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

[Petr Mladek]

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 at suse.com>

Best Regards,
Petr