Outline SkSpinlock::acquire().

include/core/SkSpinlock.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.
  */
 
 // This file is not part of the public Skia API.
 
 #ifndef SkSpinlock_DEFINED
 #define SkSpinlock_DEFINED
 
 #include "SkAtomics.h"
 
 #define SK_DECLARE_STATIC_SPINLOCK(name) namespace {} static SkPODSpinlock name
 
 // This class has no constructor and must be zero-initialized (the macro above does this).
-struct SkPODSpinlock {
+class SkPODSpinlock {
+public:
     void acquire() {
-        // To act as a mutex, we need an acquire barrier.
-        while(sk_atomic_exchange(&fLocked, true, sk_memory_order_acquire)) { /*spin*/ }
+        // To act as a mutex, we need an acquire barrier if we take the lock.
+        if (sk_atomic_exchange(&fLocked, true, sk_memory_order_acquire)) {
+            // Lock was contended.  Fall back to an out-of-line spin loop.
+            this->contendedAcquire();
+        }
     }
+
     void release() {
         // To act as a mutex, we need a release barrier.
         sk_atomic_store(&fLocked, false, sk_memory_order_release);
     }
 
+private:
+    void contendedAcquire();
     bool fLocked;
 };
 
 // For non-global-static use cases, this is normally what you want.
 class SkSpinlock : public SkPODSpinlock {
 public:
     SkSpinlock() { this->release(); }
 };
 
 #endif//SkSpinlock_DEFINED