[S390] vmur: Use wait queue instead of mutex to serialize open

If user space opens a unit record device node then vmur is leaving the kernel
with lock open_mutex still held to prevent other processes from opening the
device simultaneously. This causes lockdep to complain about a lock held when
returning to user space.
Now the mutex is replaced by a wait queue to serialize device open.

Signed-off-by: Frank Munzert <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>

drivers/s390/char/vmur.c

···
 	urd->reclen = cdev->id.driver_info;
 	ccw_device_get_id(cdev, &urd->dev_id);
 	mutex_init(&urd->io_mutex);
-	mutex_init(&urd->open_mutex);
+	init_waitqueue_head(&urd->wait);
+	spin_lock_init(&urd->open_lock);
 	atomic_set(&urd->ref_count,  1);
 	urd->cdev = cdev;
 	get_device(&cdev->dev);
···
 	if (!urd)
 		return -ENXIO;
 
-	if (file->f_flags & O_NONBLOCK) {
-		if (!mutex_trylock(&urd->open_mutex)) {
+	spin_lock(&urd->open_lock);
+	while (urd->open_flag) {
+		spin_unlock(&urd->open_lock);
+		if (file->f_flags & O_NONBLOCK) {
 			rc = -EBUSY;
 			goto fail_put;
 		}
-	} else {
-		if (mutex_lock_interruptible(&urd->open_mutex)) {
+		if (wait_event_interruptible(urd->wait, urd->open_flag == 0)) {
 			rc = -ERESTARTSYS;
 			goto fail_put;
 		}
+		spin_lock(&urd->open_lock);
 	}
+	urd->open_flag++;
+	spin_unlock(&urd->open_lock);
 
 	TRACE("ur_open\n");
 
···
 fail_urfile_free:
 	urfile_free(urf);
 fail_unlock:
-	mutex_unlock(&urd->open_mutex);
+	spin_lock(&urd->open_lock);
+	urd->open_flag--;
+	spin_unlock(&urd->open_lock);
 fail_put:
 	urdev_put(urd);
 	return rc;
···
 	struct urfile *urf = file->private_data;
 
 	TRACE("ur_release\n");
-	mutex_unlock(&urf->urd->open_mutex);
+	spin_lock(&urf->urd->open_lock);
+	urf->urd->open_flag--;
+	spin_unlock(&urf->urd->open_lock);
+	wake_up_interruptible(&urf->urd->wait);
 	urdev_put(urf->urd);
 	urfile_free(urf);
 	return 0;

drivers/s390/char/vmur.h

···
 struct urdev {
 	struct ccw_device *cdev;	/* Backpointer to ccw device */
 	struct mutex io_mutex;		/* Serialises device IO */
-	struct mutex open_mutex;	/* Serialises access to device */
 	struct completion *io_done;	/* do_ur_io waits; irq completes */
 	struct device *device;
 	struct cdev *char_device;
···
 	int class;			/* VM device class */
 	int io_request_rc;		/* return code from I/O request */
 	atomic_t ref_count;		/* reference counter */
+	wait_queue_head_t wait;		/* wait queue to serialize open */
+	int open_flag;			/* "urdev is open" flag */
+	spinlock_t open_lock;		/* serialize critical sections */
 };
 
 /*