Re: [RFC PATCH RESEND 16/28] kernel/fork: assert no VMA readers during its destruction

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Sep 9, 2022 at 6:56 AM Laurent Dufour <ldufour@linux.ibm.com> wrote:
>
> Le 01/09/2022 à 19:35, Suren Baghdasaryan a écrit :
> > Assert there are no holders of VMA lock for reading when it is about to be
> > destroyed.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  include/linux/mm.h | 8 ++++++++
> >  kernel/fork.c      | 2 ++
> >  2 files changed, 10 insertions(+)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index dc72be923e5b..0d9c1563c354 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -676,6 +676,13 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos)
> >       VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
> >  }
> >
> > +static inline void vma_assert_no_reader(struct vm_area_struct *vma)
> > +{
> > +     VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock) &&
> > +                   vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq),
> > +                   vma);
> > +}
> > +
> >  #else /* CONFIG_PER_VMA_LOCK */
> >
> >  static inline void vma_init_lock(struct vm_area_struct *vma) {}
> > @@ -685,6 +692,7 @@ static inline bool vma_read_trylock(struct vm_area_struct *vma)
> >  static inline void vma_read_unlock(struct vm_area_struct *vma) {}
> >  static inline void vma_assert_locked(struct vm_area_struct *vma) {}
> >  static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos) {}
> > +static inline void vma_assert_no_reader(struct vm_area_struct *vma) {}
> >
> >  #endif /* CONFIG_PER_VMA_LOCK */
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 1872ad549fed..b443ba3a247a 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -487,6 +487,8 @@ static void __vm_area_free(struct rcu_head *head)
> >  {
> >       struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
> >                                                 vm_rcu);
> > +     /* The vma should either have no lock holders or be write-locked. */
> > +     vma_assert_no_reader(vma);
>
> I'm wondering if this can be hit in the case the thread freeing a VMA is
> preempted before incrementing the mm ref count, like this:
>
> VMA is about to be freed
> write lock VMA
> free vma -> call_rcu
> ..
> <--- thread preempted
>         rcu handler runs
>         rcu calls __vm_area_free() <<<<<<

At this point the VMA is still write-locked (mm seq count hasn't been
incremented yet), correct? If so then vma_assert_no_reader() will not
assert because the second condition of VMA being write-locked is
satisfied. Did I miss anything?

> unlock mmap_lock and increase the mm seq count
>
>
> >       kmem_cache_free(vm_area_cachep, vma);
> >  }
> >  #endif
>

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