March
21st,
2015
Linux kernel程式裡利用current來指向當前的process的data structure，在
Linux Device Drivers chapter 2面提到，
The current pointer refers to the user process currently executing.
During the execution of a system call, such as open or read, the current
process is the one that invoked the call. Kernel code can use process-specific
information by using current, if it needs to do so. [...]
實際上它在整個kernel裡面並非唯一的global variable，而是每個thread有自己的storage。
他是怎麼實現的呢？在include/asm-generic/current.h裡，
#define get_current() (current_thread_info()->task)
#define current get_current()
所以最終是透過每個thread的current_thread_info()來找到task pointer。記得以前看過mips是
透過特殊的register gp 來實現，所以在kernel中gp也是有特殊含義的。
static inline struct thread_info *current_thread_info(void)
{
register struct thread_info *__current_thread_info __asm__("$28");
return __current_thread_info;
}
來看一下arm的case，
static inline struct thread_info *current_thread_info(void)
{
register unsigned long sp asm ("sp");
return (struct thread_info *)(sp & ~(THREAD_SIZE - 1));
}
看起來是直接透過原本的計算取得task information pointer。另外arm64與arm的實現相同。 關於task structure的描述可見Process Descriptor and the Task Structure