rcu: Remove references to old grace-period-wait primitives

The rcu_barrier_sched(), synchronize_sched(), and synchronize_rcu_bh()
RCU API members have been gone for many years. This commit therefore
removes non-historical instances of them.

Reported-by: Joe Perches <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Signed-off-by: Boqun Feng <[e-mail]>

Documentation/RCU/rcubarrier.rst

···
 	was first added back in 2005.  This is because on_each_cpu()
 	disables preemption, which acted as an RCU read-side critical
 	section, thus preventing CPU 0's grace period from completing
-	until on_each_cpu() had dealt with all of the CPUs.  However,
-	with the advent of preemptible RCU, rcu_barrier() no longer
-	waited on nonpreemptible regions of code in preemptible kernels,
-	that being the job of the new rcu_barrier_sched() function.
+	until on_each_cpu() had dealt with all of the CPUs.
 
 	However, with the RCU flavor consolidation around v4.20, this
 	possibility was once again ruled out, because the consolidated

include/linux/rcupdate.h

···
  * sections, invocation of the corresponding RCU callback is deferred
  * until after the all the other CPUs exit their critical sections.
  *
- * In v5.0 and later kernels, synchronize_rcu() and call_rcu() also
- * wait for regions of code with preemption disabled, including regions of
- * code with interrupts or softirqs disabled.  In pre-v5.0 kernels, which
- * define synchronize_sched(), only code enclosed within rcu_read_lock()
- * and rcu_read_unlock() are guaranteed to be waited for.
+ * Both synchronize_rcu() and call_rcu() also wait for regions of code
+ * with preemption disabled, including regions of code with interrupts or
+ * softirqs disabled.
  *
  * Note, however, that RCU callbacks are permitted to run concurrently
  * with new RCU read-side critical sections.  One way that this can happen
···
  * rcu_read_unlock() - marks the end of an RCU read-side critical section.
  *
  * In almost all situations, rcu_read_unlock() is immune from deadlock.
- * In recent kernels that have consolidated synchronize_sched() and
- * synchronize_rcu_bh() into synchronize_rcu(), this deadlock immunity
- * also extends to the scheduler's runqueue and priority-inheritance
- * spinlocks, courtesy of the quiescent-state deferral that is carried
- * out when rcu_read_unlock() is invoked with interrupts disabled.
+ * This deadlock immunity also extends to the scheduler's runqueue
+ * and priority-inheritance spinlocks, courtesy of the quiescent-state
+ * deferral that is carried out when rcu_read_unlock() is invoked with
+ * interrupts disabled.
  *
  * See rcu_read_lock() for more information.
  */