struct pe_ring { void *self; pe_ring *next, *prev; };

#define PE_RING_INIT(LNK, SELF) 		\
STMT_START {					\
  (LNK)->next = LNK;				\
  (LNK)->prev = LNK;				\
  (LNK)->self = SELF;				\
} STMT_END

#define PE_RING_EMPTY(LNK) ((LNK)->next == LNK)

#define PE_RING_UNSHIFT(LNK, ALL)		\
STMT_START {					\
  assert((LNK)->next==LNK);			\
  (LNK)->next = (ALL)->next;			\
  (LNK)->prev = ALL;				\
  (LNK)->next->prev = LNK;			\
  (LNK)->prev->next = LNK;			\
} STMT_END

#define PE_RING_ADD_BEFORE(L1,L2)		\
STMT_START {					\
  assert((L1)->next==L1);			\
  (L1)->next = L2;				\
  (L1)->prev = (L2)->prev;			\
  (L1)->next->prev = L1;			\
  (L1)->prev->next = L1;			\
} STMT_END

#define PE_RING_DETACH(LNK)			\
STMT_START {					\
  if ((LNK)->next != LNK) {			\
    (LNK)->next->prev = (LNK)->prev;		\
    (LNK)->prev->next = (LNK)->next;		\
    (LNK)->next = LNK;				\
  }						\
} STMT_END