linux-kernel - Re: [PATCH printk v3 09/19] printk: nbcon: Introduce printer kthreads

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Message-ID: <ZqkD096ddCv_4PgB@pathway.suse.cz>
Date: Tue, 30 Jul 2024 17:16:35 +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>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v3 09/19] printk: nbcon: Introduce printer kthreads

On Mon 2024-07-22 19:25:29, John Ogness wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> Provide the main implementation for running a printer kthread
> per nbcon console that is takeover/handover aware. This
> includes:
> 
> - new mandatory write_thread() callback
> - kthread creation
> - kthread main printing loop
> - kthread wakeup mechanism
> - kthread shutdown

> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -985,17 +986,10 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_a
>  	/* Initialize the write context for driver callbacks. */
>  	nbcon_write_context_set_buf(wctxt, &pmsg.pbufs->outbuf[0], pmsg.outbuf_len);
>  
> -	if (use_atomic) {
> +	if (use_atomic)
>  		con->write_atomic(con, wctxt);
> -	} else {
> -		/*
> -		 * This function should never be called for legacy consoles.
> -		 * Handle it as if ownership was lost and try to continue.
> -		 */
> -		WARN_ON_ONCE(1);
> -		nbcon_context_release(ctxt);
> -		return false;
> -	}

This "else" code path should have been removed in the previous patch (8th).

> +	else
> +		con->write_thread(con, wctxt);

We need to make sure that this callback exists by extending the check
at the beginning of this function:

	/*
	 * This function should never be called for consoles that have not
	 * implemented the necessary callback for writing: i.e. legacy
	 * consoles and, when atomic, nbcon consoles with no write_atomic().
	 * Handle it as if ownership was lost and try to continue.
	 */
	if (WARN_ON_ONCE((use_atomic && !con->write_atomic) ||
+			 (!use_atomic && !con->write_thread) ||
			 !(console_srcu_read_flags(con) & CON_NBCON))) {
		nbcon_context_release(ctxt);
		return false;
	}

>  
>  	if (!wctxt->outbuf) {
>  		/*

Otherwise, it looks good.

Best Regards,
Petr