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

Jason Gunthorpe
2020-02-28T15:13:43Z

On Fri, Feb 28, 2020 at 04:04:27PM +0100, Jean-Philippe Brucker wrote:
> 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().

I usually advocate for simple use of these APIs. The mm_notifier_get()
should happen in bind() and the matching put should happen in the
call_rcu callbcak that does the kfree. Then you can never get a stale
pointer. Don't worry about exit_mmap().

release() is an unusual callback and I see alot of places using it
wrong. The purpose of release is to invalidate_all, that is it.

Also, confusingly release may be called multiple times in some
situations, so it shouldn't disturb anything that might impact a 2nd
call.

Jason