fix race to set the new gen ID

src/core/SkPixelRef.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmapCache.h"
 #include "SkPixelRef.h"
 #include "SkThread.h"
@@ skipping 189 matching lines @@
     return this->onLockPixelsAreWritable();
 }
 
 bool SkPixelRef::onLockPixelsAreWritable() const {
     return true;
 }
 
 uint32_t SkPixelRef::getGenerationID() const {
     uint32_t id = fTaggedGenID.load();
     if (0 == id) {
-        id = next_gen_id();
-        fTaggedGenID.store(id | 1u);  // TODO(mtklein): should become a compare-and-swap
-        SkASSERT(this->genIDIsUnique());
+        uint32_t next = next_gen_id() | 1u;
+        if (fTaggedGenID.compare_exchange(&id, next)) {
+            id = next;  // There was no race or we won the race.  fTaggedGenID is next now.
+        } else {
+            // We lost a race to set fTaggedGenID. compare_exchange() filled id with the winner.
+        }
+        // We can't quite SkASSERT(this->genIDIsUnique()). It could be non-unique
+        // if we got here via the else path (pretty unlikely, but possible).
     }
     return id & ~1u;  // Mask off bottom unique bit.
 }
 
 void SkPixelRef::addGenIDChangeListener(GenIDChangeListener* listener) {
     if (NULL == listener || !this->genIDIsUnique()) {
         // No point in tracking this if we're not going to call it.
         SkDELETE(listener);
         return;
     }
@@ skipping 50 matching lines @@
 
 bool SkPixelRef::onGetYUV8Planes(SkISize sizes[3], void* planes[3], size_t rowBytes[3],
                                  SkYUVColorSpace* colorSpace) {
     return false;
 }
 
 size_t SkPixelRef::getAllocatedSizeInBytes() const {
     return 0;
 }