Re: [PATCH v2 -mm 2/2] current_is_single_threaded: don't use ->mmap_sem

> current_is_single_threaded() can safely miss a freshly forked CLONE_VM
> task, but in this case it must not miss its parent. That is why we take
> mm->mmap_sem for writing to make sure a thread/task with the same ->mm
> can't pass exit_mm() and disappear.
>
> However we can avoid ->mmap_sem and rely on rcu/barriers:
>
> - if we do not see the exiting parent on thread/process list
>  we see the result of list_del_rcu(), in this case we must
>  also see the result of list_add_rcu() which does wmb().
>
> - if we do see the parent but its ->mm == NULL, we need rmb()
>  to make sure we can't miss the child.
>
> Signed-off-by: Oleg Nesterov <oleg@...>