Extract the spinlock from SkOnce as SkSpinlock.

include/ports/SkAtomics_sync.h

+/*
+ * 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
 
 // This file is mostly a shim.  We'd like to delete it.  Please don't put much
 // effort into maintaining it, and if you find bugs in it, the right fix is to
 // delete this file and upgrade your compiler to something that supports
 // __atomic builtins or std::atomic.
 
 static inline void barrier(sk_memory_order mo) {
     asm volatile("" : : : "memory");  // Prevents the compiler from reordering code.
@@ skipping 30 matching lines @@
 template <typename T>
 bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired, sk_memory_order, sk_memory_order) {
     T prev = __sync_val_compare_and_swap(ptr, *expected, desired);
     if (prev == *expected) {
         return true;
     }
     *expected = prev;
     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