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=-15.7 required=3.0 tests=BAYES_00,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 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 76AFAC433E9 for ; Wed, 3 Mar 2021 00:19:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 479EE64FC8 for ; Wed, 3 Mar 2021 00:19:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234682AbhCCAJj (ORCPT ); Tue, 2 Mar 2021 19:09:39 -0500 Received: from mx2.suse.de ([195.135.220.15]:56646 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240483AbhCBKDw (ORCPT ); Tue, 2 Mar 2021 05:03:52 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1614679359; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=OSC0RMb4jQ8vLtAy/2lrY+WzohUmliZWQjgjoo9BBmk=; b=uB2FnJ8mEyD7HKxvihmKbQodVAWJkZJFF2EaBg++M9iORUmfZrdofkznU5gEHi+NAHq/sd iWMMAMVvA/OvAa2jLsk3aLN18x9XtJuVnl3s3ZAN6ecZE2ZXQ9tuB3RYfi3fEubIAHtRBz JKcpzIIXogKu7NMCbwrh6PvpY4bHpoQ= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 83B29ABF4; Tue, 2 Mar 2021 10:02:39 +0000 (UTC) Date: Tue, 2 Mar 2021 11:02:39 +0100 From: Petr Mladek To: gregkh@linuxfoundation.org Cc: songmuchun@bytedance.com, sergey.senozhatsky@gmail.com, stable@vger.kernel.org Subject: Re: FAILED: patch "[PATCH] printk: fix deadlock when kernel panic" failed to apply to 4.9-stable tree Message-ID: References: <1614604832185174@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1614604832185174@kroah.com> Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Mon 2021-03-01 14:20:32, gregkh@linuxfoundation.org wrote: > > The patch below does not apply to the 4.9-stable tree. > If someone wants it applied there, or to any other stable or longterm > tree, then please email the backport, including the original git commit > id to . Please, find below the backport for stable-4.9: >From bfc246e5ed94300431bc8d74b67c74b6f66e75ff Mon Sep 17 00:00:00 2001 From: Muchun Song Date: Wed, 10 Feb 2021 11:48:23 +0800 Subject: [PATCH] printk: fix deadlock when kernel panic printk_nmi_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_nmi_flush __printk_nmi_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_nmi_flush_on_panic printk_nmi_flush __printk_nmi_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_nmi_flush(). Handle the lock the same way as logbuf_lock. The printk_nmi 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_nmi 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_nmi buffers have been obsoleted by the lockless ring buffer. This is a backport of the commit 8a8109f303e25a27f92c ("printk: fix deadlock when kernel panic") for the stable 4.9 kernel. 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 --- kernel/printk/nmi.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c index 2c3e7f024c15..7a50b405ad28 100644 --- 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(struct nmi_seq_buf *s, */ 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 irq_work *work) * 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 @@ static void __printk_nmi_flush(struct irq_work *work) 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(); } -- 2.16.4