Re: [PATCH printk] printk: Avoid multiple unregistration in unregister_console_locked()

[John Ogness <john.ogness@linutronix.de>]

On 2026-10-29, Sashiko wrote:
>> @@ -4471,6 +4493,9 @@ static int __init printk_late_init(void)
>>  		}
>>  
>>  		unregister_console_locked(con);
>> +
>> +		/* Changed console list, may require printer threads to start/stop. */
>> +		printk_kthreads_check_locked();
>>  	}
>
> Should printk_kthreads_check_locked() be called after the
> hlist_for_each_entry_safe(con, tmp, &console_list, node) loop in
> printk_late_init() rather than inside the loop body?
>
> In kernel/printk/printk.c:printk_late_init():
>
>	hlist_for_each_entry_safe(con, tmp, &console_list, node) {
>		if (!(con->flags & CON_BOOT))
>			continue;
>		...
>		unregister_console_locked(con);
>
>		/* Changed console list, may require printer threads to start/stop. */
>		printk_kthreads_check_locked();
>	}
>	console_list_unlock();
>
> Because printk_kthreads_check_locked() is placed inside the loop,
> unregistering the last boot console sets have_boot_console to false and
> immediately attempts to start printer kthreads while the loop is still
> iterating over console_list.
>
> If kthread creation in legacy_kthread_create() or nbcon_kthread_create()
> fails for the console stored in tmp, printk_kthreads_check_locked() calls
> unregister_console_locked() on that console, removing it with
> hlist_del_init_rcu(), freeing nbcon resources, and invoking console->exit().
> On the next iteration of printk_late_init(), con is set to tmp, accessing the
> already-unregistered console and terminating the walk early because
> hlist_del_init_rcu() cleared tmp->next.

Geez, sorry. Obviously printk_kthreads_check_locked() needs to be
outside of the hlist_for_each_entry_safe(console_list) loop. That was
the whole point of the series!

I will wait a day for v2 in case there is any feedback regarding my
comments (or lack thereof).

John