From: Stephen Toub <stoub@microsoft.com>
Date: Tue, 15 Jun 2021 00:15:17 +0000 (-0400)
Subject: Fix thread-safety of DiagnosticListener.AllListeners (#54142)
X-Git-Tag: submit/tizen/20210909.063632~776
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=90c1750027a0935a40f1db85d084261ea11b0d08;p=platform%2Fupstream%2Fdotnet%2Fruntime.git

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

diff --git a/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagnosticListener.cs b/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagnosticListener.cs
index cd588e6..e06a927 100644
--- a/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagnosticListener.cs
+++ b/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagnosticListener.cs
@@ -39,12 +39,10 @@ namespace System.Diagnostics
 #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 @@ namespace System.Diagnostics
             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 @@ namespace System.Diagnostics
         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");