[PATCH 2/7] rcu: limit PREEMPT_RCU configurations

[Ankur Arora]

PREEMPT_LAZY can be enabled stand-alone or alongside PREEMPT_DYNAMIC
which allows for dynamic switching of preemption models.

The choice of preemptible RCU or not, however, is fixed at compile
time. Given the trade-offs made to have a preemptible RCU, some
configurations which have limited preemption might prefer the
stronger forward-progress guarantees of PREEMPT_RCU=n.

Accordingly, explicitly limit PREEMPT_RCU=y to PREEMPT_DYNAMIC,
PREEMPT, PREEMPT_RT.

This means that (PREEMPT_LAZY=y, PREEMPT_DYNAMIC=n), which selects
PREEMPTION will run with PREEMPT_RCU=n. The combination (PREEMPT_LAZY=y,
PREEMPT_DYNAMIC=y), will run with PREEMPT_RCU=y.

Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 kernel/rcu/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 3e079de0f5b4..1bfe7016724f 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -18,7 +18,7 @@ config TREE_RCU
 
 config PREEMPT_RCU
 	bool
-	default y if PREEMPTION
+	default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
 	select TREE_RCU
 	help
 	  This option selects the RCU implementation that is
-- 
2.43.5