rcu: Remove now-unused ->b.exp_need_qs field from the rcu_special union

The ->b.exp_need_qs field is now set only to false, so this commit
removes it.  The job this field used to do is now done by the rcu_data
structure's ->deferred_qs field, which is a consequence of a better
split between task-based (the rcu_node structure's ->exp_tasks field) and
CPU-based (the aforementioned rcu_data structure's ->deferred_qs field)
tracking of quiescent states for RCU-preempt expedited grace periods.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/sched.h

@@ -571,12 +571,8 @@ union rcu_special {
 	struct {
 		u8			blocked;
 		u8			need_qs;
-		u8			exp_need_qs;
-
-		/* Otherwise the compiler can store garbage here: */
-		u8			pad;
 	} b; /* Bits. */
-	u32 s; /* Set of bits. */
+	u16 s; /* Set of bits. */
 };
 
 enum perf_event_task_context {

kernel/rcu/tree_plugin.h

@@ -284,13 +284,10 @@ static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
 	 * no need to check for a subsequent expedited GP.  (Though we are
 	 * still in a quiescent state in any case.)
 	 */
-	if (blkd_state & RCU_EXP_BLKD &&
-	    t->rcu_read_unlock_special.b.exp_need_qs) {
-		t->rcu_read_unlock_special.b.exp_need_qs = false;
+	if (blkd_state & RCU_EXP_BLKD && rdp->deferred_qs)
 		rcu_report_exp_rdp(rdp->rsp, rdp, true);
-	} else {
-		WARN_ON_ONCE(t->rcu_read_unlock_special.b.exp_need_qs);
-	}
+	else
+		WARN_ON_ONCE(rdp->deferred_qs);
 }
 
 /*
@@ -509,9 +506,7 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
 	 * tasks are handled when removing the task from the
 	 * blocked-tasks list below.
 	 */
-	if (special.b.exp_need_qs || rdp->deferred_qs) {
-		t->rcu_read_unlock_special.b.exp_need_qs = false;
-		rdp->deferred_qs = false;
+	if (rdp->deferred_qs) {
 		rcu_report_exp_rdp(rcu_state_p, rdp, true);
 		if (!t->rcu_read_unlock_special.s) {
 			local_irq_restore(flags);