uclinux-2008R1.5-RC3(bf561)到VDSP5的移植(47)：per_cpu的奇怪问题

快乐虾

http://blog.youkuaiyun.com/lights_joy/

lights@hb165.com

本文适用于

ADI bf561 DSP

优视BF561EVB开发板

uclinux-2008r1.5-rc3(smp patch)

Visual DSP++ 5.0(update 5)

欢迎转载，但请保留作者信息

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)