Man page 9 : threadgroup_lock

threadgroup_lock - lock threadgroup

void threadgroup_lock(struct task_struct *  tsk );

tsk
    member task of the threadgroup to lock

Lock the threadgroup tsk belongs to. No new task is allowed to enter and member tasks arenAqt allowed to exit (as indicated by PF_EXITING) or change ->group_leader/pid. This is useful for cases where the threadgroup needs to stay stable across blockable operations.

fork and exit paths explicitly call threadgroup_change_{begin|end}() for synchronization. While held, no new task will be added to threadgroup and no existing live task will have its PF_EXITING set.

de_thread does threadgroup_change_{begin|end}() when a non-leader sub-thread becomes a new leader.