[PATCH 3/4] exit: avoid sig->count in de_thread/__exit_signal synchronization

[Oleg Nesterov <oleg@redhat.com>]

de_thread() and __exit_signal() use signal_struct->count/notify_count
for synchronization. We can simplify the code and use ->notify_count
only. Instead of comparing these two counters, we can change de_thread()
to set ->notify_count = nr_of_sub_threads, then change __exit_signal()
to dec-and-test this counter and notify group_exit_task.

Note that __exit_signal() checks "notify_count > 0" just for symmetry
with exit_notify(), we could just check it is != 0.

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

 fs/exec.c     |   11 +++++------
 kernel/exit.c |    2 +-
 2 files changed, 6 insertions(+), 7 deletions(-)

--- 34-rc1/fs/exec.c~3_DE_THREAD_AND_EXIT_SIGNAL	2010-03-17 19:10:27.000000000 +0100
+++ 34-rc1/fs/exec.c	2010-03-17 19:37:18.000000000 +0100
@@ -763,7 +763,6 @@ static int de_thread(struct task_struct 
 	struct signal_struct *sig = tsk->signal;
 	struct sighand_struct *oldsighand = tsk->sighand;
 	spinlock_t *lock = &oldsighand->siglock;
-	int count;
 
 	if (thread_group_empty(tsk))
 		goto no_thread_group;
@@ -780,13 +779,13 @@ static int de_thread(struct task_struct 
 		spin_unlock_irq(lock);
 		return -EAGAIN;
 	}
+
 	sig->group_exit_task = tsk;
-	zap_other_threads(tsk);
+	sig->notify_count = zap_other_threads(tsk);
+	if (!thread_group_leader(tsk))
+		sig->notify_count--;
 
-	/* Account for the thread group leader hanging around: */
-	count = thread_group_leader(tsk) ? 1 : 2;
-	sig->notify_count = count;
-	while (atomic_read(&sig->count) > count) {
+	while (sig->notify_count) {
 		__set_current_state(TASK_UNINTERRUPTIBLE);
 		spin_unlock_irq(lock);
 		schedule();
--- 34-rc1/kernel/exit.c~3_DE_THREAD_AND_EXIT_SIGNAL	2010-03-17 19:17:39.000000000 +0100
+++ 34-rc1/kernel/exit.c	2010-03-17 19:37:18.000000000 +0100
@@ -98,7 +98,7 @@ static void __exit_signal(struct task_st
 		 * If there is any task waiting for the group exit
 		 * then notify it:
 		 */
-		if (sig->group_exit_task && atomic_read(&sig->count) == sig->notify_count)
+		if (sig->notify_count > 0 && !--sig->notify_count)
 			wake_up_process(sig->group_exit_task);
 
 		if (tsk == sig->curr_target)