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

[Jason Gunthorpe]

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?

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.

Jason