为什么不调用`poll`在sysfs设备属性文件上正确阻塞？

Question

我有一个简单的sysfs设备属性,显示在我的sysfs目录下,并在调用时read返回一个kernelspace变量的值.我想调用poll此属性以允许我的用户空间线程阻塞,直到属性显示的值发生更改.

我的问题是poll似乎没有阻止我的属性 - POLLPRI即使属性显示的值没有改变,它仍然会返回.实际上,我sysfs_notify在内核模块中根本没有调用,但用户空间调用poll仍然没有阻塞.

也许我应该检查比其他东西返回值POLLPRI-但根据文档中的Linux内核,sysfs_poll应该返回POLLERR|POLLPRI:

/* ... When the content changes (assuming the
 * manager for the kobject supports notification), poll will
 * return POLLERR|POLLPRI ...
 */

有什么我忘了做的事poll吗？

---

1. device属性位于:/ sys/class/vilhelm/foo/blah.

2. 我加载了一个名为foo的内核模块,它注册了一个设备,并创建了一个类和这个设备属性.

3. 名为bar的用户空间应用程序生成一个调用poll设备属性的线程,检查POLLPRI.

   • 如果poll返回正数,POLLPRI则返回.
   • 使用fopen和fscan从设备属性文件中读取值.
   • 如果值是42,打印FROM THREAD !!! .

问题是当我期望调用poll无限期阻塞时,消息会不间断地打印出来.问题必然在于poll(其他调用成功42从设备属性获取正确的值).

---

userspace app - bar.c:

#include <stdio.h>

#include <fcntl.h>
#include <poll.h>
#include <pthread.h>
#include <unistd.h>

static void handle_val(unsigned val, FILE *fp);
void * start_val_service(void *arg);

int main(void){
        pthread_t val_serv;
        pthread_create(&val_serv, NULL, &start_val_service, NULL);

        pthread_exit(NULL);
        return 0;

}

static void handle_val(unsigned val, FILE *fp){
        switch(val){
                case 42:
                {
                        printf("FROM THREAD!!!\n");
                        break;
                }

                default:
                        break;
        }
}


void * start_val_service(void *arg){
        struct pollfd fds;
        fds.fd = open("/sys/class/vilhelm/foo/blah", O_RDONLY);
        fds.events = POLLPRI;

        do{
                int ret = poll(&fds, 1, -1);
                if(ret > 0){
                        FILE *fp = fopen("/sys/class/vilhelm/foo/blah", "r");

                        unsigned val;
                        fscanf(fp, "%u", &val);

                        handle_val(val, fp);

                        fclose(fp);
                }
        }while(1);

        close(fds.fd);

        pthread_exit(NULL);
}

内核模块 - foo.c:

#include <linux/device.h>
#include <linux/fs.h>
#include <linux/module.h>
#include <linux/kernel.h>

static dev_t foo_dev;
static struct class *vilhelm;

static unsigned myvar = 42;

static ssize_t unsigned_dev_attr_show(struct device *dev, struct device_attribute *attr, char *buf);

struct unsigned_device_attribute{
        struct device_attribute dev_attr;
        unsigned *ptr;
};

static struct unsigned_device_attribute unsigned_dev_attr_blah = {
        .dev_attr = __ATTR(blah, S_IRUGO, unsigned_dev_attr_show, NULL)
};

static int __init foo_init(void){
        int retval = 0;

        printk(KERN_INFO "HELLO FROM MODULE 1");

        if(alloc_chrdev_region(&foo_dev, 0, 1, "vilhelm") < 0){
                printk(KERN_ERR "foo: unable to register device");
                retval = -1;
                goto out_alloc_chrdev_region;
        }

        vilhelm = class_create(THIS_MODULE, "vilhelm");
        if(IS_ERR(vilhelm)){
                printk(KERN_ERR "foo: unable to create device class");
                retval = PTR_ERR(vilhelm);
                goto out_class_create;
        }

        struct device *foo_device = device_create(vilhelm, NULL, foo_dev, NULL, "foo");
        if(IS_ERR(foo_device)){
                printk(KERN_ERR "foo: unable to create device file");
                retval = PTR_ERR(foo_device);
                goto out_device_create;
        }

        unsigned_dev_attr_blah.ptr = &myvar;
        retval = device_create_file(foo_device, &unsigned_dev_attr_blah.dev_attr);
        if(retval){
                printk(KERN_ERR "foo: unable to create device attribute files");
                goto out_create_foo_dev_attr_files;
        }

        return 0;

        out_create_foo_dev_attr_files:
                device_destroy(vilhelm, foo_dev);
        out_device_create:
                class_destroy(vilhelm);
        out_class_create:
                unregister_chrdev_region(foo_dev, 1);
        out_alloc_chrdev_region:
                return retval;
}

static void __exit foo_exit(void){
        printk(KERN_INFO "BYE FROM MODULE 1");

        device_destroy(vilhelm, foo_dev);
        class_destroy(vilhelm);
        unregister_chrdev_region(foo_dev, 1);
}

static ssize_t unsigned_dev_attr_show(struct device *dev, struct device_attribute *attr, char *buf){
        struct unsigned_device_attribute *tmp = container_of(attr, struct unsigned_device_attribute, dev_attr);

        unsigned value = *(tmp->ptr);

        return scnprintf(buf, PAGE_SIZE, "%u\n", value);
}

module_init(foo_init);
module_exit(foo_exit);

MODULE_LICENSE("GPL");

---

也可以看看

使用Linux sysfs_notify调用

Answer 1

从您引用的评论中引用更多内容:

一旦poll/select指示值已更改,您需要关闭并重新打开该文件,或者搜索0并再次读取.

但你什么都不做fds.fd.

另外,read()在打电话前做一个假人poll(); 任何新打开的文件都被视为已更改.