linux-kernel - Re: [RFC PATCH v1 00/25] printk: new implementation

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Date:   Wed, 13 Feb 2019 15:15:42 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 00/25] printk: new implementation

On 2019-02-13, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:
>> - console_flush_on_panic() currently is a NOP. It is pretty clear how
>>   this could be implemented if atomic_write was available. But if no
>>   such console is registered, it is not clear what should be done. Is
>>   this function really even needed?
>
> If you now rely on a fully preemptible printk kthread to flush
> pending logbuf messages, then console_flush_on_panic() is your
> only chance to see those pending logbuf messages on the serial
> console when the system dies.

Anything critical would have already been immediately print to the
emergency consoles. And if an emergency console was available,
console_flush_on_panic() could be a special case where _all_ unseen
messages (regardless of importance) are printed to the emergency
console.

> Non-atomic consoles should become atomic once you call bust_spinlocks(1),
> this is what we currently have:
>
> 	panic()
> 	bust_spinlocks(1)                       // sets oops_in_progress
> 	console_flush_on_panic()
> 	 call_console_drivers()
> 	  -> serial_driver_write()
> 	      if (oops_in_progress)
> 	        locked = spin_trylock_irqsave(&port->lock);
> 	      uart_console_write();
> 	      if (locked)
> 	        spin_unlock_irqrestore(&port->lock);

I don't like bust_spinlocks() because drivers end up implementing
oops_in_progress with exactly that... ignoring their own locks. I prefer
consoles are provided with a locking mechanism that they can use to
support a separate NMI-safe write function. My series introduces
console_atomic_lock() for exactly this purpose.

But this doesn't help here. Here we are talking about a crashing system
that does _not_ have an emergency console. And in this case I would say
messages would be lost (just like they are now if all you have is a vt
console and it was busy).

I suppose we could keep the current bust_spinlocks() stuff for the
special case that there are no emergency consoles available. It's better
than nothing, but also not really reliable. Preferrably we figure out
how to implement write_atomic for all console drivers.

John Ogness