rcu: add rcu_migrate_enable and rcu_migrate_disable

migrate_disable() is called to disable migration in the kernel, and it is
used togather with rcu_read_lock() oftenly.

However, with PREEMPT_RCU disabled, it's unnecessary, as rcu_read_lock()
will disable preemption, which will also disable migration.

Introduce rcu_migrate_enable() and rcu_migrate_disable(), which will do
the migration enable and disable only when the rcu_read_lock() can't do
it.

Signed-off-by: Menglong Dong <[email protected]>

Author: image-dragon

include/linux/rcupdate.h

@@ -72,6 +72,16 @@ static inline bool same_state_synchronize_rcu(unsigned long oldstate1, unsigned
 void __rcu_read_lock(void);
 void __rcu_read_unlock(void);
 
+static inline void rcu_migrate_enable(void)
+{
+	migrate_enable();
+}
+
+static inline void rcu_migrate_disable(void)
+{
+	migrate_disable();
+}
+
 /*
  * Defined as a macro as it is a very low level header included from
  * areas that don't even know about current.  This gives the rcu_read_lock()
@@ -105,6 +115,14 @@ static inline int rcu_preempt_depth(void)
 	return 0;
 }
 
+static inline void rcu_migrate_enable(void)
+{
+}
+
+static inline void rcu_migrate_disable(void)
+{
+}
+
 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 
 #ifdef CONFIG_RCU_LAZY