Hi,
After more detailed runtime testing I discovered that I didn't re-insert
the console to the correct place in the list. More below...
On 2022-11-14, John Ogness <john.ogness@xxxxxxxxxxxxx> wrote:
> diff --git a/include/linux/console.h b/include/linux/console.h
> index f716e1dd9eaf..9cea254b34b8 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -291,6 +291,7 @@ enum con_flush_mode {
> };
>
> extern int add_preferred_console(char *name, int idx, char *options);
> +extern void console_force_preferred_locked(struct console *con);
> extern void register_console(struct console *);
> extern int unregister_console(struct console *);
> extern void console_lock(void);
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index e770b1ede6c9..dff76c1cef80 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3461,6 +3462,48 @@ int unregister_console(struct console *console)
> }
> EXPORT_SYMBOL(unregister_console);
>
> +/**
> + * console_force_preferred_locked - force a registered console preferred
> + * @con: The registered console to force preferred.
> + *
> + * Must be called under console_list_lock().
> + */
> +void console_force_preferred_locked(struct console *con)
> +{
> + struct console *cur_pref_con;
> +
> + if (!console_is_registered_locked(con))
> + return;
> +
> + cur_pref_con = console_first();
> +
> + /* Already preferred? */
> + if (cur_pref_con == con)
> + return;
> +
> + /*
> + * Delete, but do not re-initialize the entry. This allows the console
> + * to continue to appear registered (via any hlist_unhashed_lockless()
> + * checks), even though it was briefly removed from the console list.
> + */
> + hlist_del_rcu(&con->node);
> +
> + /*
> + * Ensure that all SRCU list walks have completed so that the console
> + * can be added to the beginning of the console list and its forward
> + * list pointer can be re-initialized.
> + */
> + synchronize_srcu(&console_srcu);
> +
> + con->flags |= CON_CONSDEV;
> + WARN_ON(!con->device);
> +
> + /* Only the new head can have CON_CONSDEV set. */
> + console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV);
> + hlist_add_behind_rcu(&con->node, console_list.first);
This is adding the console as the 2nd item. It should be the new
head. The patch below fixes it.
I have done careful runtime testing with this fixup. After the
force_preferred, the console is the new head and sending data to
/dev/console redirects to that console.
It would be nice if we could fold this in. Sorry.
John Ogness
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 8d635467882f..4b77586cf4cb 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3494,7 +3494,7 @@ void console_force_preferred_locked(struct console *con)
/* Only the new head can have CON_CONSDEV set. */
console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV);
- hlist_add_behind_rcu(&con->node, console_list.first);
+ hlist_add_head_rcu(&con->node, &console_list);
}
EXPORT_SYMBOL(console_force_preferred_locked);
