fix multithread related crashes in skpdiff

tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmap.h"
 
 #include "SkDifferentPixelsMetric.h"
@@ skipping 34 matching lines @@
 
 const char* SkDifferentPixelsMetric::getName() {
     return "different_pixels";
 }
 
 bool SkDifferentPixelsMetric::enablePOIAlphaMask() {
     return false;
 }
 
 int SkDifferentPixelsMetric::queueDiff(SkBitmap* baseline, SkBitmap* test) {
+    double startTime = get_seconds();
+
+    // since we currently synchronously do the diff within queueDiff we only need
+    // to protect the createion of the ID and queuedDiff object.

[scroggo, 2013/11/07 21:43:23]

creation*

+    SkAutoMutexAcquire queueLock(fQueueMutex);
     int diffID = fQueuedDiffs.count();
-    double startTime = get_seconds();
     QueuedDiff* diff = fQueuedDiffs.push();
+    queueLock.release();
 
     // If we never end up running the kernel, include some safe defaults in the result.
     diff->finished = false;
     diff->result = -1.0;
     diff->numDiffPixels = 0;
     diff->poi = NULL;
 
     // Ensure the images are comparable
     if (baseline->width() != test->width() || baseline->height() != test->height() ||
         baseline->width() <= 0 || baseline->height() <= 0 ||
@@ skipping 97 matching lines @@
     return fQueuedDiffs[id].poi;
 }
 
 bool SkDifferentPixelsMetric::onInit() {
     if (!this->loadKernelSource(kDifferentPixelsKernelSource, "diff", &fKernel)) {
         return false;
     }
 
     return true;
 }