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

On Mon 2022-11-14 20:57:18, John Ogness wrote:

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...

quoted

--- 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.

Great catch!

It would be nice if we could fold this in. Sorry.

I have missed it as well :-/

With this change:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr