Re: [PATCH -mm 2/2] ptrace_untrace: fix the SIGNAL_STOP_STOPPED check

[Oleg Nesterov <oleg@redhat.com>]

On 02/19, Roland McGrath wrote:
>
> > This bug is ancient too. ptrace_untrace() must not resume the task
> > if the group stop in progress, we should set TASK_STOPPED instead.
>
> Right, the change looks correct.
>
> > Unfortunately, we still have problems here:
> >
> > 	- if the process/thread was traced, SIGNAL_STOP_STOPPED
> > 	  does not necessary means this thread group is stopped.
> >
> > 	- ptrace breaks the bookkeeping of ->group_stop_count.
>
> Neither of these should be true today as I understand the code.

Suppose we have

	int main(void)
	{
		kill(getpid(), SIGSTOP);

		for (;;)
			;
	}

If you strace this task nobody clears SIGNAL_STOP_STOPPED.

As for bookkeeping of ->group_stop_count, I think ptrace_stop() is wrong
in many ways. For example, it decrements ->group_stop_count but doesn't
set SIGNAL_STOP_STOPPED if it becomes zero.

Oleg.