What is the type of elements of thread_group list in sched.h?

In sched.h, task_struct has following 2 fields: thread_group & thread_node. They keep the first element of their list but I could not find which type of variables they contain.

1511 struct task_struct {
....
1657         /* PID/PID hash table linkage. */
1658         struct pid_link pids[PIDTYPE_MAX];
1659         struct list_head thread_group;
1660         struct list_head thread_node;
.... 
     }

Answer

thread_group and thread_node are both intrusive linked-lists of all the threads in a thread group – they are use to link together task_structs, they don’t “contain” anything.

The difference between the two is that thread_group has its head in the task_struct of the thread group leader, whereas thread_node has its head in the signal_struct shared by the thread group. In the medium-term thread_group is going away.