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
