Fix thread-safety of DiagnosticListener.AllListeners (#54142)

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagnosticListener.cs

@@ -39,12 +39,10 @@ public static IObservable<DiagnosticListener> AllListeners
 #if ENABLE_HTTP_HANDLER
                 GC.KeepAlive(HttpHandlerDiagnosticListener.s_instance);
 #endif
-
-                if (s_allListenerObservable == null)
-                {
-                    s_allListenerObservable = new AllListenerObservable();
-                }
-                return s_allListenerObservable;
+                return
+                    s_allListenerObservable ??
+                    Interlocked.CompareExchange(ref s_allListenerObservable, new AllListenerObservable(), null) ??
+                    s_allListenerObservable;
             }
         }
 
@@ -137,9 +135,7 @@ public DiagnosticListener(string name)
             lock (s_allListenersLock)
             {
                 // Issue the callback for this new diagnostic listener.
-                var allListenerObservable = s_allListenerObservable;
-                if (allListenerObservable != null)
-                    allListenerObservable.OnNewDiagnosticListener(this);
+                s_allListenerObservable?.OnNewDiagnosticListener(this);
 
                 // And add it to the list of all past listeners.
                 _next = s_allListeners;
@@ -460,7 +456,7 @@ public void Dispose()
         private bool _disposed;                        // Has Dispose been called?
 
         private static DiagnosticListener? s_allListeners;               // linked list of all instances of DiagnosticListeners.
-        private static AllListenerObservable? s_allListenerObservable;   // to make callbacks to this object when listeners come into existence.
+        private static volatile AllListenerObservable? s_allListenerObservable;   // to make callbacks to this object when listeners come into existence.
         private static readonly object s_allListenersLock = new object();
 #if false
         private static readonly DiagnosticListener s_default = new DiagnosticListener("DiagnosticListener.DefaultListener");