Quoting Chris Wilson (2019-08-14 18:06:18) > 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. The problem is that the point of the loop is that we do need a check on the fences after the full memory barrier. Thinking of the rationale beaten out for dma_fence_get_excl_rcu_safe() We don't have a full memory barrier here, so this cannot be used safely in light of fence reallocation. -Chris _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
