rcu: Remove full ordering on second EQS snapshot

When the grace period kthread checks the extended quiescent state
counter of a CPU, full ordering is necessary to ensure that either:

* If the GP kthread observes the remote target in an extended quiescent
  state, then that target must observe all accesses prior to the current
  grace period, including the current grace period sequence number, once
  it exits that extended quiescent state. Also the GP kthread must
  observe all accesses performed by the target prior it entering in
  EQS.

or:

* If the GP kthread observes the remote target NOT in an extended
  quiescent state, then the target further entering in an extended
  quiescent state must observe all accesses prior to the current
  grace period, including the current grace period sequence number, once
  it enters that extended quiescent state. Also the GP kthread later
  observing that EQS must also observe all accesses performed by the
  target prior it entering in EQS.

This ordering is explicitly performed both on the first EQS snapshot
and on the second one as well through the combination of a preceding
full barrier followed by an acquire read. However the second snapshot's
full memory barrier is redundant and not needed to enforce the above
guarantees:

    GP kthread                  Remote target
    ----                        -----
    // Access prior GP
    WRITE_ONCE(A, 1)
    // first snapshot
    smp_mb()
    x = smp_load_acquire(EQS)
                               // Access prior GP
                               WRITE_ONCE(B, 1)
                               // EQS enter
                               // implied full barrier by atomic_add_return()
                               atomic_add_return(RCU_DYNTICKS_IDX, EQS)
                               // implied full barrier by atomic_add_return()
                               READ_ONCE(A)
    // second snapshot
    y = smp_load_acquire(EQS)
    z = READ_ONCE(B)

If the GP kthread above fails to observe the remote target in EQS
(x not in EQS), the remote target will observe A == 1 after further
entering in EQS. Then the second snapshot taken by the GP kthread only
need to be an acquire read in order to observe z == 1.

Therefore remove the needless full memory barrier on second snapshot.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -321,7 +321,15 @@ static bool rcu_dynticks_in_eqs(int snap)
  */
 static bool rcu_dynticks_in_eqs_since(struct rcu_data *rdp, int snap)
 {
-	return snap != rcu_dynticks_snap(rdp->cpu);
+	/*
+	 * The first failing snapshot is already ordered against the accesses
+	 * performed by the remote CPU after it exits idle.
+	 *
+	 * The second snapshot therefore only needs to order against accesses
+	 * performed by the remote CPU prior to entering idle and therefore can
+	 * rely solely on acquire semantics.
+	 */
+	return snap != ct_dynticks_cpu_acquire(rdp->cpu);
 }
 
 /*