Herbert says (in bold font) “do not ever use sync-all”, in this post http://www.reddit.com/r/haskell/comments/2hes8m/the_ghc_source_code_contains_1088_todos_please/.

If that’s really true, we should either nuke it altogether, or change it to do something correct.  The idea that it might “set up your tree in subtly different ways” is alarming.

Simon