The write() callback of legacy consoles usually makes use of
spinlocks. This is not permitted with PREEMPT_RT in atomic
contexts.

For PREEMPT_RT, create a new kthread to handle printing of all
the legacy consoles (and nbcon consoles if boot consoles are
registered). This allows legacy consoles to work on PREEMPT_RT
without requiring modification. (However they will not have
the reliability properties guaranteed by nbcon atomic
consoles.)

Use the existing printk_kthreads_check_locked() to start/stop
the legacy kthread as needed.

Introduce the macro force_legacy_kthread() to query if the
forced threading of legacy consoles is in effect. Although
currently only enabled for PREEMPT_RT, this acts as a simple
mechanism for the future to allow other preemption models to
easily take advantage of the non-interference property provided
by the legacy kthread.

When force_legacy_kthread() is true, the legacy kthread
fulfills the role of the console_flush_type @legacy_offload by
waking the legacy kthread instead of printing via the
console_lock in the irq_work. If the legacy kthread is not
yet available, no legacy printing takes place (unless in
panic).

If for some reason the legacy kthread fails to create, any
legacy consoles are unregistered. With force_legacy_kthread(),
the legacy kthread is a critical component for legacy consoles.

These changes only affect CONFIG_PREEMPT_RT.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-16-john.ogness@linutronix.deSigned-off-by: Petr Mladek <pmladek@suse.com>