Tools for checking incorrect usage of locking techniques in k-space

Hi,

I recently came across this tool called KEDR[1] for checking memory leak in kernel modules. I'm using it to check If my trivial kernel modules (based on LDD3 examples) are leaking memory. I was wondering if there exist a similar tool for checking incorrect usage of locking techniques implemented by kernel module.

Quoted 8 lines · Kumar amit mehta

Hi,

I recently came across this tool called KEDR[1] for checking memory leak in kernel modules. I'm using it to check If my trivial kernel modules (based on LDD3 examples) are leaking memory. I was wondering if there exist a similar tool for checking incorrect usage of locking techniques implemented by kernel module.

You can make use of the in-kernel "lockdep" framework to check your locking. Just enable it in your .config, and build and boot your kernel. Lockdep detects a variety of locking problems at run time and warns you if there is a possibility
of deadlock etc.

Regards,
Srivatsa S. Bhat

Thank you Srivatsa. It seems that lockdep framework is enabled on my running kernel.

<snip>
amit at ubuntu:/boot$ egrep -i "debug_kernel|lockdep" config-3.2.0-29-generic-pae CONFIG_LOCKDEP_SUPPORT=y
CONFIG_DEBUG_KERNEL=y

<snip>

If above configuration is all that I need, then should I be seeing warning/error messages in kernel logs(/var/log/kern.log) when there is inconsistency in locking ? To test my hypothesis, I modified my simple kernel module to deliberately induce locking error (After initializing read-write semaphore, I call down_write() and do not free this semaphore lock by commenting out up_write() invocation). But still I don't see any error or warning message trace in kernel logs, I think, I'm missing something.

You'll need CONFIG_LOCKDEP=y as well. An easy way to configure lock debugging checks is to run 'make menuconfig' and enable the required options under the "Kernel hacking" section.

Quoted 8 lines

If above configuration is all that I need, then should I be seeing warning/error messages in kernel logs(/var/log/kern.log) when there is inconsistency in locking ? To test my hypothesis, I modified my simple kernel module to deliberately induce locking error (After initializing read-write semaphore, I call down_write() and do not free this semaphore lock by commenting out up_write() invocation). But still I don't see any error or warning message trace in kernel logs, I think, I'm missing something.

Quoted 12 lines · Srivatsa Bhat

You'll need CONFIG_LOCKDEP=y as well. An easy way to configure lock debugging checks is to run 'make menuconfig' and enable the required options under the "Kernel hacking" section.

Quoted 8 lines

If above configuration is all that I need, then should I be seeing warning/error messages in kernel logs(/var/log/kern.log) when there is inconsistency in locking ? To test my hypothesis, I modified my simple kernel module to deliberately induce locking error (After initializing read-write semaphore, I call down_write() and do not free this semaphore lock by commenting out up_write() invocation). But still I don't see any error or warning message trace in kernel logs, I think, I'm missing something.

Hi Srivatsa,

Thank you for your mail. As per your suggestion, this time I've enabled CONFIG_LOCKDEP aslo in my running kernel and did the same experiment, but still I dont't see any warning/error messages in the kernel log. To give you more idea about what I'm doing, Please see the code below.(This is a simple char driver based on LDD3 examples)

<echo.c>

#include <linux/module.h>
#include <linux/init.h>
#include <linux/types.h> //MAJOR, MINOR
#include <linux/fs.h> //register_chrdev_region, file_operations #include <linux/moduleparam.h>
#include <linux/kernel.h> //container_of
#include <linux/slab.h> //kmalloc
#include <linux/cdev.h> //struct cdev
#include <linux/version.h>
#include <linux/uaccess.h> //copy_from/to_user() #include <linux/errno.h> //error code

ssize_t echo_read(struct file *, char __user *, size_t, loff_t *); ssize_t echo_write(struct file *, const char __user *, size_t, loff_t *); int echo_open(struct inode *, struct file *);
int echo_release(struct inode *, struct file *);

struct echo_cdev {
        char *data;
        unsigned long size; //amount of data stored
        struct semaphore sem;
        struct cdev cdev;
};

