Merge pull request #3198 from johnou/contention-JENKINS-48505

[JENKINS-48505] - Invoke optimistic get before computeIfAbsent to avoid contention.

core/src/main/java/jenkins/model/Jenkins.java

@@ -2595,7 +2595,8 @@ public class Jenkins extends AbstractCIBase implements DirectlyModifiableTopLeve
      */
     @SuppressWarnings({"unchecked"})
     public <T> ExtensionList<T> getExtensionList(Class<T> extensionType) {
-        return extensionLists.computeIfAbsent(extensionType, key -> ExtensionList.create(this, key));
+        ExtensionList<T> extensionList = extensionLists.get(extensionType);
+        return extensionList != null ? extensionList : extensionLists.computeIfAbsent(extensionType, key -> ExtensionList.create(this, key));
     }
 
     /**