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 23:43:26 +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:
>>>> +	/* 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, 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?

Yes.

> 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.

Since the only flag that is ever checked in this way is CON_ENABLED, I
was planning on calling it console_srcu_is_enabled(). But I suppose I
could have it be: (console_srcu_read_flags() & CON_ENABLED). That would
keep it more abstract in case anyone should want to read other flag bits
under SRCU.

There are only 4 call sites that need this, so I suppose we don't need a
special _is_enabled() variant. Thanks for the suggestion!

John