在C中实现互斥锁

Question

我试图在C中制作一个非常简单的自旋锁互斥锁,由于某种原因我得到两个线程同时获得锁定的情况,这是不可能的.它运行在多处理器系统上,这可能是出现问题的原因.任何想法为什么它不起作用？

void mutexLock(mutex_t *mutexlock, pid_t owner)
{
int failure = 1;
while(mutexlock->mx_state == 0 || failure || mutexlock->mx_owner != owner)
{
    failure = 1;
    if (mutexlock->mx_state == 0)
    {
        asm(
        "movl    $0x01,%%eax\n\t"      // move 1 to eax
        "xchg    %%eax,%0\n\t"         // try to set the lock bit
        "mov     %%eax,%1\n\t"         // export our result to a test var
        :"=r"(mutexlock->mx_state),"=r"(failure)
        :"r"(mutexlock->mx_state)
        :"%eax"
        );
    }
    if (failure == 0)
    {
        mutexlock->mx_owner = owner; //test to see if we got the lock bit
    }
    } 
}

Answer 1

好吧,首先你要failure在第一次while()执行条件时测试一个未初始化的变量().

你的实际问题是你告诉gcc使用寄存器mx_state- 这显然不适用于自旋锁.尝试:

    asm volatile (
    "movl    $0x01,%%eax\n\t"      // move 1 to eax
    "xchg    %%eax,%0\n\t"         // try to set the lock bit
    "mov     %%eax,%1\n\t"         // export our result to a test var
    :"=m"(mutexlock->mx_state),"=r"(failure)
    :"m"(mutexlock->mx_state)
    :"%eax"
    );

请注意,asm volatile此处也很重要,以确保它不会从while循环中提升.