rcu: Make RCU able to tolerate undefined CONFIG_RCU_KTHREAD_PRIO

This commit updates the initialization of the kthread_prio boot parameter
so that RCU will build even when CONFIG_RCU_KTHREAD_PRIO is undefined.
The kthread_prio boot parameter is set to CONFIG_RCU_KTHREAD_PRIO if
that is defined, otherwise to 1 if CONFIG_RCU_BOOST is defined and
to zero otherwise.  This commit then makes CONFIG_RCU_KTHREAD_PRIO
depend on CONFIG_RCU_EXPERT, so that Kconfig users won't be asked about
CONFIG_RCU_KTHREAD_PRIO unless they want to be.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>

2 files changed, 5 insertions, 0 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 78176001f73b..af2c93c4a105 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -671,6 +671,7 @@ config RCU_KTHREAD_PRIO
         range 0 99 if !RCU_BOOST
         default 1 if RCU_BOOST
         default 0 if !RCU_BOOST
+        depends on RCU_EXPERT
         help
           This option specifies the SCHED_FIFO priority value that will be
           assigned to the rcuc/n and rcub/n threads and is also the value
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1edd11298224..0e9ce1272971 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -165,7 +165,11 @@ static void invoke_rcu_core(void);
 static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);
 
 /* rcuc/rcub kthread realtime priority */
+#ifdef CONFIG_RCU_KTHREAD_PRIO
 static int kthread_prio = CONFIG_RCU_KTHREAD_PRIO;
+#else /* #ifdef CONFIG_RCU_KTHREAD_PRIO */
+static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
+#endif /* #else #ifdef CONFIG_RCU_KTHREAD_PRIO */
 module_param(kthread_prio, int, 0644);
 
 /* Delay in jiffies for grace-period initialization delays, debug only. */