deadlock.py

from torch import Tensor
from torch.autograd import Function, Variable

class Foo(Function):
  def forward(self, x):
    return x

  def backward(self, dz_star):
    dphidz = Variable(Tensor(1), requires_grad=True)
    dphidz.backward()
    return None

x = Variable(Tensor(1), requires_grad=True)
Foo()(x).backward()