The barrier code you quote is for our old generational GC. Keep reading to see what the riptide barrier looks like. See the section on retreating wavefront.
Aha, thanks; I didn't realize the first generational section was for an older version.
However, the riptide barrier description still isn't 100% clear how it avoids terminating a sweep (I believe "drain" in your termination) in the middle of a barrier execution; in particular I'm not concerned about the data race between the field write and the mark bit, but the GC reading mark bits, believing the drain has terminated, and freeing memory while a mutator thread has written a field but not written the mark bit.
Are you just relying on the stop-the-world behavior relative to roots that you mention briefly?
The collector can only flip at safepoints. Safepoints cannot happen inside the barrier or in between the store the barrier protects and the barrier itself.
Flip = term of art for when the GC terminates and decides which objects are free.