MODULE_LICENSE("GPL v2");
MODULE_AUTHOR("amit");

int nr_major;
module_param(nr_major, int, S_IRUGO);
MODULE_PARM_DESC(nr_major, "major number");

int nr_minor;
char *chrdev_name = "echo";

static dev_t device;
static int echo_dev_count = 1;
struct echo_cdev *echo_dev = NULL;

static struct file_operations echo_fs_ops = {
.open = echo_open,
.release = echo_release,
.read = echo_read,
.write = echo_write,
.owner = THIS_MODULE,
};

int echo_open(struct inode *inode, struct file *filp)

{
        struct echo_cdev *dev;
        pr_debug("%s: f_flags: 0x%x\n",__FUNCTION__,filp->f_flags);
        //container_of(pointer, container_type, container_field);
        dev = container_of(inode->i_cdev, struct echo_cdev, cdev);
        filp->private_data = dev;
        if ((filp->f_flags & O_ACCMODE) == O_WRONLY) {
                //trim the device size to 0
                dev->size = 0;
        }
        return 0;
}

int echo_release(struct inode *inode, struct file *filp)

{
        return 0;
}

ssize_t echo_read(struct file *filp, char __user *ubuff, size_t count, loff_t *poffset)

{
        struct echo_cdev *dev = filp->private_data;
        pr_debug("%s: f_flags: 0x%x\n",__FUNCTION__,filp->f_flags);

        //user trying to access an offset which is beyond the end of file
        if (down_interruptible(&dev->sem))
                return -ERESTARTSYS;
        if (*poffset >= dev->size) {
                up(&dev->sem);
                return 0;
        }

        //user trying to access more than eof, return bytes read till the eof
        if (*poffset + count >= dev->size)
                //count = dev->size - *poffset;
                count = dev->size;
        //kspace --> uspace
        if (copy_to_user(ubuff, (dev->data + *poffset), count) < 0) {
                up(&dev->sem);
                return -EFAULT;
        }
        //update the offset
        *poffset += count;
        up(&dev->sem);
        return count;
}

//count is the size of requested data transfer
ssize_t echo_write(struct file *filp, const char __user *ubuff, size_t count, loff_t *poffset)

{
        int ret;
        struct echo_cdev *dev = filp->private_data;
        pr_debug("%s: f_flags: 0x%x\n",__FUNCTION__,filp->f_flags);
        if (down_interruptible(&dev->sem))
                return -ERESTARTSYS;
        if (dev->data == NULL) {
                dev->data = (char *)kmalloc(count, GFP_KERNEL);
                if (!dev->data) {
                        up(&dev->sem);
                        return -ENOMEM;
                } else {
                        memset(dev->data, 0, sizeof(count));
                }
        }
        dev->size = count;
        //uspace --> kspace
        if (copy_from_user(dev->data, ubuff, count) < 0) {
                up(&dev->sem);
                return -EFAULT;
        }

        *poffset += count;
        ret = count;

        if (dev->size < *poffset)
                dev->size = *poffset;
        //Force lock error
        //up(&dev->sem);
        return ret;
}

static int __init echo_init(void)

{
        int ret;
        printk(KERN_EMERG "entering %s\n",__FUNCTION__);
        //let the user provide the major number.
        if (nr_major) {
                device = MKDEV(nr_major, nr_minor);
                if ((ret = register_chrdev_region(device, echo_dev_count, chrdev_name)) < 0) {
                        pr_debug("%s: failed to register %s\n",__FUNCTION__,
                                        chrdev_name);
                        return ret;
                }
        } else {
                ret = alloc_chrdev_region(&device, 0, echo_dev_count, chrdev_name);
                if (ret < 0) {
                        pr_debug("%s: failed to register %s\n",__FUNCTION__,
                                        chrdev_name);
                        return ret;
                }
        }
        nr_major = MAJOR(device);
        nr_minor = MINOR(device);
        //print the major and minor numbers
        pr_debug("%s: major/minor:: %d/%d\n",__FUNCTION__,
                        nr_major, nr_minor);
        echo_dev = (struct echo_cdev *)kmalloc(sizeof(struct echo_cdev), GFP_KERNEL);
        if (!echo_dev) {
                printk(KERN_EMERG "Not enough memory\n");
                unregister_chrdev_region(device, echo_dev_count);
                return -ENOMEM;
        }
        memset(echo_dev, 0, sizeof(struct echo_cdev));
        echo_dev->cdev.owner = THIS_MODULE;
        echo_dev->cdev.ops = &echo_fs_ops;
        //initialize the semaphore, before it is presented to the world
        sema_init(&echo_dev->sem,1);
        cdev_init(&echo_dev->cdev, &echo_fs_ops);
        device = MKDEV(nr_major, nr_minor);
        //tell the kernel about this char device
        //telling the VFS layer to associate echo driver's fs operation for file r/w etc.
        ret = cdev_add(&echo_dev->cdev, device, 1);
        if (ret) {
                kfree(echo_dev);
                unregister_chrdev_region(device, echo_dev_count);
                return ret;
        }
        return 0;
}

static void __exit echo_exit(void)

