Add "pure_initcall" for static variable initialization

This is a quick hack to overcome the fact that SRCU currently does not
allow static initializers, and we need to sometimes initialize those
things before any other initializers (even "core" ones) can do so.

Currently we don't allow this at all for modules, and the only user that
needs is right now is cpufreq. As reported by Thomas Gleixner:

   "Commit b4dfdbb ("[PATCH] cpufreq:
    make the transition_notifier chain use SRCU breaks cpu frequency
    notification users, which register the callback > on core_init
    level."

Cc: Thomas Gleixner <tglx@timesys.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Andrew Morton <akpm@osdl.org>,
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/cpufreq/cpufreq.c

@@ -59,7 +59,7 @@ static int __init init_cpufreq_transition_notifier_list(void)
 	srcu_init_notifier_head(&cpufreq_transition_notifier_list);
 	return 0;
 }
-core_initcall(init_cpufreq_transition_notifier_list);
+pure_initcall(init_cpufreq_transition_notifier_list);
 
 static LIST_HEAD(cpufreq_governor_list);
 static DEFINE_MUTEX (cpufreq_governor_mutex);

include/asm-generic/vmlinux.lds.h

@@ -215,6 +215,8 @@
 		.notes : { *(.note.*) } :note
 
 #define INITCALLS							\
+  	*(.initcall0.init)						\
+  	*(.initcall0s.init)						\
   	*(.initcall1.init)						\
   	*(.initcall1s.init)						\
   	*(.initcall2.init)						\

include/linux/init.h

@@ -93,6 +93,14 @@ extern void setup_arch(char **);
 	static initcall_t __initcall_##fn##id __attribute_used__ \
 	__attribute__((__section__(".initcall" level ".init"))) = fn
 
+/*
+ * A "pure" initcall has no dependencies on anything else, and purely
+ * initializes variables that couldn't be statically initialized.
+ *
+ * This only exists for built-in code, not for modules.
+ */
+#define pure_initcall(fn)		__define_initcall("0",fn,1)
+
 #define core_initcall(fn)		__define_initcall("1",fn,1)
 #define core_initcall_sync(fn)		__define_initcall("1s",fn,1s)
 #define postcore_initcall(fn)		__define_initcall("2",fn,2)