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>

1 files changed, 6 insertions, 4 deletions

diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h
index 387c2759e1b0..22ecea0dfb62 100644
--- a/kernel/rcutiny_plugin.h
+++ b/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);