Using the Linux sysfs_notify call

Question

I am trying to communicate asynchronously between a kernel driver and a user-space program (I know there are lots of questions here that ask for similar information, but I could find none that deal with sysfs_notify).

I am leaving Vilhelm's edit here, but adding the source to both a simple driver utilizing sysfs and a user-space program to poll it. The driver works fine (I got most of it from the net; it is missing the credits, but I couldn't find them when I went back to add them). Unfortunately, the polling program does not work. It always returns success immediately. Interestingly, if I don't perform the two reads prior to the poll, the revents members are set to POLLERR | POLLIN instead of just POLLIN as seen in the program output.

Program output:

root@ubuntu:/home/wmulcahy/demo# ./readhello
triggered
Attribute file value: 74 (t) [0]
revents[0]: 00000001
revents[1]: 00000001

Here is the driver: hello.c (you can see where I started out...)

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/slab.h>

struct my_attr {
    struct attribute attr;
    int value;
};

static struct my_attr notify = {
    .attr.name="notify",
    .attr.mode = 0644,
    .value = 0,
};

static struct my_attr trigger = {
    .attr.name="trigger",
    .attr.mode = 0644,
    .value = 0,
};

static struct attribute * myattr[] = {
    &notify.attr,
    &trigger.attr,
    NULL
};

static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
{
    struct my_attr *a = container_of(attr, struct my_attr, attr);
    printk( "hello: show called (%s)\n", a->attr.name );
    return scnprintf(buf, PAGE_SIZE, "%s: %d\n", a->attr.name, a->value);
}
static struct kobject *mykobj;

static ssize_t store(struct kobject *kobj, struct attribute *attr, const char *buf, size_t len)
{
    struct my_attr *a = container_of(attr, struct my_attr, attr);

    sscanf(buf, "%d", &a->value);
    notify.value = a->value;
    printk("sysfs_notify store %s = %d\n", a->attr.name, a->value);
    sysfs_notify(mykobj, NULL, "notify");
    return sizeof(int);
}

static struct sysfs_ops myops = {
    .show = show,
    .store = store,
};

static struct kobj_type mytype = {
    .sysfs_ops = &myops,
    .default_attrs = myattr,
};

static struct kobject *mykobj;
static int __init hello_module_init(void)
{
    int err = -1;
    printk("Hello: init\n");
    mykobj = kzalloc(sizeof(*mykobj), GFP_KERNEL);
    if (mykobj) {
        kobject_init(mykobj, &mytype);
        if (kobject_add(mykobj, NULL, "%s", "hello")) {
             err = -1;
             printk("Hello: kobject_add() failed\n");
             kobject_put(mykobj);
             mykobj = NULL;
        }
        err = 0;
    }
    return err;
}

static void __exit hello_module_exit(void)
{
    if (mykobj) {
        kobject_put(mykobj);
        kfree(mykobj);
    }
    printk("Hello: exit\n");
}

module_init(hello_module_init);
module_exit(hello_module_exit);
MODULE_LICENSE("GPL");

And here is the poll program: readhello.c

#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h> 
#include <sys/stat.h> 
#include <poll.h>

#define TEST_SYSFS_TRIGGER  "/sys/hello/trigger"
#define TEST_SYSFS_NOTIFY   "/sys/hello/notify"

int main(int argc, char **argv)
{
    int cnt, notifyFd, triggerFd, rv;
    char attrData[100];
    struct pollfd ufds[2];

    // Open a connection to the attribute file.
    if ((notifyFd = open(TEST_SYSFS_NOTIFY, O_RDWR)) < 0)
    {
        perror("Unable to open notify");
        exit(1);
    }
    // Open a connection to the attribute file.
    if ((triggerFd = open(TEST_SYSFS_TRIGGER, O_RDWR)) < 0)
    {
        perror("Unable to open trigger");
        exit(1);
    }

    ufds[0].fd = notifyFd;
    ufds[0].events = POLLIN;
    ufds[1].fd = triggerFd;
    ufds[1].events = POLLIN;

    // Someone suggested dummy reads before the poll() call
    cnt = read( notifyFd, attrData, 100 );
    cnt = read( triggerFd, attrData, 100 );
    ufds[0].revents = 0;
    ufds[1].revents = 0;
    if (( rv = poll( ufds, 2, 10000)) < 0 )
    {
        perror("poll error");
    }
    else if (rv == 0)
    {
        printf("Timeout occurred!\n");
    }
    else if (ufds[0].revents & POLLIN)
    {
        printf("triggered\n");
        cnt = read( notifyFd, attrData, 1 );
        printf( "Attribute file value: %02X (%c) [%d]\n", attrData[0], attrData[0], cnt );
    }
    printf( "revents[0]: %08X\n", ufds[0].revents );
    printf( "revents[1]: %08X\n", ufds[1].revents );

    close( triggerFd );
    close( notifyFd );
}

Some upcoming sysfs enhancements.

Internally, the patch adds a wait queue head to every kobject on the system; that queue is inserted into a poll table in response to a poll() call. The sysfs code has no way of knowing, however, when the value of any given sysfs attribute has changed, so the subsystem implementing a pollable attribute must make explicit calls to:

void sysfs_notify(struct kobject *kobj, char *dir, char *attr);

Thanks, Lee

Answer 1

The blocking poll is from the user side. User code can simply tell the kernel which attributes it's interested in, then block in a poll() until one of them has changed.

The sysfs_notify() is a kernel-side call that releases the user-space poll(). After you adjust your kernel attribute value, just call sysfs_notify() to allow any user-space applications to respond to their outstanding poll().

Think of the poll() as "subscribing" to notices of a change in an attribute of interest, and sysfs_notify() as "publishing" the change to any subscribers.