[PATCH RFC] rcu: Don't disable preemption for Tiny and Tree RCU readers

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

rcu: Don't disable preemption for Tiny and Tree RCU readers

Because preempt_disable() maps to barrier() for non-debug builds,
it forces the compiler to spill and reload registers.  Because Tree
RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these
barrier() instances generate needless extra code for each instance of
rcu_read_lock() and rcu_read_unlock().  This extra code slows down Tree
RCU and bloats Tiny RCU (though probably not significantly).

This commit therefore removes the preempt_disable() and preempt_enable()
from the non-preemptible implementations of __rcu_read_lock() and
__rcu_read_unlock(), respectively.

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

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 78af46c98e55..927c5e60c1dd 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -298,12 +298,14 @@ void synchronize_rcu(void);
 
 static inline void __rcu_read_lock(void)
 {
-	preempt_disable();
+	if (!IS_ENABLED(CONFIG_TINY_RCU))
+		preempt_disable();
 }
 
 static inline void __rcu_read_unlock(void)
 {
-	preempt_enable();
+	if (!IS_ENABLED(CONFIG_TINY_RCU))
+		preempt_enable();
 }
 
 static inline void synchronize_rcu(void)