Re: [PATCH printk v2 05/38] printk: use console_is_enabled()

John Ogness
2022-10-31T15:39:11Z

On 2022-10-21, Petr Mladek <pmladek@suse.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