Use a spinlock in SkOnce.

include/core/SkThread.h

Index: include/core/SkThread.h
diff --git a/include/core/SkThread.h b/include/core/SkThread.h
index 412ace31eb24f64e31c1d2e9529d24a3c004e95a..127960be077145e904500cd0464a1b25237e5bb8 100644
--- a/include/core/SkThread.h
+++ b/include/core/SkThread.h
@@ -33,6 +33,11 @@ static int32_t sk_atomic_dec(int32_t* addr);
  */
 static int32_t sk_atomic_conditional_inc(int32_t* addr);
 
+/** Atomic compare and set.
+ *  If *addr == before, set *addr to after and return true, otherwise return false.
+ */
+static bool sk_atomic_cas(int32_t* addr, int32_t before, int32_t after);
+
 /** If sk_atomic_dec does not act as an acquire (L/SL) barrier,
  *  this must act as an acquire (L/SL) memory barrier and as a compiler barrier.
  */
@@ -45,6 +50,29 @@ static void sk_membar_acquire__after_atomic_conditional_inc();
 
 #include SK_ATOMICS_PLATFORM_H
 
+// This is POD and must be zero-initialized.

[reed1, 2014/01/02 13:53:34]

Is it possible to add another field, just for debugging, that we assert upon in
SkAutoSpinLock, to ensure that someone zero-initialized the SkSpinLock?
Basically, this comment looks scary.

[mtklein, 2014/01/02 16:28:16]

Yes indeed.  Done.

+struct SkSpinlock {
+    void acquire() {
+        while (!sk_atomic_cas(&thisIsPrivate, 0, 1)) {
+            // spin
+        }
+    }
+
+    void release() {
+        SkAssertResult(sk_atomic_cas(&thisIsPrivate, 1, 0));
+    }
+
+    int32_t thisIsPrivate;
+};
+
+class SkAutoSpinlock : SkNoncopyable {
+public:
+    explicit SkAutoSpinlock(SkSpinlock* lock) : fLock(lock) { fLock->acquire(); }
+    ~SkAutoSpinlock() { fLock->release(); }
+private:
+    SkSpinlock* fLock;
+};
+#define SkAutoSpinlock(...) SK_REQUIRE_LOCAL_VAR(SkAutoSpinlock)
 
 /** SK_MUTEX_PLATFORM_H must provide the following (or equivalent) declarations.