printk_safe_flush(struct irq_work *work) { - static raw_spinlock_t read_lock = - __RAW_SPIN_LOCK_INITIALIZER(read_lock); struct printk_safe_seq_buf *s = container_of(work, struct printk_safe_seq_buf, work); unsigned long flags; @@ -195,7 +195,7 @@ static void __printk_safe_flush(struct i * different CPUs. This is especially important when printing * a backtrace. */ - raw_spin_lock_irqsave(&read_lock, flags); + raw_spin_lock_irqsave(&safe_read_lock, flags); i = 0; more: @@ -232,7 +232,7 @@ more: out: report_message_lost(s); - raw_spin_unlock_irqrestore(&read_lock, flags); + raw_spin_unlock_irqrestore(&safe_read_lock, flags); } /** @@ -278,6 +278,14 @@ void printk_safe_flush_on_panic(void) raw_spin_lock_init(&logbuf_lock); } + if (raw_spin_is_locked(&safe_read_lock)) { + if (num_online_cpus() > 1) + return; + + debug_locks_off(); + raw_spin_lock_init(&safe_read_lock); + } + printk_safe_flush(); }