linux-kernel - [Question] Is there a race window between __anon_vma_prepare() with page_lock_anon_vma

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Date:   Fri, 14 May 2021 17:16:12 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Linux-MM <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>
Subject: [Question] Is there a race window between __anon_vma_prepare() with
 page_lock_anon_vma_read() ?

Hi all,
I am investigating the rmap code, and I found the below possible race window:

CPU 1								CPU 2
-----								-----
page_lock_anon_vma_read
  rcu_read_lock
  /* We assume anon_vam == root_anon_vma in this case. */
  root_anon_vma = READ_ONCE(anon_vma->root);
  root_anon_vma is *released* somewhere unfortunately.
  down_read_trylock(&root_anon_vma->rwsem)
								__anon_vma_prepare
								  anon_vma_alloc
								    root_anon_vma is *allocated* here.
								    init_rwsem(&anon_vma->rwsem);
  !page_mapped(page)
    up_read(&root_anon_vma->rwsem); -- *Oops!*

root_anon_vma->rwsem is reinitialized after locked. And reinitialized anon_vma->rwsem will be
unlocked without lock first.

I think this could happen due to the subtle SLAB_TYPESAFE_BY_RCU. But only can occur when anon_vma
is root anon_vma or they won't operate on the same rwsem.
Is this will really happen or Am I miss something ? Any reply would be very grateful.
Many Thanks! :)