[PATCH] pid: implement access helpers for a tacks various process groups

In the last round of cleaning up the pid hash table a more general struct pid
was introduced, that can be referenced counted.

With the more general struct pid most if not all places where we store a pid_t
we can now store a struct pid * and remove the need for a hash table lookup,
and avoid any possible problems with pid roll over.

Looking forward to the pid namespaces struct pid * gives us an absolute form a
pid so we can compare and use them without caring which pid namespace we are
in.

This patchset introduces the infrastructure needed to use struct pid instead
of pid_t, and then it goes on to convert two different kernel users that
currently store a pid_t value.

There are a lot more places to go but this is enough to get the basic idea.

Before we can merge a pid namespace patch all of the kernel pid_t users need
to be examined.  Those that deal with user space processes need to be
converted to using a struct pid *.  Those that deal with kernel processes need
to converted to using the kthread api.  A rare few that only use their current
processes pid values get to be left alone.

This patch:

task_session returns the struct pid of a tasks session.
task_pgrp    returns the struct pid of a tasks process group.
task_tgid    returns the struct pid of a tasks thread group.
task_pid     returns the struct pid of a tasks process id.

These can be used to avoid unnecessary hash table lookups, and to implement
safe pid comparisions in the face of a pid namespace.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/sched.h

@@ -1020,6 +1020,26 @@ static inline pid_t process_group(struct task_struct *tsk)
 	return tsk->signal->pgrp;
 }
 
+static inline struct pid *task_pid(struct task_struct *task)
+{
+	return task->pids[PIDTYPE_PID].pid;
+}
+
+static inline struct pid *task_tgid(struct task_struct *task)
+{
+	return task->group_leader->pids[PIDTYPE_PID].pid;
+}
+
+static inline struct pid *task_pgrp(struct task_struct *task)
+{
+	return task->group_leader->pids[PIDTYPE_PGID].pid;
+}
+
+static inline struct pid *task_session(struct task_struct *task)
+{
+	return task->group_leader->pids[PIDTYPE_SID].pid;
+}
+
 /**
  * pid_alive - check that a task structure is not stale
  * @p: Task structure to be checked.