linux-kernel - Re: [PATCH printk v3 11/19] printk: nbcon: Rely on kthreads for normal operation

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Message-ID: <ZqpE9urpKZxv4Ks3@pathway.suse.cz>
Date: Wed, 31 Jul 2024 16:06:46 +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
Subject: Re: [PATCH printk v3 11/19] printk: nbcon: Rely on kthreads for
 normal operation

On Mon 2024-07-22 19:25:31, John Ogness wrote:
> Once the kthread is running and available
> (i.e. @printk_kthreads_running is set), the kthread becomes
> responsible for flushing any pending messages which are added
> in NBCON_PRIO_NORMAL context. Namely the legacy
> console_flush_all() and device_release() no longer flush the
> console. And nbcon_atomic_flush_pending() used by
> nbcon_cpu_emergency_exit() no longer flushes messages added
> after the emergency messages.
> 
> The console context is safe when used by the kthread only when
> one of the following conditions are true:
> 
>   1. Other caller acquires the console context with
>      NBCON_PRIO_NORMAL with preemption disabled. It will
>      release the context before rescheduling.
> 
>   2. Other caller acquires the console context with
>      NBCON_PRIO_NORMAL under the device_lock.
> 
>   3. The kthread is the only context which acquires the console
>      with NBCON_PRIO_NORMAL.
> 
> This is satisfied for all atomic printing call sites:
> 
> nbcon_legacy_emit_next_record() (#1)
> 
> nbcon_atomic_flush_pending_con() (#1)
> 
> nbcon_device_release() (#2)
> 
> It is even double guaranteed when @printk_kthreads_running
> is set because then _only_ the kthread will print for
> NBCON_PRIO_NORMAL. (#3)
> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -4102,8 +4139,10 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
>  			 * that they make forward progress, so only increment
>  			 * @diff for usable consoles.
>  			 */
> -			if (!console_is_usable(c, flags, true))
> +			if (!console_is_usable(c, flags, true) &&
> +			    !console_is_usable(c, flags, false)) {

This looks weird. nbcon console can't make progress when
"write_atomic" is not implemented and the kthreads are not
running.

I should be:

			if (!((console_is_usable(c, flags, true)) ||
			      (console_is_usable(c, flags, false) && printk_kthreads_running))) {

That said. Do we really want to support nbcon consoles without
@write_atomic() callback?

It would make things easier when both callbacks are mandatory.

>  				continue;
> +			}
>  
>  			if (flags & CON_NBCON) {
>  				printk_seq = nbcon_seq_read(c);

Best Regards,
Petr