[PATCH 8/8] redefine thread_group_leader() as exit_signal >= 0

[Oleg Nesterov <oleg@redhat.com>]

Change de_thread() to set old_leader->exit_signal = -1. This is
good for the consistency, it is no longer the leader and all
sub-threads have exit_signal = -1 set by copy_process(CLONE_THREAD).

And this allows us to micro-optimize thread_group_leader(), it can
simply check exit_signal >= 0. This also makes sense because we
should move ->group_leader from task_struct to signal_struct.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 include/linux/sched.h |    6 ++++--
 fs/exec.c             |    1 +
 2 files changed, 5 insertions(+), 2 deletions(-)

--- ptrace/include/linux/sched.h~8_thread_group_leader	2011-06-22 22:47:13.000000000 +0200
+++ ptrace/include/linux/sched.h	2011-06-22 22:47:14.000000000 +0200
@@ -2285,8 +2285,10 @@ static inline int get_nr_threads(struct 
 	return tsk->signal->nr_threads;
 }
 
-/* de_thread depends on thread_group_leader not being a pid based check */
-#define thread_group_leader(p)	(p == p->group_leader)
+static inline bool thread_group_leader(struct task_struct *p)
+{
+	return p->exit_signal >= 0;
+}
 
 /* Do to the insanities of de_thread it is possible for a process
  * to have the pid of the thread group leader without actually being
--- ptrace/fs/exec.c~8_thread_group_leader	2011-06-22 22:47:00.000000000 +0200
+++ ptrace/fs/exec.c	2011-06-22 22:47:14.000000000 +0200
@@ -963,6 +963,7 @@ static int de_thread(struct task_struct 
 		leader->group_leader = tsk;
 
 		tsk->exit_signal = SIGCHLD;
+		leader->exit_signal = -1;
 
 		BUG_ON(leader->exit_state != EXIT_ZOMBIE);
 		leader->exit_state = EXIT_DEAD;