linux-kernel - Re: [PATCH printk v1 1/1] printk: nbcon: Allow unsafe write

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Message-ID: <845xdj8rd1.fsf@jogness.linutronix.de>
Date: Mon, 15 Sep 2025 21:15:46 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Breno Leitao <leitao@...ian.org>
Cc: Petr Mladek <pmladek@...e.com>, Sergey Senozhatsky
 <senozhatsky@...omium.org>, Steven Rostedt <rostedt@...dmis.org>, Mike
 Galbraith <efault@....de>, linux-kernel@...r.kernel.org, Greg
 Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v1 1/1] printk: nbcon: Allow unsafe
 write_atomic() for panic

On 2025-09-15, Breno Leitao <leitao@...ian.org> wrote:
> If there is a printk() inside an IRQ and the host is not panicking, then
> the message will be deferred to the kthread, which will print through
> ->write_thread.
>
> So, from a user/netconsole perspective, assuming the no panic
> (allow_unsafe_takeover=false) all the messages will be transmitted
> (always from a thread context), even if the printk() happens on an IRQ.
> So, no message will be lost.
>
> Is my understanding right?

Yes. So you will always have a safe context to write from (except in
panic, where you will do your unsafe code).

John