{
        printk(KERN_EMERG "entering %s\n",__FUNCTION__);
        if (echo_dev->data) {
                printk("Inside %s: kfree()\n",__FUNCTION__);
                kfree(echo_dev->data);
        }
        cdev_del(&echo_dev->cdev);
        kfree(echo_dev);
        unregister_chrdev_region(device, echo_dev_count);
}

module_init(echo_init);
module_exit(echo_exit);

<echo.c>

<Makefile>
obj-m := echo.o

CFLAGS_echo.o := -DDEBUG -Wall -Werror -Wmissing-prototypes \
        -Wstrict-prototypes -Wunused-variable -O2 \
        -Wunused-function -g

modules:
make -C /lib/modules/uname -r/build M=pwd
clean:
make -C /lib/modules/uname -r/build M=pwd clean
<Makefile>

Logs:
root at ubuntu:/boot# egrep -i "debug_kernel|lockdep" config-3.7.0-rc3-next-20121029 CONFIG_LOCKDEP_SUPPORT=y CONFIG_DEBUG_KERNEL=y
CONFIG_LOCKDEP=y
# CONFIG_DEBUG_LOCKDEP is not set

root at ubuntu:/home/amit/ldd3/misc-modules/echo# uname -a
Linux ubuntu 3.7.0-rc3-next-20121029 #1 SMP Mon Oct 29 21:42:20 PDT 2012 i686 i686 i386 GNU/Linux

root at ubuntu:/home/amit/ldd3/misc-modules/echo# dmesg

root at ubuntu:/home/amit/ldd3/misc-modules/echo# insmod echo.ko

root at ubuntu:/home/amit/ldd3/misc-modules/echo# dmesg

[ 1156.704072] entering echo_init
[ 1156.704914] echo_init: major/minor:: 249/0

root at ubuntu:/home/amit/ldd3/misc-modules/echo# mknod /dev/echo c 249 0

root at ubuntu:/home/amit/ldd3/misc-modules/echo# ll /dev/echo crw-r--r-- 1 root root 249, 0 Oct 29 22:29 /dev/echo

root at ubuntu:/home/amit/ldd3/misc-modules/echo# echo "kernel newbiee" >/dev/echo

root at ubuntu:/home/amit/ldd3/misc-modules/echo# dmesg

[ 1156.704072] entering echo_init
[ 1156.704914] echo_init: major/minor:: 249/0
[ 1237.377205] echo_open: f_flags: 0x8241
[ 1237.377392] echo_write: f_flags: 0x8001

root at ubuntu:/home/amit/ldd3/misc-modules/echo# cat /dev/echo --> Here the driver read routine gets stuck as I've not reliquished the semaphore lock

root at ubuntu:/home/amit/ldd3/misc-modules/echo# cat /dev/echo ^C <-- killing the user process

root at ubuntu:/home/amit/ldd3/misc-modules/echo# rmmod echo

root at ubuntu:/home/amit/ldd3/misc-modules/echo# dmesg

[ 1156.704072] entering echo_init
[ 1156.704914] echo_init: major/minor:: 249/0
[ 1237.377205] echo_open: f_flags: 0x8241
[ 1237.377392] echo_write: f_flags: 0x8001
[ 1270.622337] echo_open: f_flags: 0x8000
[ 1270.622477] echo_read: f_flags: 0x8000
[ 1394.180962] entering echo_exit
[ 1394.180996] Inside echo_exit: kfree()

As you can see, no warning/error messages reported by kernel. -Amit

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Quoted 12 lines · Srivatsa Bhat

You'll need CONFIG_LOCKDEP=y as well. An easy way to configure lock debugging checks is to run 'make menuconfig' and enable the required options under the "Kernel hacking" section.

