From d51507098ff91e863b6e0a8047507741d59b8175 Mon Sep 17 00:00:00 2001
From: Stephen Brennan <stephen.s.brennan@oracle.com>
Date: Wed, 2 Feb 2022 09:18:19 -0800
Subject: [PATCH] printk: disable optimistic spin during panic
Git-commit: d51507098ff91e863b6e0a8047507741d59b8175
Patch-mainline: v5.18-rc1
References: bsc#1197894

A CPU executing with console lock spinning enabled might be halted
during a panic. Before the panicking CPU calls console_flush_on_panic(),
it may call console_trylock(), which attempts to optimistically spin,
deadlocking the panic CPU:

CPU 0 (panic CPU)             CPU 1

-----------------             ------
                              printk() {
                                vprintk_func() {
                                  vprintk_default() {
                                    vprintk_emit() {
                                      console_unlock() {
                                        console_lock_spinning_enable();
                                        ... printing to console ...
panic() {
  crash_smp_send_stop() {
    NMI  -------------------> HALT
  }
  atomic_notifier_call_chain() {
    printk() {
      ...
      console_trylock_spinnning() {
        // optimistic spin infinitely

This hang during panic can be induced when a kdump kernel is loaded, and
crash_kexec_post_notifiers=1 is present on the kernel command line. The
following script which concurrently writes to /dev/kmsg, and triggers a
panic, can result in this hang:

    #!/bin/bash
    date
    # 991 chars (based on log buffer size):
    chars="$(printf 'a%.0s' {1..991})"
    while :; do
        echo $chars > /dev/kmsg
    done &
    echo c > /proc/sysrq-trigger &
    date
    exit

To avoid this deadlock, ensure that console_trylock_spinning() does not
allow spinning once a panic has begun.

Fixes: dbdda842fe96 ("printk: Add console owner and waiter logic to load balance console writes")

Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220202171821.179394-3-stephen.s.brennan@oracle.com
---
 kernel/printk/printk.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f04bbed0aa79..e83c12770104 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1847,6 +1847,16 @@ static int console_trylock_spinning(void)
 	if (console_trylock())
 		return 1;
 
+	/*
+	 * It's unsafe to spin once a panic has begun. If we are the
+	 * panic CPU, we may have already halted the owner of the
+	 * console_sem. If we are not the panic CPU, then we should
+	 * avoid taking console_sem, so the panic CPU has a better
+	 * chance of cleanly acquiring it later.
+	 */
+	if (panic_in_progress())
+		return 0;
+
 	printk_safe_enter_irqsave(flags);
 
 	raw_spin_lock(&console_owner_lock);
-- 
2.26.2