rcu: Regularize resetting of rcu_data wrap indicator

The rcu_data structure's ->gpwrap indicator is currently reset only
when the CPU in question detects a new grace period.  This is in theory
sufficient because any CPU that has been out of action for long enough
that its ->gpwrap indicator is set is guaranteed to see both the end
of an old grace period and the start of a new one.

However, the current code leaves a short window during which the ->gpwrap
indicator has been reset but the corresponding ->gp_seq counter has not
yet been brought up to date.  This is harmless because interrupts are
disabled, but it is likely to (at the very least) cause confusion.

This commit therefore moves the resetting of ->gpwrap to follow the
updating of ->gp_seq.  While in the area, it also resets ->gp_seq_needed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -1807,10 +1807,12 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
 		rdp->rcu_qs_ctr_snap = __this_cpu_read(rcu_dynticks.rcu_qs_ctr);
 		rdp->core_needs_qs = need_gp;
 		zero_cpu_stall_ticks(rdp);
-		WRITE_ONCE(rdp->gpwrap, false);
-		rcu_gpnum_ovf(rnp, rdp);
 	}
 	rdp->gp_seq = rnp->gp_seq;  /* Remember new grace-period state. */
+	if (ULONG_CMP_GE(rnp->gp_seq_needed, rdp->gp_seq_needed) || rdp->gpwrap)
+		rdp->gp_seq_needed = rnp->gp_seq_needed;
+	WRITE_ONCE(rdp->gpwrap, false);
+	rcu_gpnum_ovf(rnp, rdp);
 	return ret;
 }