jdk9 VarHandle and Fence methods

[Rémi Forax]

Hi Doug,
is there a plan to update AtomicReferenceFieldUpdater (and variants) implementation too, by moving the implementation in java.lang.invoke and use the very same tricks that VarHandle implementations use ?

Rémi


Le 21 août 2015 16:24:06 CEST, Doug Lea <dl at cs.oswego.edu> a écrit :
>
>For those of you who haven't been following this on the jmm-dev list
>or follow-ups on concurrency-interest:
>
>We are NOT planning a full JMM memory model update for jdk9 -- there
>are still unresolved issues in revising the formal core model.
>However, jdk9 will include APIs for VarHandles (a scaled-down version
>of
>the "enhanced volatiles" JEP), that together with some other
>support replaces the need to use of Unsafe to obtain these
>effects and does so compatibly with C/C++11 atomics. To do this, we
>must, until the next full JMM update, provide specs that are
>clear to readers (at least those familiar with the underlying
>concepts) but not formally tied to a base model, (Which limits
>how much we can say in them.)
>
>The tentative methods are pasted below and also at
>   http://gee.cs.oswego.edu/dl/wwwtmp/Fodder.java
>The javadocs currently do not include any examples helping to
>explain why you would ever want to use any of these methods.
>
>The actual VarHandle class will look a bit different because
>it will rely on specializations of polymorphic signatures.
>(Current versions can be found in the openjdk mercurial
>"jdk9/sandbox" repo.) And the "Fences" method may end up
>in a different java.lang.* utility class. We'd also retrofit
>j.u.c.atomic.Atomic* classes to use compatible methods/names.
>
>Comments welcome.
>
>...
>
>/**
>  * Stand-in for spec purposes of jdk9 java.lang.invoke.VarHandle
>  */
>abstract class NotReallyVarHandle<T> {
>     // Load
>
>     /**
>      * Returns the value, with memory semantics of reading a
>      * non-volatile variable.
>      *
>      * @return the value
>      */
>     T getRelaxed(Object owner);
>
>     /**
>      * Returns the value, with memory semantics of reading a volatile
>      * variable.
>      *
>      * @return the value
>      */
>     T getVolatile(Object owner);
>
>     /**
>      * Returns the value, and ensures that subsequent loads and stores
>      * are not reordered before this access.
>      *
>      * @apiNote Ignoring the many semantic differences from C and
>      * C++, this method has memory ordering effects compatible with
>      * memory_order_acquire ordering.
>      *
>      * @return the value
>      */
>     T getAcquire(Object owner);
>
>     /**
>      * Returns the value, accessed in program order, but with no
>      * assurance of memory ordering effects with respect to other
>      * threads.
>      *
>      * @return the value
>      */
>     T getOpaque(Object owner);
>
>     // Store
>
>     /**
>      * Sets the value, with memory semantics of setting a non-volatile
>      * variable.
>      *
>      * @param val the new value
>      */
>     void setRelaxed(Object owner, T val);
>
>     /**
>      * Sets the value, with memory semantics of setting a volatile
>      * variable.
>      *
>      * @apiNote Ignoring the many semantic differences from C and
>      * C++, this method has memory ordering effects compatible with
>      * memory_order_seq_cst.
>      *
>      * @param val the new value
>      */
>     void setVolatile(Object owner, T val);
>
>     /**
>      * Sets the value, and ensures that prior loads and stores are not
>      * reordered after this access.
>      *
>      * @apiNote Ignoring the many semantic differences from C and
>      * C++, this method has memory ordering effects compatible with
>      * memory_order_release ordering.
>      *
>      * @param val the new value
>      */
>     void setRelease(Object owner, T val);
>
>     /**
>      * Sets the value, in program order, but with no assurance of
>      * memory ordering effects with respect to other threads.
>      *
>      * @param val the new value
>      */
>     void setOpaque(Object owner, T val);
>
>     // CAS
>
>     /**
>      * Atomically sets the value to the given updated value with the
>      * memory semantics of setVolatile if the current value {@code ==}
>      * the expected value, as accessed with the memory semantics of
>      * getVolatile.
>      *
>      * @param expected the expected value
>      * @param val the new value
>      * @return {@code true} if successful. False return indicates that
>      * the actual value was not equal to the expected value.
>      */
>     boolean compareAndSet(Object owner, T expected, T val);
>
>     // Value-returning compare and exchange
>
>     /**
>      * Atomically sets the value to the given updated value with the
>      * memory semantics of setVolatile if the current value {@code ==}
>      * the expected value, as accessed with the memory semantics of
>      * getVolatile.
>      *
>      * @param expected the expected value
>      * @param val the new value
>  * @return the current value, which will be the same as {@code val} if
>      * successful.
>      */
>     T compareAndExchangeVolatile(Object owner, T expected, T val);
>
>     /**
>      * Atomically sets the value to the given updated value with the
>      * memory semantics of setRelaxed if the current value {@code ==}
>      * the expected value, as accessed with the memory semantics of
>      * getAcquire.
>      *
>      * @param expected the expected value
>      * @param val the new value
>  * @return the current value, which will be the same as {@code val} if
>      * successful.
>      */
>     T compareAndExchangeAcquire(Object owner, T expected, T val);
>
>     /**
>      * Atomically sets the value to the given updated value with the
>      * memory semantics of setRelease if the current value {@code ==}
>      * the expected value, as accessed with the memory samantics of
>      * getRelease.
>      *
>      * @param expected the expected value
>      * @param val the new value
>  * @return the current value, which will be the same as {@code val} if
>      * successful.
>      */
>     T compareAndExchangeRelease(Object owner, T expected, T val);
>
>     // Weak (spurious failures allowed)
>
>     /**
>      * Possibly atomically sets the value to the given updated value
>      * with the semantics of setRelaxed if the current value {@code
>      * ==} the expected value, as as accessed with the memory
>      * semantics of getRelaxed.  This operation may fail spuriously
>      * (typically, due to memory contention) even if the current value
>      * does match the expected value.
>      *
>      * @param expected the expected value
>      * @param val the new value
>      * @return {@code true} if successful
>      */
>     boolean weakCompareAndSetRelaxed(Object owner, T expected, T val);
>
>     /**
>      * Possibly atomically sets the value to the given updated value
>      * with the memory semantics of setRelaxed if the current value
>      * {@code ==} the expected value, as as accessed with the memory
>      * semantics of getAcquire.  This operation may fail spuriously
>      * (typically, due to memory contention) even if the current value
>      * does match the expected value.
>      *
>      * @param expected the expected value
>      * @param val the new value
>      * @return {@code true} if successful
>      */
>     boolean weakCompareAndSetAcquire(Object owner, T expected, T val);
>
>     /**
>      * Possibly atomically sets the value to the given updated value
>      * with the memory semantics of setRelease if the current value
>      * {@code ==} the expected value, as as accessed with the memory
>      * semantics of getRelaxed.  This operation may fail spuriously
>      * (typically, due to memory contention) even if the current value
>      * does match the expected value.
>      *
>      * @param expected the expected value
>      * @param val the new value
>      * @return {@code true} if successful
>      */
>     boolean weakCompareAndSetRelease(Object owner, T expected, T val);
>
>     // special RMW
>
>     /**
>      * Atomically sets to the given value with the memory semantics of
>      * setVolatile and returns the old value.
>      *
>      * @param newValue the new value
>      * @return the previous value
>      */
>     T getAndSet(Object owner, T val);
>
>     /**
>      * Atomically adds the given value to the current value with the
>      * memory semantics of setVolatile.
>      *
>      * @param delta the value to add
>      * @return the previous value
>      */
>     T getAndAdd(Object owner, T delta);
>
>     /**
>      * Atomically adds the given value to the current value with the
>      * memory semantics of setVolatile.
>      *
>      * @param delta the value to add
>      * @return the current value
>      */
>     T addAndGet(Object owner, T delta);
>}
>
>/**
>  * A set of methods providing fine-grained control of memory ordering.
>  *
>  * <p>The Java Language Specification permits operations to be
>  * executed in orders different than are apparent in program source
>  * code, subject to constraints mainly arising from the use of locks
>  * and volatile fields. The methods of this class can also be used to
>  * impose constraints. Their specifications are phrased in terms of
>  * the lack of "reorderings" -- observable ordering effects that might
>  * otherwise occur if the fence were not present.
>  *
>  * @apiNote More precise phrasing of these specifications may
>  * accompany future updates of the Java Language Specification.
>  */
>public class Fences {
>
>     /**
>      * Ensures that loads and stores before the fence will not be
>      * reordered with loads and stores after the fence.
>      *
>      * @apiNote Ignoring the many semantic differences from C and
>      * C++, this method has memory ordering effects compatible with
>      * atomic_thread_fence(memory_order_seq_cst)
>      */
>     public static void fullFence() {}
>
>     /**
>      * Ensures that loads before the fence will not be reordered with
>      * loads and stores after the fence.
>      *
>      * @apiNote Ignoring the many semantic differences from C and
>      * C++, this method has memory ordering effects compatible with
>      * atomic_thread_fence(memory_order_acquire)
>      */
>     public static void acquireFence() {}
>
>     /**
>      * Ensures that loads and stores before the fence will not be
>      * reordered with stores after the fence.
>      *
>      * @apiNote Ignoring the many semantic differences from C and
>      * C++, this method has memory ordering effects compatible with
>      * atomic_thread_fence(memory_order_release)
>      */
>     public static void releaseFence() {}
>
>     /**
>      * Ensures that loads before the fence will not be reordered with
>      * loads after the fence.
>      */
>     public static void loadLoadFence() {}
>
>     /**
>      * Ensures that stores before the fence will not be reordered with
>      * stores after the fence.
>      */
>     public static void storeStoreFence() {}
>
>}
>
>class java.lang.ref.Reference {
>     // add:
>
>     /**
>      * Ensures that the object referenced by the given reference
>      * remains <em>strongly reachable</em> (as defined in the {@link
>      * java.lang.ref} package documentation), regardless of any prior
>      * actions of the program that might otherwise cause the object to
>      * become unreachable; thus, the referenced object is not
>      * reclaimable by garbage collection at least until after the
>      * invocation of this method. Invocation of this method does not
>      * itself initiate garbage collection or finalization.
>      *
>      * @param ref the reference. If null, this method has no effect.
>      */
>     public static void reachabilityFence(Object ref) {}
>
>}