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, 1 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index d2952162399b..cc5be0d01ce6 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1946,6 +1946,7 @@ static __latent_entropy struct task_struct *copy_process(
 
                 init_task_pid(p, PIDTYPE_PID, pid);
                 if (thread_group_leader(p)) {
+                        init_task_pid(p, PIDTYPE_TGID, pid);
                         init_task_pid(p, PIDTYPE_PGID, task_pgrp(current));
                         init_task_pid(p, PIDTYPE_SID, task_session(current));
 
@@ -1954,7 +1955,6 @@ static __latent_entropy struct task_struct *copy_process(
                                 p->signal->flags |= SIGNAL_UNKILLABLE;
                         }
 
-                        p->signal->leader_pid = pid;
                         p->signal->tty = tty_kref_get(current->signal->tty);
                         /*
                          * Inherit has_child_subreaper flag under the same
@@ -1965,6 +1965,7 @@ static __latent_entropy struct task_struct *copy_process(
                                                          p->real_parent->signal->is_child_subreaper;
                         list_add_tail(&p->sibling, &p->real_parent->children);
                         list_add_tail_rcu(&p->tasks, &init_task.tasks);
+                        attach_pid(p, PIDTYPE_TGID);
                         attach_pid(p, PIDTYPE_PGID);
                         attach_pid(p, PIDTYPE_SID);
                         __this_cpu_inc(process_counts);