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 30 matching lines @@
     cl_mem test;
     cl_mem resultsBuffer;
     cl_mem poiBuffer;
 };
 
 const char* SkDifferentPixelsMetric::getName() {
     return "different_pixels";
 }
 
 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.
+    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 ||
         baseline->config() != test->config()) {
         diff->finished = true;
         return diffID;
     }
 
     // Upload images to the CL device
     if (!this->makeImage2D(baseline, &diff->baseline) || !this->makeImage2D(test, &diff->test)) {
         diff->finished = true;
         fIsGood = false;

[mtklein, 2013/11/06 14:24:36]

I doubt it's going to cause much trouble, but fIsGood is also not protected by
any synchronization.

[djsollen, 2013/11/06 15:01:58]

Yes, it currently isn't read by anything so I was going to refactor it or remove
it in a future CL.

         return -1;
     }
 
     // A small hack that makes calculating percentage difference easier later on.
     diff->result = 1.0 / ((double)baseline->width() * baseline->height());
 
     // Make a buffer to store results into. It must be initialized with pointers to memory.
     static const int kZero = 0;
     // We know OpenCL won't write to it because we use CL_MEM_COPY_HOST_PTR
     diff->resultsBuffer = clCreateBuffer(fContext, CL_MEM_READ_WRITE | CL_MEM_COPY_HOST_PTR,
@@ skipping 78 matching lines @@
     return fQueuedDiffs[id].poi;
 }
 
 bool SkDifferentPixelsMetric::onInit() {
     if (!this->loadKernelSource(kDifferentPixelsKernelSource, "diff", &fKernel)) {
         return false;
     }
 
     return true;
 }