Make mutex semaphore based.

src/core/SkSemaphore.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 SkSemaphore_DEFINED
-#define SkSemaphore_DEFINED
-
-#include "SkTypes.h"
-#include "SkAtomics.h"
-
-/**
- * SkSemaphore is a fast mostly-user-space semaphore.
- *
- * A semaphore is logically an atomic integer with a few special properties:
- *   - The integer always starts at 0.
- *   - You can only increment or decrement it, never read or write it.
- *   - Increment is spelled 'signal()'; decrement is spelled 'wait()'.
- *   - If a call to wait() decrements the counter to <= 0,
- *     the calling thread sleeps until another thread signal()s it back above 0.
- */
-class SkSemaphore : SkNoncopyable {
-public:
-    // Initializes the counter to 0.
-    // (Though all current implementations could start from an arbitrary value.)
-    SkSemaphore();
-    ~SkSemaphore();
-
-    // Increment the counter N times.
-    // Generally it's better to call signal(N) instead of signal() N times.
-    void signal(int N = 1);
-
-    // Decrement the counter by 1,
-    // then if the counter is <= 0, sleep this thread until the counter is > 0.
-    void wait();
-
-private:
-    // This implementation follows the general strategy of
-    //     'A Lightweight Semaphore with Partial Spinning'
-    // found here
-    //     http://preshing.com/20150316/semaphores-are-surprisingly-versatile/
-    // That article (and entire blog) are very much worth reading.
-    //
-    // We wrap an OS-provided semaphore with a user-space atomic counter that
-    // lets us avoid interacting with the OS semaphore unless strictly required:
-    // moving the count from >0 to <=0 or vice-versa, i.e. sleeping or waking threads.
-    struct OSSemaphore;
-
-    SkAtomic<int>  fCount;
-    OSSemaphore*   fOSSemaphore;
-};
-
-#endif//SkSemaphore_DEFINED