[英]Linux Kernel Where to lock and unlock semaphores?
在 Linux 內核中(特別是對於設備驅動程序),我如何知道要鎖定哪些變量以及何時需要鎖定? 特別是,為什么下面代碼中的鎖定只發生在 dev 被設置之后,即使 dev 指向一個全局變量 scull_devices?
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 的代碼以獲得更完整的圖片,這里是:
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);
}
示例中的鎖定與全局scull_devices
變量無關,但鎖定用於保護一個scull_dev
屬性。
例如,假設存在read()
操作從data
復制size
字節,而提到的scroll_trim()
操作釋放data
。
所以,當進程#1通話open()
和處理#2試圖read()
在同一時間從已經打開的裝置,所述read()
運算可存取釋放data
和oopses。
這就是您需要保護數據免受競爭的原因。 信號量是一種方式; 互斥另一個通常更合適的。 自旋鎖和原子變量也可能起作用。
鎖定 - 這是保護臨界區的方法
臨界區 - 在您的驅動程序代碼中,如果多個實例正在訪問同一區域,那就是臨界區。
多個實例 - 它可以是線程、常規 ioctl cmd(來自用戶空間)以及 softirq 和 irq。 這取決於您的驅動程序實現。
基於“上下文”,您也應該使用不同的鎖。
可以休眠的線程上下文 -> 信號量/互斥量非休眠上下文 -> 自旋鎖軟中斷,tasklet -> spin_lock_bh irq -> spin_lock_irq,spin_lock_irqsave
它完全基於您的要求。
讓我們舉個例子。 如果您正在開發網絡驅動程序,您的 netdev 具有統計信息和數據包緩沖區,並且這些需要由鎖保護,因為它可以由多個實例更新,例如來自用戶空間的 net_rx_softirq、net_tx_softirq、ioctl/netlink 請求等等。
在這種情況下,根據資源的上下文,您需要使用不同的鎖/互斥鎖,有時您需要 1 個以上的鎖。
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