[PATCH for 2.6.30] ptrace: revert "ptrace_detach: the wrong wakeup breaks the ERESTARTxxx logic"

[Oleg Nesterov <oleg@redhat.com>]

Commit 95a3540da9c81a5987be810e1d9a83640a366bd5 removed the "extra"
wake_up_process() from ptrace_detach(), but as Jan pointed out this breaks
the compatibility.

I believe the changelog is right and this wake_up() is wrong in many ways.
But GDB assumes that ptrace(PTRACE_DETACH, child, 0, 0) always wakes up the
tracee. Despite the fact this breaks SIGNAL_STOP_STOPPED/group_stop_count logic,
and despite the fact this wake_up_process() can break another assumption:
PTRACE_DETACH with SIGSTOP should leave the tracee in TASK_STOPPED case.
Because the untraced child can dequeue SIGSTOP and call do_signal_stop()
before ptrace_detach() calls wake_up_process().

Revert this change for now. We need some fixes even if we we want to keep
the current behaviour, but these fixes are not for 2.6.30.

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

--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -304,6 +304,8 @@ int ptrace_detach(struct task_struct *ch
 	if (child->ptrace) {
 		child->exit_code = data;
 		dead = __ptrace_detach(current, child);
+		if (!child->exit_state)
+			wake_up_process(child);
 	}
 	write_unlock_irq(&tasklist_lock);