Unnecessary Relocation Hiding?

From: Dong Feng
Date: Wed Aug 23 2006 - 08:26:34 EST

Hi, all,

I have a question. Why shall we need a RELOC_HIDE() macro in the
definition of per_cpu()? Maybe the question is actually why we need
macro RELOC_HIDE() at all. I changed the following line in
include/asm-generic/percpu.h, from

#define per_cpu(var, cpu) (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]))


#define per_cpu(var, cpu) (*((unsigned long)(&per_cpu__##var) +

I recompiled the code and it works well on my Intel Dual-core laptop.
It essentially the same as to change the definition of RELOC_HIDE(),

#define RELOC_HIDE(ptr, off) \
({ unsigned long __ptr; \
__asm__ ("" : "=r"(__ptr) : "0"(ptr)); \
(typeof(ptr)) (__ptr + (off)); })


#define RELOC_HIDE(ptr, off) \
({ unsigned long __ptr; \
__ptr = (unsigned long)ptr; \
(typeof(ptr)) (__ptr + (off)); })

Why shouldn't we have a pure C solution in this part?

Best Regards.
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