[tip:core/urgent] objtool: Prevent GCC from merging annotate_unreachable(), take 2

[tip-bot for Josh Poimboeuf]

Commit-ID:  ec1e1b6109171d1890a437481c35b2b56d2327b8
Gitweb:     https://git.kernel.org/tip/ec1e1b6109171d1890a437481c35b2b56d2327b8
Author:     Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate: Fri, 3 Nov 2017 17:19:41 -0500
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sat, 4 Nov 2017 15:03:39 +0100

objtool: Prevent GCC from merging annotate_unreachable(), take 2

This fixes the following warning with GCC 4.6:

  mm/migrate.o: warning: objtool: migrate_misplaced_transhuge_page()+0x71: unreachable instruction

The problem is that the compiler merged identical annotate_unreachable()
inline asm blocks, resulting in a missing 'unreachable' annotation.

This problem happened before, and was partially fixed with:

  3d1e236022cc ("objtool: Prevent GCC from merging annotate_unreachable()")

That commit tried to ensure that each instance of the
annotate_unreachable() inline asm statement has a unique label.  It used
the __LINE__ macro to generate the label number.  However, even the line
number isn't necessarily unique when used in an inline function with
multiple callers (in this case, __alloc_pages_node()'s use of
VM_BUG_ON).

Reported-by: kbuild test robot <fengguang.wu@xxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: kbuild-all@xxxxxx
Cc: tipbuild@xxxxxxxxx
Fixes: 3d1e236022cc ("objtool: Prevent GCC from merging annotate_unreachable()")
Link: 20171103221941.cajpwszir7ujxyc4@treble">http://lkml.kernel.org/r/20171103221941.cajpwszir7ujxyc4@treble
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 include/linux/compiler.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index fd8697a..2027104 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -191,13 +191,13 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	asm("%c0:\n\t"							\
 	    ".pushsection .discard.reachable\n\t"			\
 	    ".long %c0b - .\n\t"					\
-	    ".popsection\n\t" : : "i" (__LINE__));			\
+	    ".popsection\n\t" : : "i" (__COUNTER__));			\
 })
 #define annotate_unreachable() ({					\
 	asm("%c0:\n\t"							\
 	    ".pushsection .discard.unreachable\n\t"			\
 	    ".long %c0b - .\n\t"					\
-	    ".popsection\n\t" : : "i" (__LINE__));			\
+	    ".popsection\n\t" : : "i" (__COUNTER__));			\
 })
 #define ASM_UNREACHABLE							\
 	"999:\n\t"							\