printk: nbcon: Introduce printer kthreads

Provide the main implementation for running a printer kthread
per nbcon console that is takeover/handover aware. This
includes:

- new mandatory write_thread() callback
- kthread creation
- kthread main printing loop
- kthread wakeup mechanism
- kthread shutdown

kthread creation is a bit tricky because consoles may register
before kthreads can be created. In such cases, registration
will succeed, even though no kthread exists. Once kthreads can
be created, an early_initcall will set @printk_kthreads_ready.
If there are no registered boot consoles, the early_initcall
creates the kthreads for all registered nbcon consoles. If
kthread creation fails, the related console is unregistered.

If there are registered boot consoles when
@printk_kthreads_ready is set, no kthreads are created until
the final boot console unregisters.

Once kthread creation finally occurs, @printk_kthreads_running
is set so that the system knows kthreads are available for all
registered nbcon consoles.

If @printk_kthreads_running is already set when the console
is registering, the kthread is created during registration. If
kthread creation fails, the registration will fail.

Until @printk_kthreads_running is set, console printing occurs
directly via the console_lock.

kthread shutdown on system shutdown/reboot is necessary to
ensure the printer kthreads finish their printing so that the
system can cleanly transition back to direct printing via the
console_lock in order to reliably push out the final
shutdown/reboot messages. @printk_kthreads_running is cleared
before shutting down the individual kthreads.

The kthread uses a new mandatory write_thread() callback that
is called with both device_lock() and the console context
acquired.

The console ownership handling is necessary for synchronization
against write_atomic() which is synchronized only via the
console context ownership.

The device_lock() serializes acquiring the console context with
NBCON_PRIO_NORMAL. It is needed in case the device_lock() does
not disable preemption. It prevents the following race:

CPU0				CPU1

 [ task A ]

 nbcon_context_try_acquire()
   # success with NORMAL prio
   # .unsafe == false;  // safe for takeover

 [ schedule: task A -> B ]

				WARN_ON()
				  nbcon_atomic_flush_pending()
				    nbcon_context_try_acquire()
				      # success with EMERGENCY prio

				      # flushing
				      nbcon_context_release()

				      # HERE: con->nbcon_state is free
				      #       to take by anyone !!!

 nbcon_context_try_acquire()
   # success with NORMAL prio [ task B ]

 [ schedule: task B -> A ]

 nbcon_enter_unsafe()
   nbcon_context_can_proceed()

BUG: nbcon_context_can_proceed() returns "true" because
     the console is owned by a context on CPU0 with
     NBCON_PRIO_NORMAL.

     But it should return "false". The console is owned
     by a context from task B and we do the check
     in a context from task A.

Note that with these changes, the printer kthreads do not yet
take over full responsibility for nbcon printing during normal
operation. These changes only focus on the lifecycle of the
kthreads.

Co-developed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-7-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>

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