[PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock

[Matthew Wilcox]

On Tue, Jan 17, 2023 at 10:26:32AM -0800, Suren Baghdasaryan wrote:
> On Tue, Jan 17, 2023 at 10:12 AM Jann Horn <jannh at google.com> wrote:
> >
> > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <surenb at google.com> wrote:
> > > rw_semaphore is a sizable structure of 40 bytes and consumes
> > > considerable space for each vm_area_struct. However vma_lock has
> > > two important specifics which can be used to replace rw_semaphore
> > > with a simpler structure:
> > [...]
> > >  static inline void vma_read_unlock(struct vm_area_struct *vma)
> > >  {
> > > -       up_read(&vma->vm_lock->lock);
> > > +       if (atomic_dec_and_test(&vma->vm_lock->count))
> > > +               wake_up(&vma->vm_mm->vma_writer_wait);
> > >  }
> >
> > I haven't properly reviewed this, but this bit looks like a
> > use-after-free because you're accessing the vma after dropping your
> > reference on it. You'd have to first look up the vma->vm_mm, then do
> > the atomic_dec_and_test(), and afterwards do the wake_up() without
> > touching the vma. Or alternatively wrap the whole thing in an RCU
> > read-side critical section if the VMA is freed with RCU delay.
> 
> vm_lock->count does not control the lifetime of the VMA, it's a
> counter of how many readers took the lock or it's negative if the lock
> is write-locked.

Yes, but ...
	
	Task A:
	atomic_dec_and_test(&vma->vm_lock->count)
			Task B:
			munmap()
			write lock
			free VMA
			synchronize_rcu()
			VMA is really freed
        wake_up(&vma->vm_mm->vma_writer_wait);

... vma is freed.

Now, I think this doesn't occur.  I'm pretty sure that every caller of
vma_read_unlock() is holding the RCU read lock.  But maybe we should
have that assertion?