The big kernel lock (BKL) was removed in Jan 2011 for 2.6.39, and
before that, trivial users of the BKL were migrated via script to use
a mutex in Jun 2010 via commit 2a48fc0ab2424.

The changes in 35 follow suit.

The mutex core was added by Ingo Molnar in commit 6053ee3b32e3437 in
January 2006 for 2.6.16-rc1.  We could tell from the test 6 results
whether we have a really old kernel without mutexes (but with the
BKL), but we'd better just use the BKL when it's in the kernel, since
we were originally advised to use it even though it was already
planned for removal at the time.