[Ksummit-discuss] [TECH TOPIC] asynchronous printk

[Sergey Senozhatsky]

On (07/19/16 09:38), Hannes Reinecke wrote:
[..]
> > yes, panic() must be in sync printk mode. but we do it a
> > bit differently: console_verbose() forces printk to switch
> > to sync mode.
> > 
> > so panic() goes like this:
> > 
> > 	panic()
> > 	{
> > 		console_verbose(); # switch to sync printk. forever.
> > 		bust_spinlocks();
> > 		pr_emerg("Kernel panic - not syncing...");
> > 
> > 		....
> > 		debug_locks_off();
> > 		console_flush_on_panic();  # flushes kernel log_bug in
> > 					   # sync mode
> > 
> > 		// the rest of panic()
> > 
> > 	}
> > 
> > 	-ss
> > 
> but this is precisely what I meant by priority inversion:
> If there are lots of messages in the printk buffer we might not be
> _able_ to print out everything as the machine died before the entire
> printk buffer could be printed.

why would it die? the same CPU that panics the system flushes the
log_buf in sync mode. console_flush_on_panic() is invoked after
panic_smp_send_stop(), which is supposed to IPI_STOP other CPUs on the
system, but before panic() reboots the system (if it must reboot it).
and it does not really care whether console_sem is available, it goes
to console_unlock() in any case

void console_flush_on_panic(void)
{
	/*
	 * If someone else is holding the console lock, trylock will fail
	 * and may_schedule may be set.  Ignore and proceed to unlock so
	 * that messages are flushed out.  As this can be called from any
	 * context and we don't want to get preempted while flushing,
	 * ensure may_schedule is cleared.
	 */
	console_trylock();
	console_may_schedule = 0;
	console_unlock();
}

so why would the system die before we console_unlock()?

	-ss