NVIDIA has been working on X Synchronization Fences for a while and was publicly talked about at least year's X Developers' Summit by Aaron Plattner, but now they are ready with a code drop for this new type of X Synchronization counters. As said in the mailing list announcement, the new X Synchronization Fences "can be used to synchronize X rendering with direct rendering X clients such as OpenGL and vice versa. They are especially useful in synchronizing GL-based composite managers¹ screen updates with Xs rendering."
NVIDIA's X Synchronization Fences support requires changes to the X Server, xextproto, libXext, damageproto, and libXdamage. NVIDIA's James Jones is also working on XCB support. NVIDIA is hoping the X Synchronization Fences will be ready to merge in X.Org Server 1.10.