The problem was due to a misuse of GCC asm constraints used to
implement a atomic load. On x86_64, the load was implemented
as a cmpxchg which implicitly uses the eax register as a
source and destination operand, yet the dummy value used for
comparison wasn't being properly loaded into eax (and other
problems).

The core problem is that cmpxchg is unnecessary as a load
on x86_64 as there are other simpler instructions such
as xadd. Even though, such instructions are only used to
have a memory barrier as load and stores are atomic by
definition. Hence, the solution is to explicitly issue the
required CPU and compiler barriers.