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假设我有一个 (base,offset) 元组指向的值。

例如

class Data{
    int x = 0;
}

class Accessor{
    public Data data;
    public Object base$x;
    public long off$x;
    public static final Unsafe unsafe;

    public void run(){
        data = new Data();

        base$x = data;
        off$x = 12;
        unsafe.putInt(base$x,off$x,1);

        assert(data.x == 1);
    }

    static{
        try {
            Constructor<Unsafe> unsafeConstructor = Unsafe.class.getDeclaredConstructor();
            unsafeConstructor.setAccessible(true);
            unsafe = unsafeConstructor.newInstance();
        } catch (NoSuchMethodException | InvocationTargetException | IllegalAccessException | InstantiationException e) {
            throw new RuntimeException(e);
        }
    }
}

（可运行版本：https ://ideone.com/OasQrh ）

我现在想执行

unsafe.setInt(base$x,off$x,1);

在C.

为什么？因为不同的进程可能会移动数据并改变base并off指向新的位置。我想利用英特尔 RTM 来确保移动数据不会干扰对该字段的常规访问。

所以让我们创建一个新类

class Transactionally{
     static{
         System.loadLibrary("RTM_transact");
     }
     public static native void setInt(Object target, String fieldname);
}

并更换

class Accessor{
    public Data data;
    public Object base;
    public long off;
    public static final Unsafe unsafe;

    public void run(){
        data = new Data();

        base$x = data;
        off$x = 12;

        Transactionally.setInt(this,"x",1);

        assert(data.x == 1);
    }

    static{
        try {
            Constructor<Unsafe> unsafeConstructor = Unsafe.class.getDeclaredConstructor();
            unsafeConstructor.setAccessible(true);
            unsafe = unsafeConstructor.newInstance();
        } catch (NoSuchMethodException | InvocationTargetException | IllegalAccessException | InstantiationException e) {
            throw new RuntimeException(e);
        }
    }
}

我们通过 javah 运行它以获取标头，将方法签名复制到 anRTM_transct.cpp并获得类似

#include<jni.h>
#include "rtmbenchmark_Transactionally.h"
#include <string>

std::string fldBase("base$");
std::string fldOff("off$");

/*
* Class:     rtmbenchmark_Transactionally
* Method:    setInt
* Signature: (Ljava/lang/Object;Ljava/lang/String;I)V
*/
JNIEXPORT void JNICALL Java_rtmbenchmark_Transactionally_setInt(JNIEnv *env, jclass _obsolete, jobject target, jstring fieldname, jint value){
    jclass targetClass = (*env)->GetObjClass(env,target);
    jfieldID fidFLDbase = (*env)->GetFieldID(env, targetClass, fldBase + fieldname, "Ljava.lang.Object");
    jfieldid fidFLDoff = (*env)->GetFieldID(env, targetClass, fldOff + fieldname, "J");


    volatile int tries = 0;
    volatile boolean success = 0;
    while(!success){/*TODO: maybe switch to a different strategy when tries grows too large?*/
        __asm__ __volatile__ (
            "xbegin 1f" /*1f: local label 1, look forward to find first*/
                :"+rm"(tries) /*artificial dependency to prevent re-ordering*/
        );
        ++tries;

        jobject base = (*env)->GetIntField(env,targetClass,fidFLDbase);
        jlong offset = (*env)->GetLongField(env,targetClass,fidFLDoff);
        //??? ==> unsafe.setLong(base,offset,value)

        __asm__ __volatile__ (
            "xend\n\t"
            "incl %0\n" /*increment success ==> break out of loop*/
            "jmp 2f\n" /*jump to end of loop*/
            "1:\n\t" /*local label 1 (jumped to when transaction is aborted)*/
            "2:" /*local label 2 (jumped to after transaction succeeded)*/
            :"+rm"(success)
            :"rm"(tries) /*artificial dependency*/
        );
    }
}

在交易主体中，我用什么来处理不安全的？

事务的“核心”是（应该）对应于 Accessor 过去所做的这三行：

        jobject base = (*env)->GetIntField(env,targetClass,fidFLDbase);
        jlong offset = (*env)->GetLongField(env,targetClass,fidFLDoff);
        //??? ==> unsafe.setInt(base,offset,value)

)

理想情况下，当然，我想做类似的事情base[offset] = value，但我非常怀疑这会奏效。

我在这里为 unsafe.setInt 放什么？

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0 回答 0