linux內核函數kernel_thread
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設備驅動程序中,如果需要幾個併發執行的人物,可以啓動內核線程,啓動內和縣城的函數爲:
int kernel_thread (int ( * fn )( void * ), void * arg, unsigned long flags);
kernel_thread函數的作用是產生一個新的線程
內核線程實際上就是一個共享父進程地址空間的進程,它有自己的系統堆棧.
內核線程和進程都是通過do_fork()函數來產生的,系統中規定的最大進程數與
線程數由fork_init來決定:
[/arch/kernel/process.c/fork_init()]
void __init fork_init(unsigned long mempages)
{
#ifndef __HAVE_ARCH_TASK_STRUCT_ALLOCATOR
#ifndef ARCH_MIN_TASKALIGN
#define ARCH_MIN_TASKALIGN L1_CACHE_BYTES
#endif
/* 在slab高速緩存中建立task_struct結構專用的緩衝區隊列 */
task_struct_cachep =
kmem_cache_create("task_struct", sizeof(struct task_struct),
ARCH_MIN_TASKALIGN, SLAB_PANIC, NULL, NULL);
#endif
/*
把默認線程數設置到一個安全值,因爲內核中總的線程佔用的空間
可能要內存一半還要多.
參數mempages系統中總的物理內存結構大小,它等於mempages/PAGESIZE.
比如我機器的內存是512m,那麼在我的系統最多能同時產生線程數爲
(512*2^20/2^12) / 2^3 = 512*2^5 = 16384
*/
max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
/*
* 啓動系統的時候至少需要20個線程
*/
if(max_threads < 20)
max_threads = 20;
/*
* 每個進程最多產生max_threads/2,也就是線程總數的一半,在我的機器上爲8192.
*/
init_task.signal->rlim[RLIMIT_NPROC].rlim_cur = max_threads/2;
init_task.signal->rlim[RLIMIT_NPROC].rlim_max = max_threads/2;
}
kernel_thread原形在/arch/kernel/process.c中.
(*fn)(void *)爲要執行的函數的指針,arg爲函數參數,flags爲do_fork產生線程時的標誌.
int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
{
struct pt_regs regs;
memset(®s, 0, sizeof(regs));
regs.ebx = (unsigned long) fn; /* ebx指向函數地址 */
regs.edx = (unsigned long) arg; /* edx指向參數 */
regs.xds = __USER_DS;
regs.xes = __USER_DS;
regs.orig_eax = -1;
regs.eip = (unsigned long) kernel_thread_helper;
regs.xcs = __KERNEL_CS;
regs.eflags = X86_EFLAGS_IF | X86_EFLAGS_SF | X86_EFLAGS_PF | 0x2;
/* 利用do_fork來產生一個新的線程,共享父進程地址空間,並且不允許調試子進程 */
return do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, ®s, 0, NULL, NULL);
}
[/arch/i386/kernel/process.c/kernel_thread_helper]
extern void kernel_thread_helper(void); /* 定義成全局變量 */
__asm__(".section .text/n"
".align 4/n"
"kernel_thread_helper:/n/t"
"movl %edx,%eax/n/t"
"pushl %edx/n/t" /* edx指向參數,壓入堆棧 */
"call *%ebx/n/t" /* ebx指向函數地址,執行函數 */
"pushl %eax/n/t"
"call do_exit/n" /* 結束線程 */
".previous");
在kernel_thread中調用了do_fork,那麼do_fork是怎樣轉入kernel_thread_helper去執行的呢,繼續跟蹤下do_fork函數.
[kernel/fork.c/do_fork()]
long do_fork(unsigned long clone_flags,
unsigned
long stack_start,
struct pt_regs *regs,
unsigned long stack_size,
int __user *parent_tidptr,
int __user *child_tidptr)
{
....
....
p = copy_process(clone_flags, stack_start, regs, stack_size, parent_tidptr, child_tidptr, pid);
....
....
}
它調用copy_process函數來向子進程拷貝父進程的進程環境和全部寄存器副本.
[kernel/fork.c/do_fork()->copy_process]
static task_t *copy_process(unsigned long clone_flags,
unsigned long stack_start,
struct pt_regs *regs,
unsigned long stack_size,
int __user *parent_tidptr,
int __user *child_tidptr,
int pid)
{
...
...
retval = copy_thread(0, clone_flags, stack_start, stack_size, p, regs);
...
...
}
它又調用copy_thread來拷貝父進程的系統堆棧並做相應的調整.
[/arch/i386/kernel/process.c/copy_thread]:
int copy_thread(int nr, unsigned long clone_flags, unsigned long esp,
unsigned long unused,
struct task_struct * p, struct pt_regs * regs)
{
...
...
p->thread.eip = (unsigned long) ret_from_fork;
}
在這裏把ret_from_fork的地址賦值給p->thread.eip,p->thread.eip表示當進程下一次調度時的指令開始地址,
所以當線程創建後被調度時,是從ret_from_fork地址處開始的.
[/arch/i386/kernel/entry.s]
到這裏說明,新的線程已經產生了.
ENTRY(ret_from_fork)
pushl %eax
call schedule_tail
GET_THREAD_INFO(%ebp)
popl %eax
jmp syscall_exit
syscall_exit:
...
work_resched:
call schedule
...
當它從ret_from_fork退出時,會從堆棧中彈出原來保存的ip,而ip指向kernel_thread_helper,
至此kernel_thread_helper被調用,它就可以運行我們的指定的函數了
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