More chuckles from the source:

   /*
    * Yes, this permits 64-bit accesses on 32-bit architectures. These will
    * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
    * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
    * (e.g. a virtual address) and a strong prevailing wind.
    */