Tidy up locking usage around FFTW.

* Move QMutex into the class instead of static.
* Use QMutexLocker
* Only lock around FFTW plan & destroy functions.

gst/moodbar/gstfastspectrum.cpp

@@ -19,10 +19,12 @@
  * Boston, MA 02110-1301, USA.
  */
 
-#include <QMutex>
-
 #include <cstring>
 #include <cmath>
+
+#include <QMutex>
+#include <QMutexLocker>
+
 #include "gstfastspectrum.h"
 
 GST_DEBUG_CATEGORY_STATIC (gst_fastspectrum_debug);
@@ -50,8 +52,6 @@ enum {
   PROP_BANDS
 };
 
-static QMutex fftw_mutex;
-
 #define gst_fastspectrum_parent_class parent_class
 G_DEFINE_TYPE (GstFastSpectrum, gst_fastspectrum, GST_TYPE_AUDIO_FILTER);
 
@@ -111,6 +111,8 @@ gst_fastspectrum_class_init (GstFastSpectrumClass * klass)
   caps = gst_caps_from_string (ALLOWED_CAPS);
   gst_audio_filter_class_add_pad_templates (filter_class, caps);
   gst_caps_unref (caps);
+
+  klass->fftw_lock = new QMutex;
 }
 
 static void
@@ -127,8 +129,6 @@ gst_fastspectrum_init (GstFastSpectrum * spectrum)
 static void
 gst_fastspectrum_alloc_channel_data (GstFastSpectrum * spectrum)
 {
-  fftw_mutex.lock();
-
   guint bands = spectrum->bands;
   guint nfft = 2 * bands - 2;
 
@@ -139,31 +139,36 @@ gst_fastspectrum_alloc_channel_data (GstFastSpectrum * spectrum)
       fftw_malloc(sizeof(fftw_complex) * (nfft/2+1)));
 
   spectrum->spect_magnitude = new double[bands];
-  spectrum->plan = fftw_plan_dft_r2c_1d(
-      nfft,
-      spectrum->fft_input,
-      spectrum->fft_output,
-      FFTW_ESTIMATE);
-  spectrum->channel_data_initialised = true;
 
-  fftw_mutex.unlock();
+  GstFastSpectrumClass* klass = reinterpret_cast<GstFastSpectrumClass*>(
+      G_OBJECT_GET_CLASS(spectrum));
+  {
+    QMutexLocker l(klass->fftw_lock);
+    spectrum->plan = fftw_plan_dft_r2c_1d(
+        nfft,
+        spectrum->fft_input,
+        spectrum->fft_output,
+        FFTW_ESTIMATE);
+  }
+  spectrum->channel_data_initialised = true;
 }
 
 static void
 gst_fastspectrum_free_channel_data (GstFastSpectrum * spectrum)
 {
+  GstFastSpectrumClass* klass = reinterpret_cast<GstFastSpectrumClass*>(
+      G_OBJECT_GET_CLASS(spectrum));
   if (spectrum->channel_data_initialised) {
-    fftw_mutex.lock();
-
-    fftw_destroy_plan(spectrum->plan);
+    {
+      QMutexLocker l(klass->fftw_lock);
+      fftw_destroy_plan(spectrum->plan);
+    }
     fftw_free(spectrum->fft_input);
     fftw_free(spectrum->fft_output);
     delete[] spectrum->input_ring_buffer;
     delete[] spectrum->spect_magnitude;
 
     spectrum->channel_data_initialised = false;
-
-    fftw_mutex.unlock();
   }
 }
 
@@ -385,12 +390,11 @@ gst_fastspectrum_run_fft (GstFastSpectrum * spectrum, guint input_pos)
   guint bands = spectrum->bands;
   guint nfft = 2 * bands - 2;
 
-  fftw_mutex.lock();
-
   for (i = 0; i < nfft; i++)
     spectrum->fft_input[i] =
         spectrum->input_ring_buffer[(input_pos + i) % nfft];
 
+  // Should be safe to execute the same plan multiple times in parallel.
   fftw_execute(spectrum->plan);
 
   gdouble val;
@@ -401,8 +405,6 @@ gst_fastspectrum_run_fft (GstFastSpectrum * spectrum, guint input_pos)
     val /= nfft * nfft;
     spectrum->spect_magnitude[i] += val;
   }
-
-  fftw_mutex.unlock();
 }
 
 static GstFlowReturn

gst/moodbar/gstfastspectrum.h

@@ -43,6 +43,8 @@ G_BEGIN_DECLS
 #define GST_FASTSPECTRUM_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST((klass), GST_TYPE_FASTSPECTRUM,GstFastSpectrumClass))
 #define GST_IS_FASTSPECTRUM_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE((klass), GST_TYPE_FASTSPECTRUM))
 
+class QMutex;
+
 typedef void (*GstFastSpectrumInputData)(const guint8* in, double* out,
     guint len, double max_value, guint op, guint nfft);
 
@@ -84,6 +86,9 @@ struct GstFastSpectrum {
 
 struct GstFastSpectrumClass {
   GstAudioFilterClass parent_class;
+
+  // Static lock for creating & destroying FFTW plans.
+  QMutex* fftw_lock;
 };
 
 GType gst_fastspectrum_get_type (void);