宏 for_each_process

宏 for_each_process 的功能是：遍历进程（task_struct）

这个宏从指向 &init_task 开始，把指针移到下一个任务（在 linux 底层中，进程被称为任务 task），然后继续，直到又回到 &init_task

（得益于 task_struct 中的 tasks 成员，tasks 是一个 list_head 类型，是一个双向链表，具有循环性）。

// include/linux/sched/signal.h
#define next_task(p) \
	list_entry_rcu((p)->tasks.next, struct task_struct, tasks)

#define for_each_process(p) \
	for (p = &init_task ; (p = next_task(p)) != &init_task ; )

// include/linux/rculist.h
/**
 * list_entry_rcu - get the struct for this entry
 * @ptr:        the &struct list_head pointer.
 * @type:       the type of the struct this is embedded in.
 * @member:     the name of the list_head within the struct.
 *
 * This primitive may safely run concurrently with the _rcu list-mutation
 * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
 */
#define list_entry_rcu(ptr, type, member) \
	container_of(READ_ONCE(ptr), type, member)