Re: [PATCH 2/4] printk: disable optimistic spin during panic

[Petr Mladek]

On Fri 2022-01-21 11:02:20, Stephen Brennan wrote:
> 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@xxxxxxxx>
> Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>

Looks good to me:

Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>

Best Regards,
Petr