[PATCH] do __unhash_process() under ->siglock

This patch moves __unhash_process() call from realease_task() to
__exit_signal(), so __detach_pid() is called with ->siglock held.

This means we don't need tasklist_lock to iterate over thread group anymore:

	copy_process() was already changed to do attach_pid()
	under ->siglock.

	Eric's "pidhash-kill-switch_exec_pids.patch" from -mm
	changed de_thread() so it doesn't touch PIDTYPE_TGID.

NOTE: de_thread() still needs some attention.  It still changes task->pid
lockless.  Taking ->sighand.siglock here allows to do more tasklist_lock
removals.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/exit.c

@@ -112,6 +112,8 @@ static void __exit_signal(struct task_struct *tsk)
 		sig = NULL; /* Marker for below. */
 	}
 
+	__unhash_process(tsk);
+
 	tsk->signal = NULL;
 	cleanup_sighand(tsk);
 	spin_unlock(&sighand->siglock);
@@ -140,8 +142,6 @@ void release_task(struct task_struct * p)
 	BUG_ON(!list_empty(&p->ptrace_list) || !list_empty(&p->ptrace_children));
 	__exit_signal(p);
 
-	__unhash_process(p);
-
 	/*
 	 * If we are the last non-leader member of the thread
 	 * group, and the leader is zombie, then notify the