That should be:

  Parent* p = container_of(field,Parent,pa_somefield);
  access(p->pa_otherfield);

You'd usually define container_of using subtraction (not negative offset per se):

  #define container_of(FIELD,TYPE,MEMB) ({ \
   const typeof( ((TYPE*)0)->MEMB )* _mptr = (FIELD); \
   (TYPE*)( (char*)_mptr - __builtin_offsetof(TYPE,MEMB) ); \
   })

but you shouldn't actually be using that directly, because thats what the macro is for.