linux-kernel - Re: [PATCH printk v2 04/18] printk: nbcon: Introduce printing kthreads

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Date: Wed, 12 Jun 2024 13:24:24 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
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 04/18] printk: nbcon: Introduce printing kthreads

On 2024-06-12, Petr Mladek <pmladek@...e.com> wrote:
>> > After all, I would add two comments, like these:
>> >
>> > <proposal-2>
>> > 	/*
>> > 	 * Any access to the console device is serialized either by
>> > 	 * device_lock() or console context or both.
>> > 	 */
>> > 	kt = kthread_run(nbcon_kthread_func, con, "pr/%s%d", con->name,
>> > con->index);
>> > [...]
>> >
>> > 	/*
>> > 	 * Some users check con->kthread to decide whether to flush
>> > 	 * the messages directly using con->write_atomic(). But they
>> > 	 * do so only when the console is already in @console_list.
>> > 	 */
>> 
>> I do not understand how @console_list is related to racing between
>> non-thread and thread. kthreads are not only created during
>> registration. For example, they can be created much later when the last
>> boot console unregisters.
>
> I had in mind two particular code paths:
>
>   1. The check of con->kthread in nbcon_device_release() before
>      calling __nbcon_atomic_flush_pending_con().
>
>      But it is called only when __uart_port_using_nbcon() returns true.
>      And it would fail when nbcon_kthread_create() is called because
>
> 		checks hlist_unhashed_lockless(&up->cons->node)
>
>      would fail. Which checks of the console is in @console_list
>
>
>   2. The following check in console_flush_all()
>
> 			if ((flags & CON_NBCON) && con->kthread)
> 				continue;
>
>      The result affects whether the legacy flush would call
>      nbcon_legacy_emit_next_record().
>
>      But this is called only for_each_console_srcu(con)
>      => it could not race with nbcon_kthread_create()
> 	because this console is not in @console_list at this moment.
>
> By other words, I was curious whether some other code paths might
> call con->write_atomic() while the kthread is already running.
>
> It is not that important because it would be safe anyway.
> I was checking this before I realized that it would be safe.

Yes, it must be safe because it can happen at any time. For example,
when flushing after an emergency section.

> Anyway, the information about that the console is not in @console_list
> when we set con->kthread still looks useful.

Except that it is not always true. If boot consoles are registered, the
kthread is created later, after the console _is_ in
@console_list. Setting con->kthread really has nothing to do with
@console_list.

> At minimum, the check would be racy if the console was on the list.

The con->kthread check _is_ racey, but it doesn't matter. Perhaps you
just want to make it clear that it is racey but it does not matter.

How about:

	/*
	 * Some users check con->kthread to decide whether to flush
	 * the messages directly using con->write_atomic(). Although
	 * racey, such a check for that purpose is safe because both
	 * threaded and atomic printing are serialized by the
	 * console context.
	 */
	con->kthread = kt;

John