linux-kernel - Re: [PATCH printk v2 05/38] printk: use console_is

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Date:   Mon, 31 Oct 2022 16:45:00 +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
Subject: Re: [PATCH printk v2 05/38] printk: use console_is_enabled()

On 2022-10-21, Petr Mladek <pmladek@...e.com> wrote:
>>  static inline bool console_is_usable(struct console *con)
>>  {
>> -	if (!(con->flags & CON_ENABLED))
>> +	if (!console_is_enabled(con))
>>  		return false;
>
> This allows to use console_is_usable() without synchronization against
> modification of the state. I guess that it will be needed for the
> printk kthreads. But it is not needed at the moment.

It will be needed once console_lock is no longer providing the
synchronization for console->flags (later in _this_ series).

I will add to the commit message that this is a preparatory change for
when console_lock no longer provides this synchronization.

>> @@ -2944,7 +2944,7 @@ void console_unblank(void)
>>  	console_locked = 1;
>>  	console_may_schedule = 0;
>>  	for_each_console(c)
>> -		if ((c->flags & CON_ENABLED) && c->unblank)
>> +		if (console_is_enabled(c) && c->unblank)
>>  			c->unblank();
>>  	console_unlock();
>
> The reading of the flag is actually synchronized against modifications
> here. I guess that we need this because c->unblank() probably is not safe
> against other operations with the console, e.g. c->write().

Again, we will need it later in this series when holding console_lock
does not provide the necessary synchronization.

>> @@ -3098,7 +3098,7 @@ static int try_enable_preferred_console(struct console *newcon,
>>  	 * without matching. Accept the pre-enabled consoles only when match()
>>  	 * and setup() had a chance to be called.
>>  	 */
>> -	if (newcon->flags & CON_ENABLED && c->user_specified ==	user_specified)
>> +	if (console_is_enabled(newcon) && (c->user_specified == user_specified))
>
> This is not racy because newcon is not in console_list at this
> point. So that the state can't be modified by another callers.
>
> Anyway, it is not explicitly synchronized, so we need to use
> console_is_enabled with data_race() annotation.

For this case, it makes sense to _not_ use console_is_enabled(). This
code will be synchronized against writes even after console_lock has
been relieved of this responsibility.

Originally I wanted to replace _all_ checks with console_is_enabled(),
but since synchronization is rare, I'll keep the manual checks for those
(and add a comment that it is not a data race).

John