linux-kernel - Re: [BUG] Threaded printk breaks early debugging

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Date:   Fri, 10 Jun 2022 17:11:15 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Peter Geis <pgwipeout@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>
Subject: Re: [BUG] Threaded printk breaks early debugging

Hi Peter,

On 2022-06-10, Peter Geis <pgwipeout@...il.com> wrote:
> However I've run into an issue debugging early boot issues. Anything
> that causes the kernel threading system to die (for example here, a
> NPE) causes the boot console to halt before it outputs the error.

A null pointer exception should trigger a panic, which will cause printk
to enter direct mode for the oops. It should be just as effective as
before introducing the printing kthreads.

Can you please provide a test case or patch that will trigger the
problem you refer to?

> Would it be possible to have both a kconfig option and a runtime
> option to toggle printk_fallback_preferred_direct()? Currently I've
> had to force it at all times while debugging.

It would be trivial to provide a boot parameter for this behavior, but I
think we first need to see a real problem and also see if we can fix
that problem.

John Ogness