why does rwlock prefer readers by default?

[Torvald Riegel]

POSIX makes it an implementation-defined choice whether readers or
writers are preferred.  Our current implementation's default is that
readers are to be preferred.  I couldn't find the rationale for this;
does anybody know what it was?

Otherwise, if this was an arbitrary choice, what do you all think the
default should be?  Can we change it?  Should we change it to preferring
writers?