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

Jean-Philippe Brucker
2020-02-28T15:04:38Z

On Fri, Feb 28, 2020 at 10:48:44AM -0400, Jason Gunthorpe wrote:
> On Fri, Feb 28, 2020 at 03:39:35PM +0100, Jean-Philippe Brucker wrote:
> > > > + 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);
> > >
> > > A rcu_dereference isn't need here, just a normal derference is fine.
> >
> > bond->io_mm is annotated with __rcu (for iommu_sva_get_pasid_generic(),
> > which does bond->io_mm under rcu_read_lock())
>
> I'm surprised the bond->io_mm can change over the lifetime of the
> bond memory..

The normal lifetime of the bond is between device driver calls to bind()
and unbind(). If the mm exits early, though, we clear bond->io_mm. The
bond is then stale but can only be freed when the device driver releases
it with unbind().

>
> > > > If io_mm->ctx and io_mm->ops are already valid before the
> > > > mmu notifier is published, then we don't need that stuff.
> > >
> > > So, this trickyness with RCU is not a bad reason to introduce the priv
> > > scheme, maybe explain it in the commit message?
> >
> > Ok, I've added this to the commit message:
> >
> > The IOMMU SVA module, which attaches an mm to multiple devices,
> > exemplifies this situation. In essence it does:
> >
> > mmu_notifier_get()
> > alloc_notifier()
> > A = kzalloc()
> > /* MMU notifier is published */
> > A->ctx = ctx; // (1)
> > device->A = A;
> > list_add_rcu(device, A->devices); // (2)
> >
> > The invalidate notifier, which may start running before A is fully
> > initialized at (1), does the following:
> >
> > io_mm_invalidate(A)
> > list_for_each_entry_rcu(device, A->devices)
> > A = device->A; // (3)
>
> I would drop the work around from the decription, it is enough to say
> that the line below needs to observe (1) after (2) and this is
> trivially achieved by moving (1) to before publishing the notifier so
> the core MM locking can be used.

Ok, will do

Thanks,
Jean