pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
{
struct pt_regs regs;
memset(®s, 0, sizeof(regs));
regs.ARM_r1 = (unsigned long)arg;
regs.ARM_r2 = (unsigned long)fn;
regs.ARM_r3 = (unsigned long)kernel_thread_exit;
regs.ARM_pc = (unsigned long)kernel_thread_helper;
regs.ARM_cpsr = SVC_MODE | PSR_ENDSTATE | PSR_ISETSTATE;
return do_fork(flags|CLONE_VM|CLONE_UNTRACED, 0, ®s, 0, NULL, NULL);
}
kernel_thread最终是调用了do_fork实现。
struct task_struct *kthread_create(int (*threadfn)(void *data),
void *data,
const char namefmt[],
...)
{
struct kthread_create_info create;
create.threadfn = threadfn;
create.data = data;
init_completion(&create.done);
spin_lock(&kthread_create_lock);
list_add_tail(&create.list, &kthread_create_list);
spin_unlock(&kthread_create_lock);
wake_up_process(kthreadd_task);
wait_for_completion(&create.done);
if (!IS_ERR(create.result)) {
struct sched_param param = { .sched_priority = 0 };
va_list args;
va_start(args, namefmt);
vsnprintf(create.result->comm, sizeof(create.result->comm),
namefmt, args);
va_end(args);
/*
* root may have changed our (kthreadd's) priority or CPU mask.
* The kernel thread should not inherit these properties.
*/
sched_setscheduler_nocheck(create.result, SCHED_NORMAL, ¶m);
set_cpus_allowed_ptr(create.result, cpu_all_mask);
}
return create.result;
}
而kthread_create,现在还看不出怎么实现的。只能一行行读了。
1.创建并初始化一个struct kthread_create_info create；把要实现的函数赋给
create->threadfn = threadfn;
2.初始化完成量：create->done.
3.将create挂到一个叫kthread_create_list的全局链表上
4.唤醒一个叫kthreadd_task的task。
5.等待wait_for_completion(&create.done)。
------------------------------------------------------------------------
kthreadd_task是在(start_kernel->)rest_init中被赋值，其实就是被kernel_thread创建出来的kthreadd的task描述符。也就说要看kthreadd。
1.tsk = current;
2.进入for死循环，将当前task设置为TASK_INTERRUPTIBLE，判断
kthread_create_list是否为空，为空一直睡眠，否则，进入while循环，while的退出条件为kthread_create_list为空
3.在while循环中做的事情：取出一个create，调用creaet_kthread
4.create_kthread调用kernel_thread其一个kthread线程，也就是说，一个create就有一个kthread线程。
5.kthread做的事情：
a.取出create
b.定义并初始化一个struct kthread self。
struct kthread {
int should_stop;
struct completion exited;
};
c.current->vfork_done = &self.exited;
d.__set_current_state(TASK_UNINTERRUPTIBLE);
create->result = current;将当前task状态设置为TASK_UNINTERRUPTIBLE，并将task描述符赋给create->result。后面看到这就是kthread_create返回的task描述符。
e.唤醒等待creaet->done完成量的task,并通过schedule()让出cpu。
------------------------------------------------------------------------
6.返回create->result.
