linux-kernel - Re: [PATCH printk v1 0/1] Allow unsafe ->write

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Message-ID: <bqihuqqrfc4ayghycmtfjcowyflvbku4ledy7pwajryptlp2wg@hq6cctbmimi3>
Date: Fri, 26 Sep 2025 08:17:53 -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 0/1] Allow unsafe ->write_atomic() for panic

Hello John,

On Fri, Sep 26, 2025 at 11:27:49AM +0206, John Ogness wrote:
> On 2025-09-17, Breno Leitao <leitao@...ian.org> wrote:
> > Upon further consideration, it's worth noting that not all network
> > drivers rely on irq-unsafe locks. In practice, only a subset of drivers
> > use them, while most network drivers I'm familiar with maintain IRQ-safe
> > TX paths.
> >
> > If we could determine the IRQ safety characteristics (IRQ-safe vs
> > IRQ-unsafe TX) during netconsole registration, this would enable more
> > optimized behavior: netconsole could register as CON_NBCON_ATOMIC_UNSAFE
> > only when the underlying network adapter uses IRQ-unsafe locks. For
> > adapters with IRQ-safe implementations, netconsole could safely utilize
> > the ->write_atomic path without restrictions.
> 
> This is good to read. But note that if CON_NBCON_ATOMIC_UNSAFE is not
> set, it is expected that ->write_atomic() will also function in NMI. So
> being IRQ-safe may not be enough.

What are the other requirements for ->write_atomic() so it could be
executed inside a NMI?

That brings me another point, I suppose that netconsole callbacks are
currently being called from NMI, given it is registered as a legacy
console, and legacy consoles are printked from inside NMIs, right?

Thanks!
--breno