Re: [PATCH v3] printk: Remove remaining boot consoles when a real console exists

[Xiaochun Li]

On 8/21/2026 3:17 PM, Xiaochun Li wrote:
> Boot consoles are temporary and should be removed once a real console is
> available. However, the late init cleanup currently only unregisters boot
> consoles that use init section memory. Other boot consoles are expected
> to be removed when the real preferred console is registered.
>
> This does not cover cases where a real console has registered, but the
> boot console was not removed because the real console did not become the
> preferred console. For example, with multiple console= parameters using
> the same driver, a real 8250 console may be enabled while the early
> console remains registered. The result is duplicate printk output from
> both consoles.
>
> In the mailing list discussion, two possible approaches were suggested
> to fix this problem [1]. This patch implements the first one: during
> printk_late_init(), check whether at least one real console is already
> registered. If so, unregister all remaining boot consoles. If no real
> console exists yet, keep the existing behavior and unregister only boot
> consoles that reference init section memory, avoiding a period with no
> console output while waiting for a deferred or modular real console.
>
> Suggested-by: Petr Mladek <pmladek@xxxxxxxx>
> Link: https://lore.kernel.org/lkml/ahBsSW2bB2CRfW-k@xxxxxxxxxxxxxxx/ # [1]
> Signed-off-by: Xiaochun Li <lixiaochun@xxxxxxxxxxxxxx>
> ---
> Changes in v3:
> - Rename realcon_registered to have_real_console per review feedback.
> - Adjust cleanup logic to respect keep_bootcon.
>
>   kernel/printk/printk.c | 37 ++++++++++++++++++++++++++-----------
>   1 file changed, 26 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 2fe9a963c823..cd837f7db7f4 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -4420,36 +4420,51 @@ void __init console_init(void)
>    * be a brief interval in which no messages are logged to the console, which
>    * makes it difficult to diagnose problems that occur during this time.
>    *
> - * To mitigate this problem somewhat, only unregister consoles whose memory
> - * intersects with the init section. Note that all other boot consoles will
> - * get unregistered when the real preferred console is registered.
> + * If a real console has already registered, remove all remaining boot consoles.
> + * Otherwise, mitigate the no-console interval by removing only boot consoles
> + * whose memory intersects with the init section.
>    */
>   static int __init printk_late_init(void)
>   {
> +	bool have_real_console = false;
>   	struct hlist_node *tmp;
>   	struct console *con;
>   	int ret;
>   
>   	console_list_lock();
> +	for_each_console(con) {
> +		if (!(con->flags & CON_BOOT)) {
> +			have_real_console = true;
> +			break;
> +		}
> +	}
> +
>   	hlist_for_each_entry_safe(con, tmp, &console_list, node) {
>   		if (!(con->flags & CON_BOOT))
>   			continue;
>   
> -		/* Check addresses that might be used for enabled consoles. */
> -		if (init_section_intersects(con, sizeof(*con)) ||
> -		    init_section_contains(con->write, 0) ||
> -		    init_section_contains(con->read, 0) ||
> -		    init_section_contains(con->device, 0) ||
> -		    init_section_contains(con->unblank, 0) ||
> -		    init_section_contains(con->data, 0)) {
> +		/*
> +		 * Keep the boot console when requested or as a fallback
> +		 * unless it is using an init section.
> +		 */
> +		if (keep_bootcon || !have_real_console) {

Hi Petr,

Sashiko AI raised the following concern about v3:

| Does this logic unintentionally unregister boot consoles when an unrelated
| real console is present?
| If a system boots with multiple consoles (like console=tty0 console=ttyS0
| earlycon) and an unrelated real console like tty0 (or dummycon) registers
| early, have_real_console will evaluate to true here.
| Because have_real_console is true, the init-section memory check is bypassed
| entirely, and the boot console is unconditionally destroyed:
|       if (keep_bootcon || !have_real_console) {
|               // bypassed
|       }
|       unregister_console_locked(con);
| If the real driver for ttyS0 is modular and has not loaded yet, won't this
| leave the serial console dead and cause a loss of console output during the
| window between late_initcall and the module loading?

I think this concern is valid for the case where an unrelated real console
has registered while the real console corresponding to a remaining boot
console is delayed by deferred probing or module loading.

`have_real_console` is intentionally global in this patch. The purpose is to
handle the case where a real console has already been registered but has not
become `CON_CONSDEV`. In that situation, the existing registration path does
not remove the remaining boot consoles, and duplicate output may persist.

Therefore, when `printk_late_init()` observes any registered real console,
this patch deliberately removes all remaining boot consoles without trying
to establish a one-to-one correspondence between them. This does introduce
a trade-off: a boot console may be removed even though its corresponding real
console has not registered yet, creating a temporary loss of output on that
console.

Unregistering the boot console does not remove records from the printk ring
buffer. A later real console may replay some or all of those records,
depending on its flags and sequence initialization. However, this does not
guarantee that messages generated during the gap will be visible, especially
if the system fails before the real console registers or if the records are
overwritten.

This patch implements only idea 1 from [0]. It does not solve the problem
comprehensively. We plan to investigate idea 2, based on the work in [1],
which should allow the cleanup decision to be made with more precise
information about the corresponding real console.

Do you think the trade-off described above is acceptable for this patch?

Thanks,
Xiaochun

[0] https://lore.kernel.org/lkml/ahBsSW2bB2CRfW-k@xxxxxxxxxxxxxxx/
[1] https://lore.kernel.org/lkml/20260604101459.393162-1-pmladek@xxxxxxxx/

> +			if (!init_section_intersects(con, sizeof(*con)) &&
> +			    !init_section_contains(con->write, 0) &&
> +			    !init_section_contains(con->read, 0) &&
> +			    !init_section_contains(con->device, 0) &&
> +			    !init_section_contains(con->unblank, 0) &&
> +			    !init_section_contains(con->data, 0))
> +				continue;
> +
>   			/*
>   			 * Please, consider moving the reported consoles out
>   			 * of the init section.
>   			 */
>   			pr_warn("bootconsole [%s%d] uses init memory and must be disabled even before the real one is ready\n",
>   				con->name, con->index);
> -			unregister_console_locked(con);
>   		}
> +
> +		unregister_console_locked(con);
>   	}
>   	console_list_unlock();
>