* 2. holding a buffer in a validate list
* 3. holding a flushed buffer (not in any validate list) with an active fence
* 4. holding a flushed buffer with an expired fence
*
* An entry progresses from 1 -> 2 -> 3 -> 4. When we need an entry to put a
* 2. holding a buffer in a validate list
* 3. holding a flushed buffer (not in any validate list) with an active fence
* 4. holding a flushed buffer with an expired fence
*
* An entry progresses from 1 -> 2 -> 3 -> 4. When we need an entry to put a