linux-kernel - Removal of printk safe buffers delays NMI context printk

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Message-Id: <1636039236.y415994wfa.astroid@bobo.none>
Date:   Fri, 05 Nov 2021 01:54:44 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org
Subject: Removal of printk safe buffers delays NMI context printk

Hi John,

It seems printk from NMI context is now delayed indefinitely and
there is no printk_safe_flush equivalent (or I can't see one) to
allow a NMI buffer to be flushed by a different CPU.

This causes hard lockup watchdog messages to not get shown on the
console. I can call printk from a different CPU and that seems to
flush the stuck CPU's NMI buffer immediately.

What's the best way to expose this? Can we have something like tihs?

void printk_flush(void)
{
	preempt_disable();
	if (console_trylock_spinning())
		console_unlock();
	preempt_enable();
        wake_up_klogd();
}

Thanks,
Nick