Make mutex semaphore based.

include/private/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"
+#include "../private/SkOncePtr.h"
+
+struct SkBaseSemaphore {
+
+    // Increment the counter by 1.
+    // This is a specialization for supporting SkMutex.
+    void signal() {
+        // Since this fetches the value before the add, 0 indicates that this thread is running and
+        // no threads are waiting, -1 and below means that threads are waiting, but only signal 1
+        // thread to run.
+        if (sk_atomic_fetch_add(&fCount, 1, sk_memory_order_release) < 0) {
+           this->osSignal(1);
+        }
+    }
+
+    // Increment the counter N times.
+    // Generally it's better to call signal(N) instead of signal() N times.
+    void signal(int N);
+
+    // Decrement the counter by 1,
+    // then if the counter is <= 0, sleep this thread until the counter is > 0.
+    void wait() {
+        // Since this fetches the value before the subtract, zero and below means that there are no
+        // resources left, so the thread needs to wait.
+        if (sk_atomic_fetch_sub(&fCount, 1, sk_memory_order_acquire) <= 0) {
+            this->osWait();
+        }
+    }
+
+    struct OSSemaphore;
+
+    void osSignal(int n);
+    void osWait();
+    void deleteSemaphore();
+
+    // 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.
+    int                        fCount;
+    SkBaseOncePtr<OSSemaphore> fOSSemaphore;
+};
 
 /**
  * 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();
 
+    void signal(int n = 1);
+
 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;
+    SkBaseSemaphore fBaseSemaphore;
 };
 
 #endif//SkSemaphore_DEFINED