rcu: Update documentation of rcu_read_unlock()

Since commit b4abf91 ("rtmutex: Make wait_lock irq safe") the
explanation in rcu_read_unlock() documentation about irq unsafe rtmutex
wait_lock is no longer valid.

Remove it to prevent kernel developers reading the documentation to rely on
it.

Suggested-by: Eric W. Biederman <[email protected]>
Signed-off-by: Anna-Maria Gleixner <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Acked-by: "Eric W. Biederman" <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]

Author: anna-marialx

include/linux/rcupdate.h

@@ -652,9 +652,7 @@ static inline void rcu_read_lock(void)
  * Unfortunately, this function acquires the scheduler's runqueue and
  * priority-inheritance spinlocks.  This means that deadlock could result
  * if the caller of rcu_read_unlock() already holds one of these locks or
- * any lock that is ever acquired while holding them; or any lock which
- * can be taken from interrupt context because rcu_boost()->rt_mutex_lock()
- * does not disable irqs while taking ->wait_lock.
+ * any lock that is ever acquired while holding them.
  *
  * That said, RCU readers are never priority boosted unless they were
  * preempted.  Therefore, one way to avoid deadlock is to make sure