linux-kernel - Re: [PATCH printk v2 00/18] add threaded printing + the rest

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Date: Wed, 5 Jun 2024 11:32:15 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Petr Mladek <pmladek@...e.com>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Jonathan Corbet <corbet@....net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Sreenath Vijayan <sreenath.vijayan@...y.com>,
	Shimoyashiki Taichi <taichi.shimoyashiki@...y.com>,
	Tomas Mudrunka <tomas.mudrunka@...il.com>,
	linux-doc@...r.kernel.org, linux-serial@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	"Borislav Petkov (AMD)" <bp@...en8.de>,
	Xiongwei Song <xiongwei.song@...driver.com>
Subject: Re: [PATCH printk v2 00/18] add threaded printing + the rest

On 05/06/24 10:15, John Ogness wrote:

...

> Yes, that probably is a good candidate for emergency mode.
> 
> However, your report is also identifying a real issue:
> nbcon_cpu_emergency_flush() was implemented to be callable from
> non-emergency contexts (in which case it should do nothing). However, in
> order to check if it is an emergency context, migration needs to be
> disabled.

I see.

> Perhaps the below change can be made for v2 of this series?

Yes, this seems to cure it.

Thanks for the super quick reply and patch!

Best,
Juri

> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index 4b9645e7ed70..eeaf8465f492 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1581,8 +1581,19 @@ void nbcon_cpu_emergency_exit(void)
>   */
>  void nbcon_cpu_emergency_flush(void)
>  {
> +	bool is_emergency;
> +
> +	/*
> +	 * If the current context is not an emergency context, preemption
> +	 * might be enabled. To be sure, disable preemption when checking
> +	 * if this is an emergency context.
> +	 */
> +	preempt_disable();
> +	is_emergency = (*nbcon_get_cpu_emergency_nesting() != 0);
> +	preempt_enable();
> +
>  	/* The explicit flush is needed only in the emergency context. */
> -	if (*(nbcon_get_cpu_emergency_nesting()) == 0)
> +	if (!is_emergency)
>  		return;
>  
>  	nbcon_atomic_flush_pending();
>