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
