Re: [PATCH] rework/fix is_single_threaded()

> - Fix the comment, is_single_threaded(p) actually means that nobody shares
>   ->mm with p.
>
>   I think this helper should be renamed, and it should not have arguments.
>   With or without this patch it must not be used unless p == current,
>   otherwise we can't safely use p->signal or p->mm.
>
> - "if (atomic_read(&p->signal->count) != 1)" is not right when we have a
>   zombie group leader, use signal->live instead.
>
> - Add PF_KTHREAD check to skip kernel threads which may borrow p->mm,
>   otherwise we can return the wrong "false".
>
> - Use for_each_process() instead of do_each_thread(), all threads must use
>   the same ->mm.
>
> - Use down_write(mm->mmap_sem) + rcu_read_lock() instead of tasklist_lock
>   to iterate over the process list. If there is another CLONE_VM process
>   it can't pass exit_mm() which takes the same mm->mmap_sem. We can miss
>   a freshly forked CLONE_VM task, but this doesn't matter because we must
>   see its parent and return false.