linux-kernel - Re: [PATCH printk v2 8/8] printk: nbcon: Add functions for drivers to mark unsafe regions

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Message-ID: <ZNY8koUN5a9TVzS2@alley>
Date:   Fri, 11 Aug 2023 15:50:10 +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>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v2 8/8] printk: nbcon: Add functions for drivers
 to mark unsafe regions

On Fri 2023-07-28 02:08:33, John Ogness wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> For the write_atomic callback, the console driver may have unsafe
> regions that need to be appropriately marked. Provide functions
> that accept the nbcon_write_context struct to allow for the driver
> to enter and exit unsafe regions.
> 
> diff --git a/kernel/printk/printk_nbcon.c b/kernel/printk/printk_nbcon.c
> index e1f0e4278ffa..57b7539bbbb2 100644
> --- a/kernel/printk/printk_nbcon.c
> +++ b/kernel/printk/printk_nbcon.c
> @@ -761,7 +761,6 @@ EXPORT_SYMBOL_GPL(nbcon_can_proceed);
>   *
>   * Internal helper to avoid duplicated code
>   */
> -__maybe_unused
>  static bool nbcon_context_update_unsafe(struct nbcon_context *ctxt, bool unsafe)
>  {
>  	struct console *con = ctxt->console;

It might be better to define nbcon_context_update_unsafe()
in this patch. I guess that nbcon_enter_unsafe()/nbcon_exit_unsafe()
are the only callers.

> @@ -787,6 +786,46 @@ static bool nbcon_context_update_unsafe(struct nbcon_context *ctxt, bool unsafe)
>  	return true;
>  }
>  
> +/**
> + * nbcon_enter_unsafe - Enter an unsafe region in the driver
> + * @wctxt:	The write context that was handed to the write function
> + *
> + * Return:	True if the state is correct. False if ownership was
> + *		handed over or taken.

Same as in 7th patch. I would write:

 * Return:	True if this context still owns the console. False
		if the ownership has been handed over or taken.

> + *
> + * When this function returns false then the calling context is no longer
> + * the owner and is no longer allowed to go forward. In this case it must
> + * back out immediately and carefully. The buffer content is also no longer
> + * trusted since it no longer belongs to the calling context.
> + */
> +bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt)
> +{
> +	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> +
> +	return nbcon_context_update_unsafe(ctxt, true);
> +}
> +EXPORT_SYMBOL_GPL(nbcon_enter_unsafe);
> +
> +/**
> + * nbcon_exit_unsafe - Exit an unsafe region in the driver
> + * @wctxt:	The write context that was handed to the write function
> + *
> + * Return:	True if the state is correct. False if ownership was
> + *		handed over or taken.

Same here.

> + * When this function returns false then the calling context is no longer
> + * the owner and is no longer allowed to go forward. In this case it must
> + * back out immediately and carefully. The buffer content is also no longer
> + * trusted since it no longer belongs to the calling context.
> + */
> +bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt)
> +{
> +	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> +
> +	return nbcon_context_update_unsafe(ctxt, false);
> +}
> +EXPORT_SYMBOL_GPL(nbcon_exit_unsafe);
> +

Best Regards,
Petr