rcu/exp: Remove confusing needless full barrier on task unblock

A full memory barrier in the RCU-PREEMPT task unblock path advertizes
to order the context switch (or rather the accesses prior to
rcu_read_unlock()) with the expedited grace period fastpath.

However the grace period can not complete without the rnp calling into
rcu_report_exp_rnp() with the node locked. This reports the quiescent
state in a fully ordered fashion against updater's accesses thanks to:

1) The READ-SIDE smp_mb__after_unlock_lock() barrier across nodes
   locking while propagating QS up to the root.

2) The UPDATE-SIDE smp_mb__after_unlock_lock() barrier while holding the
   the root rnp to wait/check for the GP completion.

3) The (perhaps redundant given step 1) and 2)) smp_mb() in rcu_seq_end()
   before the grace period completes.

This makes the explicit barrier in this place superfluous. Therefore
remove it as it is confusing.

Acked-by: Paul E. McKenney <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Joel Fernandes <[email protected]>
Signed-off-by: Neeraj Upadhyay (AMD) <[email protected]>

Author: Frederic Weisbecker

kernel/rcu/tree_plugin.h

@@ -534,7 +534,6 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
 		WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
 			     (!empty_norm || rnp->qsmask));
 		empty_exp = sync_rcu_exp_done(rnp);
-		smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
 		np = rcu_next_node_entry(t, rnp);
 		list_del_init(&t->rcu_node_entry);
 		t->rcu_blocked_node = NULL;