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

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Message-ID: <i6qgd5wcdl5gkwfc6cru2hnokcsawdd4yzez2i6klu54uxwx23@jl44kdpph3t3>
Date: Mon, 15 Sep 2025 08:46:13 -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

On Mon, Sep 15, 2025 at 04:20:35PM +0206, John Ogness wrote:
> 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.

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?

Thanks,
--breno