linux-kernel - Re: [PATCH v3 2/4] printk: nbcon: Introduce KDB helpers

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Message-ID: <aLsOBwV6CVBwG9JV@pathway.suse.cz>
Date: Fri, 5 Sep 2025 18:21:27 +0200
From: Petr Mladek <pmladek@...e.com>
To: Marcos Paulo de Souza <mpdesouza@...e.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Jason Wessel <jason.wessel@...driver.com>,
	Daniel Thompson <danielt@...nel.org>,
	Douglas Anderson <dianders@...omium.org>,
	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net
Subject: Re: [PATCH v3 2/4] printk: nbcon: Introduce KDB helpers

On Tue 2025-09-02 15:33:53, Marcos Paulo de Souza wrote:
> These helpers will be used when calling console->write_atomic on
> KDB code in the next patch. It's basically the same implementaion
> as nbcon_device_try_acquire, but using NBCON_PORIO_EMERGENCY when
> acquiring the context.
> 
> For release we need to flush the console, since some messages could be
> added before the context was acquired, as KDB emits the messages using
> con->{write,write_atomic} instead of storing them on the ring buffer.

I am a bit confused by the last paragraph. It is a very long sentence.

Sigh, I wanted to propose a simple and clear alternative. But I ended
in a rabbit hole and with a rather complex text:

<proposal>
The atomic flush in the release function is questionable. vkdb_printf()
is primary called only when other CPUs are quiescent in kdb_main_loop()
and do not call the classic printk(). But, for example, the
write_atomic() callback might print debug messages. Or there is
one kdb_printf() called in kgdb_panic() before other CPUs are
quiescent. So the flush might be useful. Especially, when
the kdb code fails to quiescent the CPUs and returns early.

Let's keep it simple and just call __nbcon_atomic_flush_pending_con().
It uses write_atomic() callback which is used by the locked kdb code
anyway.

The legacy loop (console_trylock()/console_unlock()) is not
usable in kdb context.

It might make sense to trigger the flush via the printk kthread.
But it would not work in panic() where is the only known kdb_printf()
called when other CPUs are not quiescent. So, it does not look
worth it.
</proposal>

What do you think?

My opinion:

Honestly, I think that the flush is not much important because
it will most offten have nothing to do.

I am just not sure whether it is better to have it there
or avoid it. It might be better to remove it after all.
And just document the decision.

Best Regards,
Petr