rcu: Eliminate a few RCU_BOOST #ifdefs in favor of IS_ENABLED()

This commit removes a few RCU_BOOST #ifdefs, replacing them with
IS_ENABLED()-protected return statements.  This relies on the
optimizer to remove any resulting dead code.  There are several other
RCU_BOOST #ifdefs, however these rely on some per-CPU variables that
are available only under RCU_BOOST.  These might be converted later,
if the simplification proves to outweigh the increase in memory footprint.
One hoped-for advantage is more easily locating compiler errors in
obscure combinations of Kconfig parameters.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: <linux-rt-users@vger.kernel.org>

2 files changed, 20 insertions, 17 deletions

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index a69d3dab2ec4..dd5ce4034875 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -170,7 +170,6 @@ struct rcu_node {
                                 /*  if there is no such task.  If there */
                                 /*  is no current expedited grace period, */
                                 /*  then there can cannot be any such task. */
-#ifdef CONFIG_RCU_BOOST
         struct list_head *boost_tasks;
                                 /* Pointer to first task that needs to be */
                                 /*  priority boosted, or NULL if no priority */
@@ -208,7 +207,6 @@ struct rcu_node {
         unsigned long n_balk_nos;
                                 /* Refused to boost: not sure why, though. */
                                 /*  This can happen due to race conditions. */
-#endif /* #ifdef CONFIG_RCU_BOOST */
 #ifdef CONFIG_RCU_NOCB_CPU
         wait_queue_head_t nocb_gp_wq[2];
                                 /* Place for rcu_nocb_kthread() to wait GP. */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 9a04764dd239..8f8142778684 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -43,7 +43,17 @@ DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
 DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_loops);
 DEFINE_PER_CPU(char, rcu_cpu_has_work);
 
-#endif /* #ifdef CONFIG_RCU_BOOST */
+#else /* #ifdef CONFIG_RCU_BOOST */
+
+/*
+ * Some architectures do not define rt_mutexes, but if !CONFIG_RCU_BOOST,
+ * all uses are in dead code.  Provide a definition to keep the compiler
+ * happy, but add WARN_ON_ONCE() to complain if used in the wrong place.
+ * This probably needs to be excluded from -rt builds.
+ */
+#define rt_mutex_owner(a) ({ WARN_ON_ONCE(1); NULL; })
+
+#endif /* #else #ifdef CONFIG_RCU_BOOST */
 
 #ifdef CONFIG_RCU_NOCB_CPU
 static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */
@@ -180,10 +190,9 @@ static void rcu_preempt_note_context_switch(void)
                 if ((rnp->qsmask & rdp->grpmask) && rnp->gp_tasks != NULL) {
                         list_add(&t->rcu_node_entry, rnp->gp_tasks->prev);
                         rnp->gp_tasks = &t->rcu_node_entry;
-#ifdef CONFIG_RCU_BOOST
-                        if (rnp->boost_tasks != NULL)
+                        if (IS_ENABLED(CONFIG_RCU_BOOST) &&
+                            rnp->boost_tasks != NULL)
                                 rnp->boost_tasks = rnp->gp_tasks;
-#endif /* #ifdef CONFIG_RCU_BOOST */
                 } else {
                         list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
                         if (rnp->qsmask & rdp->grpmask)
@@ -263,9 +272,7 @@ void rcu_read_unlock_special(struct task_struct *t)
         bool empty_exp_now;
         unsigned long flags;
         struct list_head *np;
-#ifdef CONFIG_RCU_BOOST
         bool drop_boost_mutex = false;
-#endif /* #ifdef CONFIG_RCU_BOOST */
         struct rcu_node *rnp;
         union rcu_special special;
 
@@ -331,12 +338,12 @@ void rcu_read_unlock_special(struct task_struct *t)
                         rnp->gp_tasks = np;
                 if (&t->rcu_node_entry == rnp->exp_tasks)
                         rnp->exp_tasks = np;
-#ifdef CONFIG_RCU_BOOST
-                if (&t->rcu_node_entry == rnp->boost_tasks)
-                        rnp->boost_tasks = np;
-                /* Snapshot ->boost_mtx ownership with rcu_node lock held. */
-                drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx) == t;
-#endif /* #ifdef CONFIG_RCU_BOOST */
+                if (IS_ENABLED(CONFIG_RCU_BOOST)) {
+                        if (&t->rcu_node_entry == rnp->boost_tasks)
+                                rnp->boost_tasks = np;
+                        /* Snapshot ->boost_mtx ownership w/rnp->lock held. */
+                        drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx) == t;
+                }
 
                 /*
                  * If this was the last task on the current list, and if
@@ -358,11 +365,9 @@ void rcu_read_unlock_special(struct task_struct *t)
                         raw_spin_unlock_irqrestore(&rnp->lock, flags);
                 }
 
-#ifdef CONFIG_RCU_BOOST
                 /* Unboost if we were boosted. */
-                if (drop_boost_mutex)
+                if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
                         rt_mutex_unlock(&rnp->boost_mtx);
-#endif /* #ifdef CONFIG_RCU_BOOST */
 
                 /*
                  * If this was the last task on the expedited lists,