rcu: Simplify unboosting checks

This is a port of commit #82e78d80 from TREE_PREEMPT_RCU to
TINY_PREEMPT_RCU.

This commit uses the fact that current->rcu_boost_mutex is set
any time that the RCU_READ_UNLOCK_BOOSTED flag is set in the
current->rcu_read_unlock_special bitmask.  This allows tests of
the bit to be changed to tests of the pointer, which in turn allows
the RCU_READ_UNLOCK_BOOSTED flag to be eliminated.

Please note that the check of current->rcu_read_unlock_special need not
change because any time that RCU_READ_UNLOCK_BOOSTED was set, so was
RCU_READ_UNLOCK_BLOCKED.  Therefore, __rcu_read_unlock() can continue
testing current->rcu_read_unlock_special for non-zero, as before.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/sched.h

@@ -1864,8 +1864,7 @@ extern void task_clear_jobctl_pending(struct task_struct *task,
 #ifdef CONFIG_PREEMPT_RCU
 
 #define RCU_READ_UNLOCK_BLOCKED (1 << 0) /* blocked while in RCU read-side. */
-#define RCU_READ_UNLOCK_BOOSTED (1 << 1) /* boosted while in RCU read-side. */
-#define RCU_READ_UNLOCK_NEED_QS (1 << 2) /* RCU core needs CPU response. */
+#define RCU_READ_UNLOCK_NEED_QS (1 << 1) /* RCU core needs CPU response. */
 
 static inline void rcu_copy_process(struct task_struct *p)
 {

kernel/rcutiny_plugin.h

@@ -318,7 +318,6 @@ static int rcu_boost(void)
 	t = container_of(tb, struct task_struct, rcu_node_entry);
 	rt_mutex_init_proxy_locked(&mtx, t);
 	t->rcu_boost_mutex = &mtx;
-	t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BOOSTED;
 	raw_local_irq_restore(flags);
 	rt_mutex_lock(&mtx);
 	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */
@@ -550,6 +549,9 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
 	int empty_exp;
 	unsigned long flags;
 	struct list_head *np;
+#ifdef CONFIG_RCU_BOOST
+	struct rt_mutex *rbmp = NULL;
+#endif /* #ifdef CONFIG_RCU_BOOST */
 	int special;
 
 	/*
@@ -615,10 +617,10 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
 	}
 #ifdef CONFIG_RCU_BOOST
 	/* Unboost self if was boosted. */
-	if (special & RCU_READ_UNLOCK_BOOSTED) {
-		t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BOOSTED;
-		rt_mutex_unlock(t->rcu_boost_mutex);
+	if (t->rcu_boost_mutex != NULL) {
+		rbmp = t->rcu_boost_mutex;
 		t->rcu_boost_mutex = NULL;
+		rt_mutex_unlock(rbmp);
 	}
 #endif /* #ifdef CONFIG_RCU_BOOST */
 	local_irq_restore(flags);