Re: [Ksummit-discuss] [TECH TOPIC] asynchronous printk

[Hannes Reinecke <hare@suse.com>]

On 07/19/2016 09:46 AM, Sergey Senozhatsky wrote:
> 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()?
> 
Errm.
Because it doesn't have any other chance?
Like, hard lockup?
Power down?
Hardware dead?

Slightly puzzled,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
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