diff --git a/drivers/char/mmtimer.c b/drivers/char/mmtimer.c
index ea7c99f..fe46978 100644
--- a/drivers/char/mmtimer.c
+++ b/drivers/char/mmtimer.c
@@ -32,7 +32,7 @@
 #include <linux/interrupt.h>
 #include <linux/time.h>
 #include <linux/math64.h>
-#include <linux/smp_lock.h>
+#include <linux/mutex.h>
 #include <linux/slab.h>
 
 #include <asm/uaccess.h>
@@ -59,6 +59,7 @@
 
 #define rtc_time()              (*RTC_COUNTER_ADDR)
 
+static DEFINE_MUTEX(mmtimer_mutex);
 static long mmtimer_ioctl(struct file *file, unsigned int cmd,
 						unsigned long arg);
 static int mmtimer_mmap(struct file *file, struct vm_area_struct *vma);
@@ -371,7 +372,7 @@
 {
 	int ret = 0;
 
-	lock_kernel();
+	mutex_lock(&mmtimer_mutex);
 
 	switch (cmd) {
 	case MMTIMER_GETOFFSET:	/* offset of the counter */
@@ -414,7 +415,7 @@
 		ret = -ENOTTY;
 		break;
 	}
-	unlock_kernel();
+	mutex_unlock(&mmtimer_mutex);
 	return ret;
 }