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

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Date:   Thu, 10 Nov 2022 18:26:48 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Helge Deller <deller@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Tom Rix <trix@...hat.com>, linux-fbdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH printk v3 33/40] printk, xen: fbfront: create/use safe
 function for forcing preferred

On Thu 2022-11-10 17:09:12, John Ogness wrote:
> On 2022-11-10, Petr Mladek <pmladek@...e.com> wrote:
> >> +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.

Ah, I was not aware of it.

> 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.

Makes sense.

> >> +	/* 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);

I am happy that your are going to introduce an API for this.

Just to be sure. The _srcu_ in the name means that the write
will use WRITE_ONCE() so that it can be read safely in SRCU
context using READ_ONCE(). Do I get it correctly, please?

I expect that the counter part will be console_srcu_read_flags().
I like the name. It is better than _unsafe_ that I proposed earlier.

Best Regards,
Petr