Re: [PATCH hmm 02/15] mm/mmu_notifier: add an interval tree notifier

[Jerome Glisse <jglisse@redhat.com>]

On Mon, Oct 21, 2019 at 07:24:53PM +0000, Jason Gunthorpe wrote:
> On Mon, Oct 21, 2019 at 03:11:57PM -0400, Jerome Glisse wrote:
> > > Since that reader is not locked we need release semantics here to
> > > ensure the unlocked reader sees a fully initinalized mmu_notifier_mm
> > > structure when it observes the pointer.
> > 
> > I thought the mm_take_all_locks() would have had a barrier and thus
> > that you could not see mmu_notifier struct partialy initialized. 
> 
> Not sure, usually a lock acquire doesn't have a store barrier?

Yeah likely.

> 
> Even if it did, we would still need some pairing read barrier..
> 
> > having the acquire/release as safety net does not hurt. Maybe add a
> > comment about the struct initialization needing to be visible before
> > pointer is set.
> 
> Is this clear?
> 
>          * release semantics on the initialization of the
>          * mmu_notifier_mm's contents are provided for unlocked readers.
> 	 * acquire can only be used while holding the
>          * mmgrab or mmget, and is safe because once created the
>          * mmu_notififer_mm is not freed until the mm is destroyed.
>          * Users holding the mmap_sem or one of the
> 	 * mm_take_all_locks() do not need to use acquire semantics.
> 
> It also helps explain why there is no locking around the other
> readers, which has puzzled me in the past at least.

Perfect.

Jérôme