Extract the spinlock from SkOnce as SkSpinlock.

include/ports/SkAtomics_sync.h

Index: include/ports/SkAtomics_sync.h
diff --git a/include/ports/SkAtomics_sync.h b/include/ports/SkAtomics_sync.h
index 66da4d35eeb3c47524e4aeb9832aa757ee4cf8fe..7ca0b46a94a6b23f95e60d7d57109cb3325d8167 100644
--- a/include/ports/SkAtomics_sync.h
+++ b/include/ports/SkAtomics_sync.h
@@ -1,3 +1,10 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
 #ifndef SkAtomics_sync_DEFINED
 #define SkAtomics_sync_DEFINED
 
@@ -48,4 +55,14 @@ bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired, sk_memory_order,
     return false;
 }
 
+template <typename T>
+T sk_atomic_exchange(T* ptr, T val, sk_memory_order) {
+    // There is no __sync exchange.  Emulate it with a CAS loop.
+    T prev;
+    do {
+        prev = sk_atomic_load(ptr);
+    } while(!sk_atomic_compare_exchange(ptr, &prev, val));

[bungeman-skia, 2015/03/30 14:52:17]

nit: usually there is a space between the 'while' and the '('. A quick grep
shows that the usage like '} while(' in the codebase is far outnumbered by usage
like '} while ('. We should probably just change the style guide and the other
10 uses of '} while('.

+    return prev;
+}
+
 #endif//SkAtomics_sync_DEFINED