use spinlock instead of semaphore in tty_io.c

index 7a32df5..ff27587 100644
@@ -137,7 +137,7 @@ EXPORT_SYMBOL(tty_mutex);
 extern struct tty_driver *ptm_driver;	/* Unix98 pty masters; for /dev/ptmx */
 extern int pty_limit;		/* Config limit on Unix98 ptys */
 static DEFINE_IDR(allocated_ptys);
-static DECLARE_MUTEX(allocated_ptys_lock);
+static DEFINE_SPINLOCK(allocated_ptys_lock);
 static int ptmx_open(struct inode *, struct file *);
 #endif
 

@@ -2471,9 +2471,9 @@ static void release_dev(struct file * filp)
 #ifdef CONFIG_UNIX98_PTYS
 	/* Make this pty number available for reallocation */
 	if (devpts) {
-		down(&allocated_ptys_lock);
+		spin_lock(&allocated_ptys_lock);
 		idr_remove(&allocated_ptys, idx);
-		up(&allocated_ptys_lock);
+		spin_unlock(&allocated_ptys_lock);
 	}
 #endif
 

@@ -2639,24 +2639,24 @@ static int ptmx_open(struct inode * inode, struct file * filp)
 	nonseekable_open(inode, filp);
 
 	/* find a device that is not in use. */
-	down(&allocated_ptys_lock);
-	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
-		up(&allocated_ptys_lock);
+	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL))
 		return -ENOMEM;
-	}
+
+	spin_lock(&allocated_ptys_lock);
+
 	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
 	if (idr_ret < 0) {
-		up(&allocated_ptys_lock);
+		spin_unlock(&allocated_ptys_lock);
 		if (idr_ret == -EAGAIN)
 			return -ENOMEM;
 		return -EIO;
 	}
 	if (index >= pty_limit) {
 		idr_remove(&allocated_ptys, index);
-		up(&allocated_ptys_lock);
+		spin_unlock(&allocated_ptys_lock);
 		return -EIO;
 	}
-	up(&allocated_ptys_lock);
+	spin_unlock(&allocated_ptys_lock);
 
 	mutex_lock(&tty_mutex);
 	retval = init_dev(ptm_driver, index, &tty);

@@ -2681,9 +2681,9 @@ out1:
 	release_dev(filp);
 	return retval;
 out:
-	down(&allocated_ptys_lock);
+	spin_lock(&allocated_ptys_lock);
 	idr_remove(&allocated_ptys, index);
-	up(&allocated_ptys_lock);
+	spin_unlock(&allocated_ptys_lock);
 	return retval;
 }
 #endif