Re: [PATCH printk v3 33/40] printk, xen: fbfront: create/use safe function for forcing preferred

On 2022-11-10, Petr Mladek [off-list ref] wrote:

quoted

+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;
+
+	hlist_del_init_rcu(&con->node);

We actually should re-initialize the node only after all existing
console list walks are finished. Se we should use here:

	hlist_del_rcu(&con->node);

hlist_del_init_rcu() only re-initializes @pprev pointer. But maybe you
are concerned that there is a window where list_unhashed() becomes true?
I agree that it should be changed to hlist_del_rcu() because there
should not be a window where this console appears unregistered.

quoted

+	/* Only the new head can have CON_CONSDEV set. */
+	WRITE_ONCE(cur_pref_con->flags, cur_pref_con->flags & ~CON_CONSDEV);

As mentioned in the reply for 7th patch, I would prefer to hide this
WRITE_ONCE into a wrapper, e.g. console_set_flag(). It might also
check that the console_list_lock is taken...

Agreed. For v4 it will become:

console_srcu_write_flags(cur_pref_con->flags & ~CON_CONSDEV);

John