Peterson算法

Question

在经典Peterson算法，你检查2个旗帜和标志1和标志2进入关键前翻变量section.Will这个工作，如果我检查转，然后再检查标志？

Answer 1

是的，它会工作，如果你第一次检查turn，然后检查flag[0]或flag[1]在条件while()内。

原因是仅当两个条件都为真时才执行繁忙等待。

作为证明我写了一个小的C程序模拟与他们之间的随机切换两个过程。

对于临界区我用这段代码在过程0：

global ^= 0x5555; 
global ^= 0x5555; 
global++; 

并且这过程1：

global ^= 0xAAAA; 
global ^= 0xAAAA; 
global++; 

两个过程来执行这一部分每1000次。如果在两者的关键部分之间存在竞争条件，那么global可能与仿真结束时的2000不同。

代码：

#include <stdio.h> 
#include <stdlib.h> 
#include <time.h> 

typedef enum 
{ 
    InstrNop, 
    InstrHalt, 
    InstrSetVarNum, 
    InstrJumpVarZero, 
    InstrJumpVarNonzero, 
    InstrJump, 
    InstrIncVar, 
    InstrDecVar, 
    InstrXorVarNum, 
} tInstr; 

int ExecuteInstruction(unsigned* Vars, const unsigned* Program, unsigned* Position) 
{ 
    switch (Program[*Position]) 
    { 
    default: 
    case InstrHalt: 
    return 0; 

    case InstrNop: 
    (*Position)++; 
    break; 

    case InstrSetVarNum: 
    Vars[Program[*Position + 1]] = Program[*Position + 2]; 
    (*Position) += 3; 
    break; 

    case InstrXorVarNum: 
    Vars[Program[*Position + 1]] ^= Program[*Position + 2]; 
    (*Position) += 3; 
    break; 

    case InstrJumpVarZero: 
    if (Vars[Program[*Position + 1]] == 0) 
     (*Position) = Program[*Position + 2]; 
    else 
     (*Position) += 3; 
    break; 

    case InstrJumpVarNonzero: 
    if (Vars[Program[*Position + 1]] != 0) 
     (*Position) = Program[*Position + 2]; 
    else 
     (*Position) += 3; 
    break; 

    case InstrJump: 
    (*Position) = Program[*Position + 1]; 
    break; 

    case InstrIncVar: 
    Vars[Program[*Position + 1]]++; 
    (*Position) += 2; 
    break; 

    case InstrDecVar: 
    Vars[Program[*Position + 1]]--; 
    (*Position) += 2; 
    break; 
    } 

    return 1; 
} 

typedef enum 
{ 
    VarGlobal, 

    VarCnt0, 
    VarCnt1, 

    VarFlag0, 
    VarFlag1, 
    VarTurn, 

    VarIdxMax 
} tVarIdx; 

unsigned Vars[VarIdxMax]; 

#define USE_PETERSON 01 
#define SWAP_CHECKS 01 

const unsigned Program0[] = 
{ 
    // cnt0 = 1000; 
    InstrSetVarNum, VarCnt0, 1000, 

// 3: 
#if USE_PETERSON 
    // flag[0] = 1; 
    InstrSetVarNum, VarFlag0, 1, 
    // turn = 1; 
    InstrSetVarNum, VarTurn, 1, 
// 9: 
    // while (flag[1] == 1 && turn == 1) {} 
#if SWAP_CHECKS 
    InstrJumpVarZero, VarTurn, 17, 
    InstrJumpVarZero, VarFlag1, 17, 
#else 
    InstrJumpVarZero, VarFlag1, 17, 
    InstrJumpVarZero, VarTurn, 17, 
#endif 
    InstrJump, 9, 
// 17: 
#endif 

// Critical section starts 
    // global ^= 0x5555; 
    // global ^= 0x5555; 
    // global++; 
    InstrXorVarNum, VarGlobal, 0x5555, 
    InstrXorVarNum, VarGlobal, 0x5555, 
    InstrIncVar, VarGlobal, 
// Critical section ends 

#if USE_PETERSON 
    // flag[0] = 0; 
    InstrSetVarNum, VarFlag0, 0, 
#endif 

    // cnt0--; 
    InstrDecVar, VarCnt0, 
    // if (cnt0 != 0) goto 3; 
    InstrJumpVarNonzero, VarCnt0, 3, 

    // end 
    InstrHalt 
}; 

const unsigned Program1[] = 
{ 
    // cnt1 = 1000; 
    InstrSetVarNum, VarCnt1, 1000, 

// 3: 
#if USE_PETERSON 
    // flag[1] = 1; 
    InstrSetVarNum, VarFlag1, 1, 
    // turn = 0; 
    InstrSetVarNum, VarTurn, 0, 
// 9: 
    // while (flag[0] == 1 && turn == 0) {} 
#if SWAP_CHECKS 
    InstrJumpVarNonzero, VarTurn, 17, 
    InstrJumpVarZero, VarFlag0, 17, 
#else 
    InstrJumpVarZero, VarFlag0, 17, 
    InstrJumpVarNonzero, VarTurn, 17, 
#endif 
    InstrJump, 9, 
// 17: 
#endif 

// Critical section starts 
    // global ^= 0xAAAA; 
    // global ^= 0xAAAA; 
    // global++; 
    InstrXorVarNum, VarGlobal, 0xAAAA, 
    InstrXorVarNum, VarGlobal, 0xAAAA, 
    InstrIncVar, VarGlobal, 
// Critical section ends 

#if USE_PETERSON 
    // flag[1] = 0; 
    InstrSetVarNum, VarFlag1, 0, 
#endif 

    // cnt1--; 
    InstrDecVar, VarCnt1, 
    // if (cnt1 != 0) goto 3; 
    InstrJumpVarNonzero, VarCnt1, 3, 

    // end 
    InstrHalt 
}; 

void Simulate(void) 
{ 
    unsigned pos0 = 0, pos1 = 0; 

    while (Program0[pos0] != InstrHalt || 
     Program1[pos1] != InstrHalt) 
    { 
    int cnt; 

    cnt = rand() % 50; 
    while (cnt--) 
     if (!ExecuteInstruction(Vars, Program0, &pos0)) 
     break; 

    cnt = rand() % 50; 
    while (cnt--) 
     if (!ExecuteInstruction(Vars, Program1, &pos1)) 
     break; 
    } 
} 

int main(void) 
{ 
    srand(time(NULL)); 
    Simulate(); 
    printf("VarGlobal = %u\n", Vars[VarGlobal]); 
    return 0; 
} 

输出（ideone）：

VarGlobal = 2000 

现在，同样的程序与支票作为在Wikipedia，为此，我限定SWAP_CHECKS为0的顺序：输出（ ideone）：

VarGlobal = 2000 

最后，对s如何，有一个竞争条件时，Peterson的算法是残疾人，我定义为USE_PETERSON 0：输出（ideone）：

VarGlobal = 1610