Re: [PATCH v4 01/26] mm/mmu_notifiers: pass private data down to alloc_notifier()

Jean-Philippe Brucker
2020-02-25T09:24:50Z

On Mon, Feb 24, 2020 at 03:00:56PM -0400, Jason Gunthorpe wrote:
> On Mon, Feb 24, 2020 at 07:23:36PM +0100, Jean-Philippe Brucker wrote:
> > The new allocation scheme introduced by 2c7933f53f6b ("mm/mmu_notifiers:
> > add a get/put scheme for the registration") provides a convenient way
> > for users to attach notifier data to an mm. However, it would be even
> > better to create this notifier data atomically.
> >
> > Since the alloc_notifier() callback only takes an mm argument at the
> > moment, some users have to perform the allocation in two times.
> > alloc_notifier() initially creates an incomplete structure, which is
> > then finalized using more context once mmu_notifier_get() returns. This
> > second step requires carrying an initialization lock in the notifier
> > data and playing dirty tricks to order memory accesses against live
> > invalidation.
>
> This was the intended pattern. Tthere shouldn't be an real issue as
> there shouldn't be any data on which to invalidate, ie the later patch
> does:
>
> + list_for_each_entry_rcu(bond, &io_mm->devices, mm_head)
>
> And that list is empty post-allocation, so no 'dirty tricks' required.

Before introducing this patch I had the following code:

+ list_for_each_entry_rcu(bond, &io_mm->devices, mm_head) {
+ /*
+ * To ensure that we observe the initialization of io_mm fields
+ * by io_mm_finalize() before the registration of this bond to
+ * the list by io_mm_attach(), introduce an address dependency
+ * between bond and io_mm. It pairs with the smp_store_release()
+ * from list_add_rcu().
+ */
+ io_mm = rcu_dereference(bond->io_mm);
+ io_mm->ops->invalidate(bond->sva.dev, io_mm->pasid, io_mm->ctx,
+ start, end - start);
+ }

(1) io_mm_get() would obtain an empty io_mm from iommu_notifier_get().
(2) then io_mm_finalize() would initialize io_mm->ops, io_mm->ctx, etc.
(3) finally io_mm_attach() would add the bond to io_mm->devices.

Since the above code can run before (2) it needs to observe valid
io_mm->ctx, io_mm->ops initialized by (2) after obtaining the bond
initialized by (3). Which I believe requires the address dependency from
the rcu_dereference() above or some stronger barrier to pair with the
list_add_rcu(). If io_mm->ctx and io_mm->ops are already valid before the
mmu notifier is published, then we don't need that stuff.

That's the main reason I would have liked moving everything to
alloc_notifier(), the locking below isn't a big deal.

> The other op callback is release, which also cannot be called as the
> caller must hold a mmget to establish the notifier.
>
> So just use the locking that already exists. There is one function
> that calls io_mm_get() which immediately calls io_mm_attach, which
> immediately grabs the global iommu_sva_lock.
>
> Thus init the pasid for the first time under that lock and everything
> is fine.

I agree with this, can't remember why I used a separate lock for
initialization rather than reusing iommu_sva_lock.

Thanks,
Jean

>
> There is nothing inherently wrong with the approach in this patch, but
> it seems unneeded in this case..
>
> Jason