Github user jerryshao commented on a diff in the pull request:

    https://github.com/apache/spark/pull/15377#discussion_r83336101
  
    --- Diff: core/src/main/scala/org/apache/spark/util/Utils.scala ---
    @@ -2474,25 +2478,42 @@ private[spark] class CallerContext(
        val context = "SPARK_" + from + appIdStr + appAttemptIdStr +
          jobIdStr + stageIdStr + stageAttemptIdStr + taskIdStr + 
taskAttemptNumberStr
     
    +  lazy val conf = new Configuration
    +
       /**
        * Set up the caller context [[context]] by invoking Hadoop 
CallerContext API of
        * [[org.apache.hadoop.ipc.CallerContext]], which was added in hadoop 
2.8.
        */
       def setCurrentContext(): Boolean = {
    -    var succeed = false
    -    try {
    -      // scalastyle:off classforname
    -      val callerContext = 
Class.forName("org.apache.hadoop.ipc.CallerContext")
    -      val Builder = 
Class.forName("org.apache.hadoop.ipc.CallerContext$Builder")
    -      // scalastyle:on classforname
    -      val builderInst = 
Builder.getConstructor(classOf[String]).newInstance(context)
    -      val hdfsContext = Builder.getMethod("build").invoke(builderInst)
    -      callerContext.getMethod("setCurrent", callerContext).invoke(null, 
hdfsContext)
    -      succeed = true
    -    } catch {
    -      case NonFatal(e) => logInfo("Fail to set Spark caller context", e)
    +    if (!CallerContext.callerContextSupported) {
    --- End diff --
    
    Alright, so the main purpose to use this flag is to avoid re-executing the 
codes below. So AFAIK unless we can guarantee the sequentially calling of this 
method, otherwise `callerContextSupported` may introduce concurrent issue, 
especially in `Task` running code. Simultaneously started running tasks will 
possibly get the flag as `true` and executing below code, which makes this flag 
useless.
    
    Also in AM/Client, it will only be called once, so this flag is not used at 
those JVMs.


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