rcu/nocb: Remove superfluous memory barrier after bypass enqueue

Pre-GP accesses performed by the update side must be ordered against
post-GP accesses performed by the readers. This is ensured by the
bypass or nocb locking on enqueue time, followed by the fully ordered
rnp locking initiated while callbacks are accelerated, and then
propagated throughout the whole GP lifecyle associated with the
callbacks.

Therefore the explicit barrier advertizing ordering between bypass
enqueue and rcuo wakeup is superfluous. If anything, it would even only
order the first bypass callback enqueue against the rcuo wakeup and
ignore all the subsequent ones.

Remove the needless barrier.

Signed-off-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Neeraj Upadhyay <[email protected]>

Author: Frederic Weisbecker

kernel/rcu/tree_nocb.h

@@ -493,7 +493,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FirstBQ"));
 	}
 	rcu_nocb_bypass_unlock(rdp);
-	smp_mb(); /* Order enqueue before wake. */
+
 	// A wake up of the grace period kthread or timer adjustment
 	// needs to be done only if:
 	// 1. Bypass list was fully empty before (this is the first