c - Linux Kernel 在哪里锁定和解锁信号量？

Question

在 Linux 内核(专门针对设备驱动程序)中，我如何知道要锁定哪些变量以及何时需要锁定？特别是，即使 dev 指向全局变量 scull_devices，为什么以下代码中的锁定只发生在设置 dev 之后？

struct scull_qset {
    void **data;    /* pointer to an array of pointers which each point to a quantum buffer */
    struct scull_qset *next;
};

struct scull_dev {
    struct scull_qset *data;  /* Pointer to first quantum set */
    int quantum;              /* the current quantum size */
    int qset;                 /* the current array size */
    unsigned long size;       /* amount of data stored here */
    unsigned int access_key;  /* used by sculluid and scullpriv */
    struct semaphore sem;     /* mutual exclusion semaphore */
    struct cdev cdev;         /* Char device structure initialized in scull_init_module */
};

struct scull_dev *scull_devices;  /* allocated dynamically in scull_init_module */

int scull_open(struct inode *inode, struct file *filp)
{
    struct scull_dev *dev; /* device information */

    dev = container_of(inode->i_cdev, struct scull_dev, cdev);
    filp->private_data = dev; /* for other methods */

    /* now trim to 0 the length of the device if open was write-only */
    if ( (filp->f_flags & O_ACCMODE) == O_WRONLY) {
        if (down_interruptible(&dev->sem))
            return -ERESTARTSYS;
        scull_trim(dev); /* empty out the scull device */
        up(&dev->sem);
    }
    return 0;          /* success */
}

如果需要 scull_init_module 的代码以获得更完整的图片，这里是:

int scull_major = SCULL_MAJOR;
int scull_minor = 0;
int scull_quantum = SCULL_QUANTUM;
int scull_qset = SCULL_QSET;
int scull_nr_devs = SCULL_NR_DEVS;


int scull_init_module(void)
{
    int result, i;
    dev_t dev = 0;

    /* assigns major and minor numbers (left out for brevity sake) */

    /* 
     * allocate the devices -- we can't have them static, as the number
     * can be specified at load time
     */
    scull_devices = kmalloc(scull_nr_devs * sizeof(struct scull_dev), GFP_KERNEL);
    if (!scull_devices) {
        result = -ENOMEM;
        goto fail; 
    }
    memset(scull_devices, 0, scull_nr_devs * sizeof(struct scull_dev));

    /* Initialize each device. */
    for (i = 0; i < scull_nr_devs; i++) {
        scull_devices[i].quantum = scull_quantum;
        scull_devices[i].qset = scull_qset;
        init_MUTEX(&scull_devices[i].sem);
        scull_setup_cdev(&scull_devices[i], i);
    }

    /* some other stuff left out for brevity sake */

    return 0; /* succeed */

  fail:            /* isn't this a little redundant? */
    scull_cleanup_module();
    return result;
}


/*
 * Set up the char_dev structure for this device.
 */
static void scull_setup_cdev(struct scull_dev *dev, int index)
{
    int err, devno = MKDEV(scull_major, scull_minor + index);

    cdev_init(&dev->cdev, &scull_fops);
    dev->cdev.owner = THIS_MODULE;
    dev->cdev.ops = &scull_fops;
    err = cdev_add (&dev->cdev, devno, 1);
    /* Fail gracefully if need be */
    if (err)
        printk(KERN_NOTICE "Error %d adding scull%d", err, index);
}

Answer 1

示例中的锁定与全局scull_devices变量无关，但锁定用于保护一个scull_dev的属性。

例如假设存在一个 read() 操作，它从 data 复制 size 字节，而提到的 scroll_trim() 操作释放数据。

因此，当进程 #1 调用 open() 并且进程 #2 尝试同时从已打开的设备中read() 时， read() 操作可以访问释放的 data 和 oopses。

这就是为什么您需要保护数据免受竞争。信号量是一种方式；互斥另一个通常更合适的。自旋锁和原子变量也可能有效。