The author while talking about buffer cache in the book - The design of the UNIX operating system, says that
a buffer is always on a hash queue, but it may or may not be on the free list.
What about the buffers in the free list that might not have been used at all? Wouldn't they be just part of the free list?
The data structures presented in the book are similar to the ones discussed here
The book talks about UNIX System V.