Re: [PATCH 1/3] vt: order the fg_console switch against vt_console_print()

[Greg Kroah-Hartman]

On Mon, Sep 21, 2026 at 09:27:14PM -0400, Jaidev Shastri via B4 Relay wrote:
> From: Jaidev Shastri <jaidevshastri@xxxxxx>
> 
> redraw_screen() switches fg_console with a plain store once the new
> console's state is set up. vt_console_print(), the printk console
> callback, indexes vc_cons[] with fg_console before it takes
> printing_lock.
> 
> Store the new index with smp_store_release() and read it with
> smp_load_acquire(), so that the printk path cannot reach the slot before
> the console it denotes is complete.
> 
> Found with MBCheck, a static herd7-based memory consistency checker.
> 
> Signed-off-by: Jaidev Shastri <jaidevshastri@xxxxxx>
> ---
>  drivers/tty/vt/vt.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 57edf3749..3abbd6cf9 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -973,7 +973,8 @@ void redraw_screen(struct vc_data *vc, int is_switch)
>  		if (!con_is_visible(vc))
>  			redraw = 1;
>  		*vc->vc_display_fg = vc;
> -		fg_console = vc->vc_num;
> +		/* Pairs with the smp_load_acquire() in vt_console_print(). */
> +		smp_store_release(&fg_console, vc->vc_num);

Using these functions are almost always wrong.  Fix things properly, do
not pepper these types of calls all over the kernel, that way lies
madness.

greg k-h