Quoted 8 lines

If above configuration is all that I need, then should I be seeing warning/error messages in kernel logs(/var/log/kern.log) when there is inconsistency in locking ? To test my hypothesis, I modified my simple kernel module to deliberately induce locking error (After initializing read-write semaphore, I call down_write() and do not free this semaphore lock by commenting out up_write() invocation). But still I don't see any error or warning message trace in kernel logs, I think, I'm missing something.

Hi Srivatsa,

Thank you for your mail. As per your suggestion, this time I've enabled CONFIG_LOCKDEP aslo in my running kernel and did the same experiment, but still I dont't see any warning/error messages in the kernel log. To give you more idea about what I'm doing, Please see the code below.(This is a simple char driver based on LDD3 examples)

<echo.c>

#include <linux/module.h>
#include <linux/init.h>
#include <linux/types.h> //MAJOR, MINOR
#include <linux/fs.h> //register_chrdev_region, file_operations #include <linux/moduleparam.h>
#include <linux/kernel.h> //container_of
#include <linux/slab.h> //kmalloc
#include <linux/cdev.h> //struct cdev
#include <linux/version.h>
#include <linux/uaccess.h> //copy_from/to_user() #include <linux/errno.h> //error code

ssize_t echo_read(struct file *, char __user *, size_t, loff_t *); ssize_t echo_write(struct file *, const char __user *, size_t, loff_t *); int echo_open(struct inode *, struct file *);
int echo_release(struct inode *, struct file *);

struct echo_cdev {

? ? ? ?  char *data;
? ? ? ?  unsigned long size; //amount of data stored
? ? ? ?  struct semaphore sem;
? ? ? ?  struct cdev cdev;

};
MODULE_LICENSE("GPL v2");
MODULE_AUTHOR("amit");

int nr_major;
module_param(nr_major, int, S_IRUGO);
MODULE_PARM_DESC(nr_major, "major number");

int nr_minor;
char *chrdev_name = "echo";

static dev_t device;
static int echo_dev_count = 1;
struct echo_cdev *echo_dev = NULL;

static struct file_operations echo_fs_ops = {

? ? ? ?  .open = echo_open,
? ? ? ?  .release = echo_release,
? ? ? ?  .read = echo_read,
? ? ? ?  .write = echo_write,
? ? ? ?  .owner = THIS_MODULE,

};

int echo_open(struct inode *inode, struct file *filp) {

? ? ? ?  struct echo_cdev *dev;
? ? ? ?  pr_debug("%s: f_flags: 0x%x\n",__FUNCTION__,filp->f_flags);
? ? ? ?  //container_of(pointer, container_type, container_field);
? ? ? ?  dev = container_of(inode->i_cdev, struct echo_cdev, cdev);
? ? ? ?  filp->private_data = dev;
? ? ? ?  if ((filp->f_flags & O_ACCMODE) == O_WRONLY) {

? ? ? ? ? ? ? ? //trim the device size to 0
? ? ? ? ? ? ? ? dev->size = 0;
? ? ? ? }
? ? ? ? return 0;
}

int echo_release(struct inode *inode, struct file *filp) {
? ? ? ? return 0;
}

ssize_t echo_read(struct file *filp, char __user *ubuff, size_t count, loff_t *poffset) {
? ? ? ? struct echo_cdev *dev = filp->private_data;
? ? ? ? pr_debug("%s: f_flags: 0x%x\n",__FUNCTION__,filp->f_flags);

? ? ? ? //user trying to access an offset which is beyond the end of file ? ? ? ? if (down_interruptible(&dev->sem))
? ? ? ? ? ? ? ? return -ERESTARTSYS;
? ? ? ? if (*poffset >= dev->size) {
? ? ? ? ? ? ? ? up(&dev->sem);
? ? ? ? ? ? ? ? return 0;
? ? ? ? }

? ? ? ? //user trying to access more than eof, return bytes read till the eof ? ? ? ? if (*poffset + count >= dev->size)
? ? ? ? ? ? ? ? //count = dev->size - *poffset;
? ? ? ? ? ? ? ? count = dev->size;
? ? ? ? //kspace --> uspace
? ? ? ? if (copy_to_user(ubuff, (dev->data + *poffset), count) < 0) { ? ? ? ? ? ? ? ? up(&dev->sem);
? ? ? ? ? ? ? ? return -EFAULT;

? ? ? ?  }
? ? ? ?  //update the offset
? ? ? ?  *poffset += count;
? ? ? ?  up(&dev->sem);
? ? ? ?  return count;

