Fix race condition in Document.GetSemanticModelAsync

Fixes #25836

src/Workspaces/Core/Portable/Workspace/Solution/Document.cs

@@ -276,16 +276,19 @@ public async Task<SemanticModel> GetSemanticModelAsync(CancellationToken cancell
                     return result;
                 }
 
-                // it looks like someone has set it. try to reuse same semantic model
+                // It looks like someone has set it. Try to reuse same semantic model, or assign the new model if that
+                // fails. The lock is required since there is no compare-and-set primitive for WeakReference<T>.
+                lock (original)
+                {
                     if (original.TryGetTarget(out semanticModel))
                     {
                         return semanticModel;
                     }
 
-                // it looks like cache is gone. reset the cache.
                     original.SetTarget(result);
                     return result;
                 }
+            }
             catch (Exception e) when (FatalError.ReportUnlessCanceled(e))
             {
                 throw ExceptionUtilities.Unreachable;