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

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (22/01/21 11:02), 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

[..]

> +++ b/kernel/printk/printk.c
> @@ -1843,6 +1843,16 @@ 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;

Is there something that prevents panic CPU from NMI hlt CPU which is
in console_trylock() under raw_spin_lock_irqsave()?

 CPU0				CPU1
				console_trylock_spinnning()
				 console_trylock()
				  down_trylock()
				   raw_spin_lock_irqsave(&sem->lock)

 panic()
  crash_smp_send_stop()
   NMI 			-> 		HALT