}

//count is the size of requested data transfer
ssize_t echo_write(struct file *filp, const char __user *ubuff, size_t count, loff_t *poffset) {

? ? ? ?  int ret;
? ? ? ?  struct echo_cdev *dev = filp->private_data;
? ? ? ?  pr_debug("%s: f_flags: 0x%x\n",__FUNCTION__,filp->f_flags);
? ? ? ?  if (down_interruptible(&dev->sem))

? ? ? ? ? ? ? ? return -ERESTARTSYS;
? ? ? ? if (dev->data == NULL) {
? ? ? ? ? ? ? ? dev->data = (char *)kmalloc(count, GFP_KERNEL); ? ? ? ? ? ? ? ? if (!dev->data) {
? ? ? ? ? ? ? ? ? ? ? ? up(&dev->sem);
? ? ? ? ? ? ? ? ? ? ? ? return -ENOMEM;
? ? ? ? ? ? ? ? } else {
? ? ? ? ? ? ? ? ? ? ? ? memset(dev->data, 0, sizeof(count)); ? ? ? ? ? ? ? ? }

? ? ? ?  }
? ? ? ?  dev->size = count;
? ? ? ?  //uspace --> kspace
? ? ? ?  if (copy_from_user(dev->data, ubuff, count) < 0) {

? ? ? ? ? ? ? ? up(&dev->sem);
? ? ? ? ? ? ? ? return -EFAULT;
? ? ? ? }

? ? ? ? *poffset += count;
? ? ? ? ret = count;

? ? ? ? if (dev->size < *poffset)
? ? ? ? ? ? ? ? dev->size = *poffset;

? ? ? ?  //Force lock error
? ? ? ?  //up(&dev->sem);
? ? ? ?  return ret;

}

static int __init echo_init(void)
{

? ? ? ?  int ret;
? ? ? ?  printk(KERN_EMERG "entering %s\n",__FUNCTION__);
? ? ? ?  //let the user provide the major number.
? ? ? ?  if (nr_major) {

? ? ? ? ? ? ? ? device = MKDEV(nr_major, nr_minor);
? ? ? ? ? ? ? ? if ((ret = register_chrdev_region(device, echo_dev_count, chrdev_name)) < 0) { ? ? ? ? ? ? ? ? ? ? ? ? pr_debug("%s: failed to register %s\n",__FUNCTION__,
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? chrdev_name);
? ? ? ? ? ? ? ? ? ? ? ? return ret;
? ? ? ? ? ? ? ? }
? ? ? ? } else {
? ? ? ? ? ? ? ? ret = alloc_chrdev_region(&device, 0, echo_dev_count, chrdev_name); ? ? ? ? ? ? ? ? if (ret < 0) {
? ? ? ? ? ? ? ? ? ? ? ? pr_debug("%s: failed to register %s\n",__FUNCTION__,
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? chrdev_name);
? ? ? ? ? ? ? ? ? ? ? ? return ret;
? ? ? ? ? ? ? ? }

? ? ? ?  }
? ? ? ?  nr_major = MAJOR(device);
? ? ? ?  nr_minor = MINOR(device);
? ? ? ?  //print the major and minor numbers
? ? ? ?  pr_debug("%s: major/minor:: %d/%d\n",__FUNCTION__,
? ? ? ? ? ? ? ? ? ? ? ?  nr_major, nr_minor);
? ? ? ?  echo_dev = (struct echo_cdev *)kmalloc(sizeof(struct echo_cdev), GFP_KERNEL);
? ? ? ?  if (!echo_dev) {
? ? ? ? ? ? ? ?  printk(KERN_EMERG "Not enough memory\n");
? ? ? ? ? ? ? ?  unregister_chrdev_region(device, echo_dev_count);
? ? ? ? ? ? ? ?  return -ENOMEM;
? ? ? ?  }
? ? ? ?  memset(echo_dev, 0, sizeof(struct echo_cdev));
? ? ? ?  echo_dev->cdev.owner = THIS_MODULE;
? ? ? ?  echo_dev->cdev.ops = &echo_fs_ops;
? ? ? ?  //initialize the semaphore, before it is presented to the world
? ? ? ?  sema_init(&echo_dev->sem,1);
? ? ? ?  cdev_init(&echo_dev->cdev, &echo_fs_ops);
? ? ? ?  device = MKDEV(nr_major, nr_minor);
? ? ? ?  //tell the kernel about this char device
? ? ? ?  //telling the VFS layer to associate echo driver's fs operation for file r/w etc.
? ? ? ?  ret = cdev_add(&echo_dev->cdev, device, 1);
? ? ? ?  if (ret) {
? ? ? ? ? ? ? ?  kfree(echo_dev);
? ? ? ? ? ? ? ?  unregister_chrdev_region(device, echo_dev_count);
? ? ? ? ? ? ? ?  return ret;

