Implement shared locks in the style of pthread's rwlock.

src/core/SkSharedMutex.h

Index: src/core/SkSharedMutex.h
diff --git a/src/core/SkSharedMutex.h b/src/core/SkSharedMutex.h
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+/*
+ * 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 SkSharedLock_DEFINED
+#define SkSharedLock_DEFINED
+
+#include "SkAtomics.h"
+#include "SkSemaphore.h"
+#include "SkTypes.h"
+
+// This is a shared lock implementation similar to pthreads rwlocks. This implementation is
+// cribbed from Preshing's article:
+// http://preshing.com/20150316/semaphores-are-surprisingly-versatile/
+//
+// This lock does not obey strict queue ordering. It will always alternate between readers and
+// a single writer.
+class SkSharedMutex {
+public:
+    SkSharedMutex();
+
+    // Acquire lock for exclusive use.
+    void acquire();
+
+    // Release lock for exclusive use.
+    void release();
+
+    // Acquire lock for shared use.
+    void acquireShared();
+
+    // Release lock for shared use.
+    void releaseShared();
+
+private:
+    SkAtomic<int32_t> fQueueCounts;
+    SkSemaphore fSharedQueue;
+    SkSemaphore fExclusiveQueue;
+};
+
+#endif // SkSharedLock_DEFINED