Make mutex semaphore based.

include/private/SkSemaphore.h

Index: include/private/SkSemaphore.h
diff --git a/src/core/SkSemaphore.h b/include/private/SkSemaphore.h
similarity index 62%
rename from src/core/SkSemaphore.h
rename to include/private/SkSemaphore.h
index 5f4c2968a77aed4a061c2b8e4347bc7567d20813..b5b5ea5957bbc14ad070a1735e5f412aec6b66db 100644
--- a/src/core/SkSemaphore.h
+++ b/include/private/SkSemaphore.h
@@ -10,6 +10,53 @@
 
 #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.
@@ -28,28 +75,12 @@ public:
     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;
+    void signal(int n = 1);
 
-    SkAtomic<int>  fCount;
-    OSSemaphore*   fOSSemaphore;
+private:
+    SkBaseSemaphore fBaseSemaphore;
 };
 
 #endif//SkSemaphore_DEFINED