? ? ? ? }
? ? ? ? return 0;
}

static void __exit echo_exit(void)
{
? ? ? ? printk(KERN_EMERG "entering %s\n",__FUNCTION__); ? ? ? ? if (echo_dev->data) {
? ? ? ? ? ? ? ? printk("Inside %s: kfree()\n",__FUNCTION__); ? ? ? ? ? ? ? ? kfree(echo_dev->data);

? ? ? ?  }
? ? ? ?  cdev_del(&echo_dev->cdev);
? ? ? ?  kfree(echo_dev);
? ? ? ?  unregister_chrdev_region(device, echo_dev_count);

}

module_init(echo_init);
module_exit(echo_exit);

<echo.c>

<Makefile>
obj-m := echo.o
CFLAGS_echo.o := -DDEBUG -Wall -Werror -Wmissing-prototypes \
? ? ? ? -Wstrict-prototypes -Wunused-variable -O2 \
? ? ? ? -Wunused-function -g

modules:
? ? ? ? make -C /lib/modules/uname -r/build M=pwd clean:
? ? ? ? make -C /lib/modules/uname -r/build M=pwd clean <Makefile>

Logs:
root at ubuntu:/boot# egrep -i "debug_kernel|lockdep" config-3.7.0-rc3-next-20121029 CONFIG_LOCKDEP_SUPPORT=y CONFIG_DEBUG_KERNEL=y
CONFIG_LOCKDEP=y
# CONFIG_DEBUG_LOCKDEP is not set

root at ubuntu:/home/amit/ldd3/misc-modules/echo# uname -a
Linux ubuntu 3.7.0-rc3-next-20121029 #1 SMP Mon Oct 29 21:42:20 PDT 2012 i686 i686 i386 GNU/Linux

root at ubuntu:/home/amit/ldd3/misc-modules/echo# dmesg

root at ubuntu:/home/amit/ldd3/misc-modules/echo# insmod echo.ko

root at ubuntu:/home/amit/ldd3/misc-modules/echo# dmesg

[ 1156.704072] entering echo_init
[ 1156.704914] echo_init: major/minor:: 249/0

root at ubuntu:/home/amit/ldd3/misc-modules/echo# mknod /dev/echo c 249 0

root at ubuntu:/home/amit/ldd3/misc-modules/echo# ll /dev/echo crw-r--r-- 1 root root 249, 0 Oct 29 22:29 /dev/echo

root at ubuntu:/home/amit/ldd3/misc-modules/echo# echo "kernel newbiee" >/dev/echo

root at ubuntu:/home/amit/ldd3/misc-modules/echo# dmesg

[ 1156.704072] entering echo_init
[ 1156.704914] echo_init: major/minor:: 249/0
[ 1237.377205] echo_open: f_flags: 0x8241
[ 1237.377392] echo_write: f_flags: 0x8001

root at ubuntu:/home/amit/ldd3/misc-modules/echo# cat /dev/echo --> Here the driver read routine gets stuck as I've not reliquished the semaphore lock

root at ubuntu:/home/amit/ldd3/misc-modules/echo# cat /dev/echo ^C <-- killing the user process

root at ubuntu:/home/amit/ldd3/misc-modules/echo# rmmod echo

root at ubuntu:/home/amit/ldd3/misc-modules/echo# dmesg

[ 1156.704072] entering echo_init
[ 1156.704914] echo_init: major/minor:: 249/0
[ 1237.377205] echo_open: f_flags: 0x8241
[ 1237.377392] echo_write: f_flags: 0x8001
[ 1270.622337] echo_open: f_flags: 0x8000
[ 1270.622477] echo_read: f_flags: 0x8000
[ 1394.180962] entering echo_exit
[ 1394.180996] Inside echo_exit: kfree()

As you can see, no warning/error messages reported by kernel. -Amit

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I've not tried your module, but I suppose that you should get warning as soon as you try to take write semaphore once again.

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