linux-kernel - Re: [PATCH printk v1] printk: nbcon: Allow reacquire during panic

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Message-ID: <aEf3t7Z4z3g7mHdg@pathway.suse.cz>
Date: Tue, 10 Jun 2025 11:15:35 +0200
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Michael Kelley <mhklinux@...look.com>,
	Toshiyuki Sato <fj6611ie@...itsu.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v1] printk: nbcon: Allow reacquire during panic

On Fri 2025-06-06 21:01:49, John Ogness wrote:
> If a console printer is interrupted during panic, it will never
> be able to reacquire ownership in order to perform and cleanup.
> That in itself is not a problem, since the non-panic CPU will
> simply quiesce in an endless loop within nbcon_reacquire_nobuf().
> 
> However, in this state, platforms that do not support a true NMI
> to interrupt the quiesced CPU will not be able to shutdown that
> CPU from within panic(). This then causes problems for such as
> being unable to load and run a kdump kernel.
> 
> Fix this by allowing non-panic CPUs to reacquire ownership using
> a direct acquire. Then the non-panic CPUs can successfullyl exit
> the nbcon_reacquire_nobuf() loop and the console driver can
> perform any necessary cleanup. But more importantly, the CPU is
> no longer quiesced and is free to process any interrupts
> necessary for panic() to shutdown the CPU.
> 
> All other forms of acquire are still not allowed for non-panic
> CPUs since it is safer to have them avoid gaining console
> ownership that is not strictly necessary.
> 
> Reported-by: Michael Kelley <mhklinux@...look.com>
> Closes: https://lore.kernel.org/r/SN6PR02MB4157A4C5E8CB219A75263A17D46DA@SN6PR02MB4157.namprd02.prod.outlook.com
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

The patch looks good to me:

Reviewed-by: Petr Mladek <pmladek@...e.com>

I am going to wait few days for a potential feedback and then
queue it for 6.17 unless anyone complains.

Best Regards,
Petr