[PATCH 04/17] init: Add __noreorder and mark initcalls __noreorder

[Andi Kleen]

From: Andi Kleen <ak@xxxxxxxxxxxxxxx>

gcc 5 has a new no_reorder attribute that prevents top level
reordering only for that symbol.

Kernels don't like any reordering of initcalls between files, as several
initcalls depend on each other.

Add a __noreorder wrapper for the no_reorder attribute and use
it for initcalls.

LTO previously needed to use -fno-toplevel-reordering to prevent boot
failures, but with noreorder this is not needed.

I believe the patch is useful even without LTO because it clearly
documents this dependency in the source.

Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
 include/linux/compiler_attributes.h | 11 +++++++++++
 include/linux/init.h                |  2 +-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 6b318efd8a74..1c44ef9034bb 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -39,6 +39,7 @@
 # define __GCC4_has_attribute___externally_visible__  1
 # define __GCC4_has_attribute___noclone__             1
 # define __GCC4_has_attribute___nonstring__           0
+# define __GCC4_has_attribute___no_reorder__          0
 # define __GCC4_has_attribute___no_sanitize_address__ (__GNUC_MINOR__ >= 8)
 #endif
 
@@ -253,4 +254,14 @@
  */
 #define __weak                          __attribute__((__weak__))
 
+/*
+ * https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
+ */
+
+#if __has_attribute(__no_reorder__)
+#define __noreorder			__attribute__((no_reorder))
+#else
+#define __noreorder
+#endif
+
 #endif /* __LINUX_COMPILER_ATTRIBUTES_H */
diff --git a/include/linux/init.h b/include/linux/init.h
index 5255069f5a9f..49d032916604 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -190,7 +190,7 @@ extern bool initcall_debug;
 	    ".previous					\n");
 #else
 #define ___define_initcall(fn, id, __sec) \
-	static initcall_t __initcall_##fn##id __used \
+	static initcall_t __initcall_##fn##id __used __noreorder \
 		__attribute__((__section__(#__sec ".init"))) = fn;
 #endif
 
-- 
2.20.1