Unsafe.park/unpark, j.u.c.LockSupport and Thread

10 Feb 2015

      Hi,

As part of the effort around Unsafe (some may have noticed some cleanup work) i have been recently looking at the park/unpark methods.

The class java.util.concurrent.locks.LockSupport [1] has some thin public wrappers around these methods:

    public static void unpark(Thread thread) {
        if (thread != null)
            U.unpark(thread);
    }

    public static void park() {
        U.park(false, 0L);
    }

    public static void parkNanos(long nanos) {
        if (nanos > 0)
            U.park(false, nanos);
    }

    public static void parkUntil(long deadline) {
        U.park(true, deadline);
    }

Is not clear to me what is exactly unsafe about park/unpark and why they were not originally placed on Thread itself given the above wrapping.

There is mention of unpark being unsafe with regards to native code and ensuring the thread has not been destroyed: 

  /**
   * Unblock the given thread blocked on <tt>park</tt>, or, if it is
   * not blocked, cause the subsequent call to <tt>park</tt> not to
   * block.  Note: this operation is "unsafe" solely because the
   * caller must somehow ensure that the thread has not been
   * destroyed. Nothing special is usually required to ensure this
   * when called from Java (in which there will ordinarily be a live
   * reference to the thread) but this is not nearly-automatically
   * so when calling from native code.
   * @param thread the thread to unpark.
   *
   */
  public native void unpark(Object thread);

However, native code is anyway inherently unsafe.

In addition this class has a cosy relationship with Thread (it wants to poke into Thread's non-public fields):

    // Hotspot implementation via intrinsics API
    private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
    private static final long PARKBLOCKER;
    private static final long SEED;
    private static final long PROBE;
    private static final long SECONDARY;
    static {
        try {
            PARKBLOCKER = U.objectFieldOffset
                (Thread.class.getDeclaredField("parkBlocker"));
            SEED = U.objectFieldOffset
                (Thread.class.getDeclaredField("threadLocalRandomSeed"));
            PROBE = U.objectFieldOffset
                (Thread.class.getDeclaredField("threadLocalRandomProbe"));
            SECONDARY = U.objectFieldOffset
                (Thread.class.getDeclaredField("threadLocalRandomSecondarySeed"));
        } catch (ReflectiveOperationException e) {
            throw new Error(e);
        }
    }

Although only PARKBLOCKER and SECONDARY are used AFAICT.

I am sure there is some history behind all this... but in my ignorance of the past perhaps it's time to reconsider?

We could reduce the coupling on Thread and dependency on Unsafe if we consider moving park/unpark and LockSupport functionality to Thread itself.

Thoughts?

Paul.

[1] http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concu...

Unsafe.park/unpark, j.u.c.LockSupport and Thread

Paul Sandoz