Back in June there were patches published by NVIDIA for X Synchronization Fences
after it was in planning since before last year's X Developers' Summit. These synchronization fences were designed to "synchronize X rendering with direct rendering X clients such as OpenGL and vice versa. They are especially useful in synchronizing GL-based composite managers screen updates with Xs rendering." Today there are more patches out from NVIDIA this time touching xextproto to add support for the binary sync objects.
NVIDIA's James Jones' mailing list message
is short and to the point:
Adds support for binary sync objects. Objects are set to "triggered" using X commands that are executed relative to X rendering commands. Clients can wait for fence sync objcts to reach the triggered state using XSync APIs or using interop functionality in other APIs. This allows for efficient cross-API synchronization with X rendering operations.