uclinux-2008R1.5-RC3(bf561)到VDSP5的移植(47)：per_cpu的奇怪问题
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本文适用于
ADI bf561 DSP
uclinux-2008r1.5-rc3(smp patch)
Visual DSP++ 5.0(update 5)
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有一个错误：
"..\..\linux-2.6.x\include\linux/rcupdate.h", line 124: cc0144: error: a
value of type "volatile __typeof__(struct rcu_data) *" cannot be
used to initialize an entity of type "struct rcu_data *"
struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
^
错误指向的代码为：
DECLARE_PER_CPU(struct rcu_data, rcu_data);
/*
* Increment the quiescent state counter.
* The counter is a bit degenerated: We do not need to know
* how many quiescent states passed, just if there was at least
* one since the start of the grace period. Thus just a flag.
*/
static inline void rcu_qsctr_inc(int cpu)
{
struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
rdp->passed_quiesc = 1;
}
先看一下DECLARE_PER_CPU的定义：
#define DECLARE_PER_CPU(type, name) extern __typeof__(type) per_cpu__##name
代入第一行就是：
extern struct rcu_data per_cpu__ rcu_data;
再看看per_cpu的定义：
/* var is in discarded region: offset to particular copy we want */
#define per_cpu(var, cpu) (*({ \
extern int simple_identifier_##var(void); \
RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]); }))
/* This macro obfuscates arithmetic on a variable address so that gcc
shouldn't recognize the original var, and make assumptions about it */
/*
* Versions of the ppc64 compiler before 4.1 had a bug where use of
* RELOC_HIDE could trash r30. The bug can be worked around by changing
* the inline assembly constraint from =g to =r, in this particular
* case either is valid.
*/
#define RELOC_HIDE(ptr, off) \
({ unsigned long __ptr; \
__asm__ ("" : "=r"(__ptr) : "0"(ptr)); \
(__typeof__(ptr)) (__ptr + (off)); })
从头到尾都没有出现volatile，奇怪？
再看看rcu_data实际定义的地方：
DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };
使用了DEFINE_PER_CPU宏，看看它的定义：
#define DEFINE_PER_CPU(type, name) \
__attribute__((__section__(".data.percpu"))) __typeof__(type) per_cpu__##name
只不过是把这个结构体放在.data.percpu这个段里，也没有特别的地方。
百思不得其解，先放过它，直接强制转换：
/*
* Increment the quiescent state counter.
* The counter is a bit degenerated: We do not need to know
* how many quiescent states passed, just if there was at least
* one since the start of the grace period. Thus just a flag.
*/
static inline void rcu_qsctr_inc(int cpu)
{
struct rcu_data *rdp = (struct rcu_data *)&per_cpu(rcu_data, cpu);
rdp->passed_quiesc = 1;
}
1 参考资料
uclinux-2008R1.5-RC3(bf561)到VDSP5的移植(41)：bfin_write_EVT15(2009-1-17)
uclinux-2008R1.5-RC3(bf561)到VDSP5的移植(42)：.macro(2009-1-17)
uclinux-2008R1.5-RC3(bf561)到VDSP5的移植(43)：exception_stacks(2009-1-18)
uclinux-2008R1.5-RC3(bf561)到VDSP5的移植(44)：Out of memory(2009-01-19)
uclinux-2008R1.5-RC3(bf561)到VDSP5的移植(45)：未命名union的后遗症(2009-01-21)
uclinux-2008R1.5-RC3(bf561)到VDSP5的移植(46)：fls_long的问题(2009-1-21)