[PATCH printk v4 31/39] printk, xen: fbfront: create/use safe function for forcing preferred

[John Ogness]

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 at linutronix.de> 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);