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

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Message-ID: <84348n9510.fsf@jogness.linutronix.de>
Date: Mon, 15 Sep 2025 16:20:35 +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:
> 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?

@allow_unsafe_takeover is only true at the very end of panic. In all
other cases, the ->write_atomic() callback is ignored as if it wasn't
implemented. That means it will rely on the deferred printing kthread to
handle it.

> 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?

When the system is not panicing, CON_NBCON_ATOMIC_UNSAFE has the effect
of acting as if you never implemented ->write_atomic(). So yes, only
->write_thread() will handle everything in a deferred context. If the
system never panics, your ->write_atomic() will never be called.

John