rcu: Simplify unboosting checks

Commit 7765be (Fix RCU_BOOST race handling current->rcu_read_unlock_special)
introduced a new ->rcu_boosted field in the task structure.  This is
redundant because the existing ->rcu_boost_mutex will be non-NULL at
any time that ->rcu_boosted is nonzero.  Therefore, this commit removes
->rcu_boosted and tests ->rcu_boost_mutex instead.

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

2 files changed, 10 insertions, 13 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 6ee91e20353b..acca43560805 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1259,9 +1259,6 @@ struct task_struct {
 #ifdef CONFIG_PREEMPT_RCU
         int rcu_read_lock_nesting;
         char rcu_read_unlock_special;
-#if defined(CONFIG_RCU_BOOST) && defined(CONFIG_TREE_PREEMPT_RCU)
-        int rcu_boosted;
-#endif /* #if defined(CONFIG_RCU_BOOST) && defined(CONFIG_TREE_PREEMPT_RCU) */
         struct list_head rcu_node_entry;
 #endif /* #ifdef CONFIG_PREEMPT_RCU */
 #ifdef CONFIG_TREE_PREEMPT_RCU
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index ed70f6bf4c31..eeb38ee8ebba 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -306,6 +306,9 @@ 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 */
         struct rcu_node *rnp;
         int special;
 
@@ -351,6 +354,7 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
                 smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
                 np = rcu_next_node_entry(t, rnp);
                 list_del_init(&t->rcu_node_entry);
+                t->rcu_blocked_node = NULL;
                 trace_rcu_unlock_preempted_task("rcu_preempt",
                                                 rnp->gpnum, t->pid);
                 if (&t->rcu_node_entry == rnp->gp_tasks)
@@ -360,13 +364,12 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
 #ifdef CONFIG_RCU_BOOST
                 if (&t->rcu_node_entry == rnp->boost_tasks)
                         rnp->boost_tasks = np;
-                /* Snapshot and clear ->rcu_boosted with rcu_node lock held. */
-                if (t->rcu_boosted) {
-                        special |= RCU_READ_UNLOCK_BOOSTED;
-                        t->rcu_boosted = 0;
+                /* Snapshot/clear ->rcu_boost_mutex with rcu_node lock held. */
+                if (t->rcu_boost_mutex) {
+                        rbmp = t->rcu_boost_mutex;
+                        t->rcu_boost_mutex = NULL;
                 }
 #endif /* #ifdef CONFIG_RCU_BOOST */
-                t->rcu_blocked_node = NULL;
 
                 /*
                  * If this was the last task on the current list, and if
@@ -387,10 +390,8 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
 
 #ifdef CONFIG_RCU_BOOST
                 /* Unboost if we were boosted. */
-                if (special & RCU_READ_UNLOCK_BOOSTED) {
-                        rt_mutex_unlock(t->rcu_boost_mutex);
-                        t->rcu_boost_mutex = NULL;
-                }
+                if (rbmp)
+                        rt_mutex_unlock(rbmp);
 #endif /* #ifdef CONFIG_RCU_BOOST */
 
                 /*
@@ -1206,7 +1207,6 @@ static int rcu_boost(struct rcu_node *rnp)
         t = container_of(tb, struct task_struct, rcu_node_entry);
         rt_mutex_init_proxy_locked(&mtx, t);
         t->rcu_boost_mutex = &mtx;
-        t->rcu_boosted = 1;
         raw_spin_unlock_irqrestore(&rnp->lock, flags);
         rt_mutex_lock(&mtx);  /* Side effect: boosts task t's priority. */
         rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */