X-ScopedMemoryAccess.java.template

/*
 * Copyright (c) 2020, 2021, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package jdk.internal.misc;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.ref.Reference;
import java.io.FileDescriptor;

import jdk.internal.access.SharedSecrets;
import jdk.internal.util.ArraysSupport;
import jdk.internal.vm.annotation.ForceInline;


/**
 * This class defines low-level methods to access on-heap and off-heap memory. The methods in this class
 * can be thought of as thin wrappers around methods provided in the {@link Unsafe} class. All the methods in this
 * class, accept one or more {@link Scope} parameter, which is used to validate as to whether access to memory
 * can be performed in a safe fashion - more specifically, to ensure that the memory being accessed has not
 * already been released (which would result in a hard VM crash).
 * <p>
 * Accessing and releasing memory from a single thread is not problematic - after all, a given thread cannot,
 * at the same time, access a memory region <em>and</em> free it. But ensuring correctness of memory access
 * when multiple threads are involved is much trickier, as there can be cases where a thread is accessing
 * a memory region while another thread is releasing it.
 * <p>
 * This class provides tools to manage races when multiple threads are accessing and/or releasing the same memory
 * region concurrently. More specifically, when a thread wants to release a memory region, it should call the
 * {@link #closeScope(jdk.internal.misc.ScopedMemoryAccess.Scope)} method provided by this class. This method initiates
 * thread-local handshakes with all the other VM threads, which are then stopped one by one. If any thread is found
 * accessing memory that is associated to the very scope object being closed, that thread execution is asynchronously
 * interrupted with a {@link Scope.ScopedAccessError}.
 * <p>
 * This synchronization strategy relies on the idea that accessing memory is atomic with respect to checking the
 * validity of the scope associated with that memory region - that is, a thread that wants to perform memory access will be
 * suspended either <em>before</em> a scope check or <em>after</em> the memory access. To ensure this atomicity,
 * all methods in this class are marked with the special {@link Scoped} annotation, which is recognized by the VM,
 * and used during the thread-local handshake to detect (and stop) threads performing potentially problematic memory access
 * operations. Additionally, to make sure that the scope object(s) of the memory being accessed is always
 * reachable during an access operation, all the methods in this class add reachability fences around the underlying
 * unsafe access.
 * <p>
 * This form of synchronization allows APIs to use plain memory access without any other form of synchronization
 * which might be deemed to expensive; in other words, this approach prioritizes the performance of memory access over
 * that of releasing a shared memory resource.
 */
public class ScopedMemoryAccess {

    private static final Unsafe UNSAFE = Unsafe.getUnsafe();

    private static native void registerNatives();
    static {
        registerNatives();
    }

    public boolean closeScope(Scope scope) {
        return closeScope0(scope, Scope.ScopedAccessError.INSTANCE);
    }

    native boolean closeScope0(Scope scope, Scope.ScopedAccessError exception);

    private ScopedMemoryAccess() {}

    private static final ScopedMemoryAccess theScopedMemoryAccess = new ScopedMemoryAccess();

    public static ScopedMemoryAccess getScopedMemoryAccess() {
        return theScopedMemoryAccess;
    }

    /**
     * Scope interface used during scoped memory access operations. A scope can be thought of as an object
     * which embodies the temporal checks associated with a given memory region.
     */
    public interface Scope {

       interface Handle {
            Scope scope();
        }

        void checkValidState();

        Thread ownerThread();

        boolean isImplicit();

        Handle acquire();

        void release(Handle handle);

        /**
         * Error thrown when memory access fails because the memory has already been released.
         * Note: for performance reasons, this exception is never created by client; instead a shared instance
         * is thrown (sometimes, this instance can be thrown asynchronously inside VM code). For this reason,
         * it is important for clients to always catch this exception and throw a regular exception instead
         * (which contains full stack information).
         */
        final class ScopedAccessError extends Error {
            private ScopedAccessError() {
                super("Attempt to access an already released memory resource", null, false, false);
            }
            static final long serialVersionUID = 1L;

            public static final ScopedAccessError INSTANCE = new ScopedAccessError();
        }
    }

    @Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
    @Retention(RetentionPolicy.RUNTIME)
    @interface Scoped { }

    // bulk ops

    @ForceInline
    public void copyMemory(Scope srcScope, Scope dstScope,
                                   Object srcBase, long srcOffset,
                                   Object destBase, long destOffset,
                                   long bytes) {
          try {
              copyMemoryInternal(srcScope, dstScope, srcBase, srcOffset, destBase, destOffset, bytes);
          } catch (Scope.ScopedAccessError ex) {
              throw new IllegalStateException("This segment is already closed");
          }
    }

    @ForceInline @Scoped
    private void copyMemoryInternal(Scope srcScope, Scope dstScope,
                               Object srcBase, long srcOffset,
                               Object destBase, long destOffset,
                               long bytes) {
        try {
            if (srcScope != null) {
                srcScope.checkValidState();
            }
            if (dstScope != null) {
                dstScope.checkValidState();
            }
            UNSAFE.copyMemory(srcBase, srcOffset, destBase, destOffset, bytes);
        } finally {
            Reference.reachabilityFence(srcScope);
            Reference.reachabilityFence(dstScope);
        }
    }

    @ForceInline
    public void copySwapMemory(Scope srcScope, Scope dstScope,
                                   Object srcBase, long srcOffset,
                                   Object destBase, long destOffset,
                                   long bytes, long elemSize) {
          try {
              copySwapMemoryInternal(srcScope, dstScope, srcBase, srcOffset, destBase, destOffset, bytes, elemSize);
          } catch (Scope.ScopedAccessError ex) {
              throw new IllegalStateException("This segment is already closed");
          }
    }

    @ForceInline @Scoped
    private void copySwapMemoryInternal(Scope srcScope, Scope dstScope,
                               Object srcBase, long srcOffset,
                               Object destBase, long destOffset,
                               long bytes, long elemSize) {
        try {
            if (srcScope != null) {
                srcScope.checkValidState();
            }
            if (dstScope != null) {
                dstScope.checkValidState();
            }
            UNSAFE.copySwapMemory(srcBase, srcOffset, destBase, destOffset, bytes, elemSize);
        } finally {
            Reference.reachabilityFence(srcScope);
            Reference.reachabilityFence(dstScope);
        }
    }

    @ForceInline
    public void setMemory(Scope scope, Object o, long offset, long bytes, byte value) {
        try {
            setMemoryInternal(scope, o, offset, bytes, value);
        } catch (Scope.ScopedAccessError ex) {
            throw new IllegalStateException("This segment is already closed");
        }
    }

    @ForceInline @Scoped
    private void setMemoryInternal(Scope scope, Object o, long offset, long bytes, byte value) {
        try {
            if (scope != null) {
                scope.checkValidState();
            }
            UNSAFE.setMemory(o, offset, bytes, value);
        } finally {
            Reference.reachabilityFence(scope);
        }
    }

    @ForceInline
    public int vectorizedMismatch(Scope aScope, Scope bScope,
                                             Object a, long aOffset,
                                             Object b, long bOffset,
                                             int length,
                                             int log2ArrayIndexScale) {
        try {
            return vectorizedMismatchInternal(aScope, bScope, a, aOffset, b, bOffset, length, log2ArrayIndexScale);
        } catch (Scope.ScopedAccessError ex) {
            throw new IllegalStateException("This segment is already closed");
        }
    }

    @ForceInline @Scoped
    private int vectorizedMismatchInternal(Scope aScope, Scope bScope,
                                             Object a, long aOffset,
                                             Object b, long bOffset,
                                             int length,
                                             int log2ArrayIndexScale) {
        try {
            if (aScope != null) {
                aScope.checkValidState();
            }
            if (bScope != null) {
                bScope.checkValidState();
            }
            return ArraysSupport.vectorizedMismatch(a, aOffset, b, bOffset, length, log2ArrayIndexScale);
        } finally {
            Reference.reachabilityFence(aScope);
            Reference.reachabilityFence(bScope);
        }
    }

    @ForceInline
    public boolean isLoaded(Scope scope, long address, boolean isSync, long size) {
        try {
            return isLoadedInternal(scope, address, isSync, size);
        } catch (Scope.ScopedAccessError ex) {
            throw new IllegalStateException("This segment is already closed");
        }
    }

    @ForceInline @Scoped
    public boolean isLoadedInternal(Scope scope, long address, boolean isSync, long size) {
        try {
            if (scope != null) {
                scope.checkValidState();
            }
            return SharedSecrets.getJavaNioAccess().isLoaded(address, isSync, size);
        } finally {
            Reference.reachabilityFence(scope);
        }
    }

    @ForceInline
    public void load(Scope scope, long address, boolean isSync, long size) {
        try {
            loadInternal(scope, address, isSync, size);
        } catch (Scope.ScopedAccessError ex) {
            throw new IllegalStateException("This segment is already closed");
        }
    }

    @ForceInline @Scoped
    public void loadInternal(Scope scope, long address, boolean isSync, long size) {
        try {
            if (scope != null) {
                scope.checkValidState();
            }
            SharedSecrets.getJavaNioAccess().load(address, isSync, size);
        } finally {
            Reference.reachabilityFence(scope);
        }
    }

    @ForceInline
    public void unload(Scope scope, long address, boolean isSync, long size) {
        try {
            unloadInternal(scope, address, isSync, size);
        } catch (Scope.ScopedAccessError ex) {
            throw new IllegalStateException("This segment is already closed");
        }
    }

    @ForceInline @Scoped
    public void unloadInternal(Scope scope, long address, boolean isSync, long size) {
        try {
            if (scope != null) {
                scope.checkValidState();
            }
            SharedSecrets.getJavaNioAccess().unload(address, isSync, size);
        } finally {
            Reference.reachabilityFence(scope);
        }
    }

    @ForceInline
    public void force(Scope scope, FileDescriptor fd, long address, boolean isSync, long index, long length) {
        try {
            forceInternal(scope, fd, address, isSync, index, length);
        } catch (Scope.ScopedAccessError ex) {
            throw new IllegalStateException("This segment is already closed");
        }
    }

    @ForceInline @Scoped
    public void forceInternal(Scope scope, FileDescriptor fd, long address, boolean isSync, long index, long length) {
        try {
            if (scope != null) {
                scope.checkValidState();
            }
            SharedSecrets.getJavaNioAccess().force(fd, address, isSync, index, length);
        } finally {
            Reference.reachabilityFence(scope);
        }
    }
    // typed-ops here

    // Note: all the accessor methods defined below take advantage of argument type profiling
    // (see src/hotspot/share/oops/methodData.cpp) which greatly enhances performance when the same accessor
    // method is used repeatedly with different 'base' objects.