printk: nbcon: Fix illegal RCU usage on thread wakeup

BugLink: https://bugs.launchpad.net/bugs/2102249

In debug kernels, printk'ing during the SMP startup of a secondary CPU
results in a splat on boot that details illegal RCU usage from
offline CPUs.

This patch aligns rcuwait_has_sleeper() with what currently exists in
Torvalds' tree, at commit 76f258bf3f2aa, and will avoid the
aforementioned splat.

Furthermore, this patch makes it so that we avoid the use of hacky
rcu_derefrencing through the waiter's task struct,and instead leverages
the correct RCU API with the rcuwait_active() function.

Reported-by: Andrew Halaney <ahalaney@redhat.com>
Closes: https://lore.kernel.org/linux-rt-users/5x4nejpojrtny37k7l6loewqwuaituq77zc3tkkojvawcuvmml@thwg65fdb3vn
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Derek Barbosa <debarbos@redhat.com>
(cherry picked from PREEMPT_RT 6.6.78-rt51)
Link: https://lore.kernel.org/all/ZxZ5WXV6umbL9XG9@debarbos-thinkpadt14sgen2i.remote.csb/T/
Acked-by: Benjamin Wheeler <benjamin.wheeler@canonical.com>
Acked-by: Massimiliano Pellizzer <massimiliano.pellizzer@canonical.com>
Signed-off-by: Kevin Becker <kevin.becker@canonical.com>

kernel/printk/nbcon.c

@@ -1137,9 +1137,6 @@ static void nbcon_irq_work(struct irq_work *irq_work)
 
 static inline bool rcuwait_has_sleeper(struct rcuwait *w)
 {
-	bool has_sleeper;
-
-	rcu_read_lock();
 	/*
 	 * Guarantee any new records can be seen by tasks preparing to wait
 	 * before this context checks if the rcuwait is empty.
@@ -1152,10 +1149,7 @@ static inline bool rcuwait_has_sleeper(struct rcuwait *w)
 	 * This pairs with nbcon_kthread_func:A.
 	 */
 	smp_mb(); /* LMM(rcuwait_has_sleeper:A) */
-	has_sleeper = !!rcu_dereference(w->task);
-	rcu_read_unlock();
-
-	return has_sleeper;
+       return rcuwait_active(w);
 }
 
 /**