Do not acquire GIL for hough_line

skimage/transform/_hough_transform.pyx

@@ -308,14 +308,17 @@ def hough_line(cnp.ndarray img,
 
     # finally, run the transform
     cdef Py_ssize_t nidxs, nthetas, i, j, x, y, accum_idx
-    nidxs = y_idxs.shape[0]  # x and y are the same shape
-    nthetas = theta.shape[0]
-    for i in range(nidxs):
-        x = x_idxs[i]
-        y = y_idxs[i]
-        for j in range(nthetas):
-            accum_idx = <int>round((ctheta[j] * x + stheta[j] * y)) + offset
-            accum[accum_idx, j] += 1
+
+    with nogil:
+        nidxs = y_idxs.shape[0]  # x and y are the same shape
+        nthetas = theta.shape[0]
+        for i in range(nidxs):
+            x = x_idxs[i]
+            y = y_idxs[i]
+            for j in range(nthetas):
+                accum_idx = <int>round((ctheta[j] * x + stheta[j] * y)) + offset
+                accum[accum_idx, j] += 1
+
     return accum, theta, bins
 
 

skimage/transform/tests/test_hough_transform.py

@@ -16,6 +16,7 @@ def append_desc(func, description):
     return func
 
 
+@test_parallel()
 def test_hough_line():
     # Generate a test image
     img = np.zeros((100, 150), dtype=int)