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...
>
> > --- 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 :-/
> 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);
With this change:
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Best Regards,
Petr
