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 _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
