How do I safely use direct IO while forking in a multi-threaded application?

Issue

How do I safely use direct IO while forking in a multi-threaded application?

If the following conditions are met, the subsequent associated copy is not properly synchronized:

• (a) a thread within a multi-threaded process having direct IO outstanding to a buffer with a size that is not a multiple of the page size and/or is not a page-aligned memory address,...