3 files changed, 20 insertions, 7 deletions
diff --git a/include/utils/SkEventTracer.h b/include/utils/SkEventTracer.h
index f4f8676a74..a0289ce9eb 100644
--- a/include/utils/SkEventTracer.h
+++ b/include/utils/SkEventTracer.h
@@ -27,9 +27,18 @@ public:
 
     typedef uint64_t Handle;
 
-    static SkEventTracer* GetInstance();
+    /**
+     * If this is the first call to SetInstance or GetInstance then the passed instance is
+     * installed and true is returned. Otherwise, false is returned. In either case ownership of the
+     * tracer is transferred and it will be deleted when no longer needed.
+     */
+    static bool SetInstance(SkEventTracer*);
 
-    static void SetInstance(SkEventTracer*);
+    /**
+     * Gets the event tracer. If this is the first call to SetInstance or GetIntance then a default
+     * event tracer is installed and returned.
+     */
+    static SkEventTracer* GetInstance();
 
     virtual ~SkEventTracer() { }
 
diff --git a/src/utils/SkEventTracer.cpp b/src/utils/SkEventTracer.cpp
index 0a748d1464..5fb60bdf8a 100644
--- a/src/utils/SkEventTracer.cpp
+++ b/src/utils/SkEventTracer.cpp
@@ -42,11 +42,15 @@ class SkDefaultEventTracer : public SkEventTracer {
 // We prefer gUserTracer if it's been set, otherwise we fall back on a default tracer;
 static SkEventTracer* gUserTracer = nullptr;
 
-void SkEventTracer::SetInstance(SkEventTracer* tracer) {
-    SkASSERT(nullptr == sk_atomic_load(&gUserTracer, sk_memory_order_acquire));
-    sk_atomic_store(&gUserTracer, tracer, sk_memory_order_release);
+bool SkEventTracer::SetInstance(SkEventTracer* tracer) {
+    SkEventTracer* expected = nullptr;
+    if (!sk_atomic_compare_exchange(&gUserTracer, &expected, tracer)) {
+        delete tracer;
+        return false;
+    }
     // An atomic load during process shutdown is probably overkill, but safe overkill.
-    atexit([]() { delete sk_atomic_load(&gUserTracer, sk_memory_order_acquire); });
+    atexit([]() { delete sk_atomic_load(&gUserTracer); });
+    return true;
 }
 
 SkEventTracer* SkEventTracer::GetInstance() {
diff --git a/tools/viewer/Viewer.cpp b/tools/viewer/Viewer.cpp
index 7cf8cd8825..a1c0b5bbea 100644
--- a/tools/viewer/Viewer.cpp
+++ b/tools/viewer/Viewer.cpp
@@ -285,7 +285,7 @@ Viewer::Viewer(int argc, char** argv, void* platformData)
 #endif
 
     if (FLAGS_atrace) {
-        SkEventTracer::SetInstance(new SkATrace());
+        SkAssertResult(SkEventTracer::SetInstance(new SkATrace()));
     }
 
     fBackendType = get_backend_type(FLAGS_backend[0]);