kernel: fix is_single_threaded

kernel: 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.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: Roland McGrath <roland@redhat.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Morris <jmorris@namei.org>