jump_label/x86: Work around asm build bug on older/backported GCCs

Boris reported that gcc version 4.4.4 20100503 (Red Hat
4.4.4-2) fails to build linux-next kernels that have
this fresh commit via the locking tree:

  11276d5306b8 ("locking/static_keys: Add a new static_key interface")

The problem appears to be that even though @key and @branch are
compile time constants, it doesn't see the following expression
as an immediate value:

   &((char *)key)[branch]

More recent GCCs don't appear to have this problem.

In particular, Red Hat backported the 'asm goto' feature into 4.4,
'normal' 4.4 compilers will not have this feature and thus not
run into this asm.

The workaround is to supply both values to the asm as immediates
and do the addition in asm.

Suggested-by: H. Peter Anvin <[email protected]>
Reported-by: Boris Ostrovsky <[email protected]>
Tested-by: Boris Ostrovsky <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Signed-off-by: Ingo Molnar <[email protected]>

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 28d7a857f9d125b292b3ac0b56ce76e0a6a39930..5daeca3d0f9e5b04d69a183dbe06e2c68fe9c0bb 100644 (file)
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -22,9 +22,9 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
                ".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
                ".pushsection __jump_table,  \"aw\" \n\t"
                _ASM_ALIGN "\n\t"
-               _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
+               _ASM_PTR "1b, %l[l_yes], %c0 + %c1 \n\t"
                ".popsection \n\t"
-               : :  "i" (&((char *)key)[branch]) : : l_yes);
+               : :  "i" (key), "i" (branch) : : l_yes);
 
        return false;
 l_yes:
@@ -38,9 +38,9 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
                "2:\n\t"
                ".pushsection __jump_table,  \"aw\" \n\t"
                _ASM_ALIGN "\n\t"
-               _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
+               _ASM_PTR "1b, %l[l_yes], %c0 + %c1 \n\t"
                ".popsection \n\t"
-               : :  "i" (&((char *)key)[branch]) : : l_yes);
+               : :  "i" (key), "i" (branch) : : l_yes);
 
        return false;
 l_yes: