printk: disable optimistic spin during panic
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