Re: [PATCH] panic: move bust_spinlocks(0) after console_flush_on_panic() to avoid deadlocks

[Andrew Morton]

On Mon,  4 Jun 2018 14:45:57 +0900 Hoeun Ryu <hoeun.ryu@xxxxxxxxxxx> wrote:

> From: Hoeun Ryu <hoeun.ryu@xxxxxxx>
> 
>  Many console device drivers hold the uart_port->lock spinlock with irq enabled
> (using spin_lock()) while the device drivers are writing characters to their devices,
> but the device drivers just try to hold the spin lock (using spin_trylock()) if
> "oops_in_progress" is equal or greater than 1 to avoid deadlocks.
> 
>  There is a case ocurring a deadlock related to the lock and oops_in_progress. A CPU
> could be stopped by smp_send_stop() while it was holding the port lock because irq was
> enabled. Once a CPU stops, it doesn't respond interrupts anymore and the lock stays
> locked forever.
> 
>  console_flush_on_panic() is called during panic() and it eventually holds the uart
> lock but the lock is held by another stopped CPU and it is a deadlock. By moving
> bust_spinlocks(0) after console_flush_on_panic(), let the console device drivers
> think the Oops is still in progress to call spin_trylock() instead of spin_lock() to
> avoid the deadlock.

hm.  Sergey, is this at all related to the UART printk deadlock change
which you're presently discussing in
http://lkml.kernel.org/r/20180615093919.559-1-sergey.senozhatsky@xxxxxxxxx?

> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -233,8 +233,6 @@ void panic(const char *fmt, ...)
>  	if (_crash_kexec_post_notifiers)
>  		__crash_kexec(NULL);
>  
> -	bust_spinlocks(0);
> -
>  	/*
>  	 * We may have ended up stopping the CPU holding the lock (in
>  	 * smp_send_stop()) while still having some valuable data in the console
> @@ -246,6 +244,8 @@ void panic(const char *fmt, ...)
>  	debug_locks_off();
>  	console_flush_on_panic();
>  
> +	bust_spinlocks(0);
> +
>  	if (!panic_blink)
>  		panic_blink = no_blink;
>