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  1. JDK
  2. JDK-6516512

HotSpot:thread terminology should be clearer or cleaned up.

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    Details

    • Type: Enhancement
    • Status: Closed
    • Priority: P4
    • Resolution: Won't Fix
    • Affects Version/s: 7
    • Fix Version/s: 10
    • Component/s: hotspot
    • Subcomponent:
    • CPU:
      generic
    • OS:
      generic

      Description

      The terminology used in the HotSpot sources are confusing to casual readers and
      people who port HotSpot to various platforms, with the choice of terms for
      the intial thread vs main thread vs primordial thread. See comments for more
      details.

      From the licensee......
      [Edited stuff]
      B) As can be seen from the using code, the intention of the
          procedure `os::Linux::is_initial_thread' (and similar ones on the
          other platforms) is to check for the primordial thread (the thread
          that installed the pthread library) rather than for the initial
          thread (the thread that called `JNI_CreateJavaVM'). But the
          implementation erroneously checks for the latter (see code).

          Before jse 6 the launcher used the primordial thread to create the
          jvm, in which case the primordial thread was equal to the initial
          thread. So `is_initial_thread' got this case right, but could
          fail on client code that did not maintain this invariant.

          Since jse 6 the launcher does not use the primordial thread
          anymore to create the jvm, but rather uses a delegate thread. So
          `os::Linux::is_initial_thread' erroneously considers the initial
          thread as the primordial one. It thus fails to properly recognize
          the primordial thread that might still enter (despite the launcher
          avoiding it) as described in A.

      [Edited Stuff]
      When `JNI_CreateJavaVM' is called, the `capture_initial_stack'
      procedure stores the stack parameters of the current thread to
      `_initial_thread_stack_size' and `_initial_thread_stack_bottom' (if it
      does anything useful at all. It might also collect nonsense when it
      is not called from the primordial thread. Apparently it was designed
      for the case that it gets called from the primordial thread). It does
      not actually know, whether the calling thread is the primordial one or
      not.

      The procedure `is_initial_thread' will return `true' exactly if the
      stack pointer of the calling thread is within the stack region
      determined by `capture_initial_stack'.

      The procedure `AttachCurrentThread' tries to record the stack
      parameters of the calling thread. It does so using
      `record_stack_base_and_size', which in turn calls `is_initial_thread'.
      If this returns `true', then `record_stack_base_and_size' will reuse
      the cached stack parameters `_initial_thread_stack_...'. Otherwise,
      `pthread_getattr_np' will be called on `pthread_self()'.

      Now, calling `pthread_getattr_np(pthread_self(), &attr)' within the
      primordial thread is undefined, because calling `pthread_self()' is
      undefined, because the primordial thread is not a pthread, by
      definition.

      So if `JNI_CreateJavaVM' was called by a non-primordial thread
      (e.g. as done by the launcher in jse 6), subsequent calls to
      `AttachCurrentThread' from the primordial thread will lead to
      undefined behavior, because the primordial thread will be mistaken for
      a non-primordial thread.

      It is clear that the only clean solutions are

        A) to use a different thread implementation which provides
        reliable info on the primordial thread as well,

        B) to disallow the primordial thread to enter the vm.
        However, it looks as if this can not be cleanly checked
        within the code on linux and must be a precondition imposed
        on the client.
      -----------------------------

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              Assignee:
              Unassigned Unassigned
              Reporter:
              ksrini Kumar Srinivasan
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