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

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Message-ID: <74htjoj66srvussqvivbhlkdkj6lkm6ox4jdv2sedb4yzccdmr@sgzbd44mivfs>
Date: Mon, 15 Sep 2025 07:01:17 -0700
From: Breno Leitao <leitao@...ian.org>
To: John Ogness <john.ogness@...utronix.de>
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

Hello John,

First of all, thanks for this patch.

On Fri, Sep 12, 2025 at 02:24:52PM +0206, John Ogness wrote:
> @@ -1606,6 +1610,13 @@ static void __nbcon_atomic_flush_pending(u64 stop_seq, bool allow_unsafe_takeove
>  		if (!console_is_usable(con, flags, true))
>  			continue;
>  
> +		/*
> +		 * It is only allowed to use unsafe ->write_atomic() from
> +		 * nbcon_atomic_flush_unsafe().
> +		 */
> +		if ((flags & CON_NBCON_ATOMIC_UNSAFE) && !allow_unsafe_takeover)
> +			continue;

What will happen with the "message" in this case? is it lost?	

Let me clarify I understand the patch. The .write_atomic callback are
called in two cases:

	1) Inside IRQ/NMI and scheduling context
	2) During panics.

In both cases, they go throught __nbcon_atomic_flush_pending_con(),
right?

Let's say that netconsole implements CON_NBCON_ATOMIC_UNSAFE. What will
happen with printks() inside IRQs (when the system is NOT panicking).
Are they coming through __nbcon_atomic_flush_pending() and will be
skipped?

Also, are these messages even deferred for later flush?

Thanks,
--breno