[Intel-gfx] [PATCH 4/4] dma-buf: nuke reservation_object seq number

[Chris Wilson]

Quoting Chris Wilson (2019-08-14 17:42:48)
> Quoting Daniel Vetter (2019-08-14 16:39:08)
> > > > > +       } while (rcu_access_pointer(obj->fence_excl) != *excl);
> > 
> > What if someone is real fast (like really real fast) and recycles the
> > exclusive fence so you read the same pointer twice, but everything else
> > changed? reused fence pointer is a lot more likely than seqlock wrapping
> > around.
> 
> It's an exclusive fence. If it is replaced, it must be later than all
> the shared fences (and dependent on them directly or indirectly), and
> so still a consistent snapshot.

An extension of that argument says we don't even need to loop,

*list = rcu_dereference(obj->fence);
*shared_count = *list ? (*list)->shared_count : 0;
smp_rmb();
*excl = rcu_dereference(obj->fence_excl);

Gives a consistent snapshot. It doesn't matter if the fence_excl is
before or after the shared_list -- if it's after, it's a superset of all
fences, if it's before, we have a correct list of shared fences the
earlier fence_excl.
-Chris