pid: Implement PIDTYPE_TGID

Everywhere except in the pid array we distinguish between a tasks pid and
a tasks tgid (thread group id).  Even in the enumeration we want that
distinction sometimes so we have added __PIDTYPE_TGID.  With leader_pid
we almost have an implementation of PIDTYPE_TGID in struct signal_struct.

Add PIDTYPE_TGID as a first class member of the pid_type enumeration and
into the pids array.  Then remove the __PIDTYPE_TGID special case and the
leader_pid in signal_struct.

The net size increase is just an extra pointer added to struct pid and
an extra pair of pointers of an hlist_node added to task_struct.

The effect on code maintenance is the removal of a number of special
cases today and the potential to remove many more special cases as
PIDTYPE_TGID gets used to it's fullest.  The long term potential
is allowing zombie thread group leaders to exit, which will remove
a lot more special cases in the code.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 2 insertions, 3 deletions

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 2dcded16eb1e..ee30a5ba475f 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -147,7 +147,6 @@ struct signal_struct {
 #endif
 
 	/* PID/PID hash table linkage. */
-	struct pid *leader_pid;
 	struct pid *pids[PIDTYPE_MAX];
 
 #ifdef CONFIG_NO_HZ_FULL
@@ -571,7 +570,7 @@ struct pid *task_pid_type(struct task_struct *task, enum pid_type type)
 
 static inline struct pid *task_tgid(struct task_struct *task)
 {
-	return task->signal->leader_pid;
+	return task->signal->pids[PIDTYPE_TGID];
 }
 
 /*
@@ -607,7 +606,7 @@ static inline bool thread_group_leader(struct task_struct *p)
  */
 static inline bool has_group_leader_pid(struct task_struct *p)
 {
-	return task_pid(p) == p->signal->leader_pid;
+	return task_pid(p) == task_tgid(p);
 }
 
 static inline