Add comments to clarify cache handling in _raw_fft

1 files changed, 8 insertions, 0 deletions

diff --git a/numpy/fft/fftpack.py b/numpy/fft/fftpack.py
index 7ee0e4bea..236a18d7a 100644
--- a/numpy/fft/fftpack.py
+++ b/numpy/fft/fftpack.py
@@ -53,6 +53,9 @@ def _raw_fft(a, n=None, axis=-1, init_function=fftpack.cffti,
         raise ValueError("Invalid number of FFT data points (%d) specified." % n)
 
     try:
+        # Thread-safety note: We rely on list.pop() here to atomically
+        # retrieve-and-remove a wsave from the cache.  This ensures that no
+        # other thread can get the same wsave while we're using it.
         wsave = fft_cache.setdefault(n, []).pop()
     except (IndexError):
         wsave = init_function(n)
@@ -76,7 +79,12 @@ def _raw_fft(a, n=None, axis=-1, init_function=fftpack.cffti,
     r = work_function(a, wsave)
     if axis != -1:
         r = swapaxes(r, axis, -1)
+
+    # As soon as we put wsave back into the cache, another thread could pick it
+    # up and start using it, so we must not do this until after we're
+    # completely done using it ourselves.
     fft_cache[n].append(wsave)
+
     return r