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

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Message-ID: <877d026blr.fsf@jogness.linutronix.de>
Date:   Thu, 10 Nov 2022 17:09:12 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
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 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. 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.

>> +	/* 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