[v2,(resend)] media: imon: reorganize serialization

diff --git a/drivers/media/rc/imon.c b/drivers/media/rc/imon.c
index 54da6f60079b..9a4f24e294bc 100644
--- a/drivers/media/rc/imon.c
+++ b/drivers/media/rc/imon.c
@@ -153,6 +153,24 @@  struct imon_context {
 	const struct imon_usb_dev_descr *dev_descr;
 					/* device description with key */
 					/* table for front panels */
+	/*
+	 * Fields for deferring free_imon_context().
+	 *
+	 * Since reference to "struct imon_context" is stored into
+	 * "struct file_operations"->private_data, we need to remember
+	 * how many file descriptors might access this "struct imon_context".
+	 */
+	refcount_t users;
+	/*
+	 * Use a flag for telling display_open()/vfd_write()/lcd_write() that
+	 * imon_disconnect() was already called.
+	 */
+	bool disconnected;
+	/*
+	 * We need to wait for RCU grace period in order to allow
+	 * display_open() to safely check ->disconnected and increment ->users.
+	 */
+	struct rcu_head rcu;
 };
 
 #define TOUCH_TIMEOUT	(HZ/30)
@@ -160,18 +178,18 @@  struct imon_context {
 /* vfd character device file operations */
 static const struct file_operations vfd_fops = {
 	.owner		= THIS_MODULE,
-	.open		= &display_open,
-	.write		= &vfd_write,
-	.release	= &display_close,
+	.open		= display_open,
+	.write		= vfd_write,
+	.release	= display_close,
 	.llseek		= noop_llseek,
 };
 
 /* lcd character device file operations */
 static const struct file_operations lcd_fops = {
 	.owner		= THIS_MODULE,
-	.open		= &display_open,
-	.write		= &lcd_write,
-	.release	= &display_close,
+	.open		= display_open,
+	.write		= lcd_write,
+	.release	= display_close,
 	.llseek		= noop_llseek,
 };
 
@@ -439,9 +457,6 @@  static struct usb_driver imon_driver = {
 	.id_table	= imon_usb_id_table,
 };
 
-/* to prevent races between open() and disconnect(), probing, etc */
-static DEFINE_MUTEX(driver_lock);
-
 /* Module bookkeeping bits */
 MODULE_AUTHOR(MOD_AUTHOR);
 MODULE_DESCRIPTION(MOD_DESC);
@@ -481,9 +496,11 @@  static void free_imon_context(struct imon_context *ictx)
 	struct device *dev = ictx->dev;
 
 	usb_free_urb(ictx->tx_urb);
+	WARN_ON(ictx->dev_present_intf0);
 	usb_free_urb(ictx->rx_urb_intf0);
+	WARN_ON(ictx->dev_present_intf1);
 	usb_free_urb(ictx->rx_urb_intf1);
-	kfree(ictx);
+	kfree_rcu(ictx, rcu);
 
 	dev_dbg(dev, "%s: iMON context freed\n", __func__);
 }
@@ -499,9 +516,6 @@  static int display_open(struct inode *inode, struct file *file)
 	int subminor;
 	int retval = 0;
 
-	/* prevent races with disconnect */
-	mutex_lock(&driver_lock);
-
 	subminor = iminor(inode);
 	interface = usb_find_interface(&imon_driver, subminor);
 	if (!interface) {
@@ -509,13 +523,16 @@  static int display_open(struct inode *inode, struct file *file)
 		retval = -ENODEV;
 		goto exit;
 	}
-	ictx = usb_get_intfdata(interface);
 
-	if (!ictx) {
+	rcu_read_lock();
+	ictx = usb_get_intfdata(interface);
+	if (!ictx || ictx->disconnected || !refcount_inc_not_zero(&ictx->users)) {
+		rcu_read_unlock();
 		pr_err("no context found for minor %d\n", subminor);
 		retval = -ENODEV;
 		goto exit;
 	}
+	rcu_read_unlock();
 
 	mutex_lock(&ictx->lock);
 
@@ -533,8 +550,10 @@  static int display_open(struct inode *inode, struct file *file)
 
 	mutex_unlock(&ictx->lock);
 
+	if (retval && refcount_dec_and_test(&ictx->users))
+		free_imon_context(ictx);
+
 exit:
-	mutex_unlock(&driver_lock);
 	return retval;
 }
 
@@ -544,16 +563,9 @@  static int display_open(struct inode *inode, struct file *file)
  */
 static int display_close(struct inode *inode, struct file *file)
 {
-	struct imon_context *ictx = NULL;
+	struct imon_context *ictx = file->private_data;
 	int retval = 0;
 
-	ictx = file->private_data;
-
-	if (!ictx) {
-		pr_err("no context for device\n");
-		return -ENODEV;
-	}
-
 	mutex_lock(&ictx->lock);
 
 	if (!ictx->display_supported) {
@@ -568,6 +580,8 @@  static int display_close(struct inode *inode, struct file *file)
 	}
 
 	mutex_unlock(&ictx->lock);
+	if (refcount_dec_and_test(&ictx->users))
+		free_imon_context(ictx);
 	return retval;
 }
 
@@ -934,15 +948,12 @@  static ssize_t vfd_write(struct file *file, const char __user *buf,
 	int offset;
 	int seq;
 	int retval = 0;
-	struct imon_context *ictx;
+	struct imon_context *ictx = file->private_data;
 	static const unsigned char vfd_packet6[] = {
 		0x01, 0x00, 0x00, 0x00, 0x00, 0xFF, 0xFF };
 
-	ictx = file->private_data;
-	if (!ictx) {
-		pr_err_ratelimited("no context for device\n");
+	if (ictx->disconnected)
 		return -ENODEV;
-	}
 
 	mutex_lock(&ictx->lock);
 
@@ -1018,13 +1029,10 @@  static ssize_t lcd_write(struct file *file, const char __user *buf,
 			 size_t n_bytes, loff_t *pos)
 {
 	int retval = 0;
-	struct imon_context *ictx;
+	struct imon_context *ictx = file->private_data;
 
-	ictx = file->private_data;
-	if (!ictx) {
-		pr_err_ratelimited("no context for device\n");
+	if (ictx->disconnected)
 		return -ENODEV;
-	}
 
 	mutex_lock(&ictx->lock);
 
@@ -2404,7 +2412,6 @@  static int imon_probe(struct usb_interface *interface,
 	int ifnum, sysfs_err;
 	int ret = 0;
 	struct imon_context *ictx = NULL;
-	struct imon_context *first_if_ctx = NULL;
 	u16 vendor, product;
 
 	usbdev     = usb_get_dev(interface_to_usbdev(interface));
@@ -2416,17 +2423,12 @@  static int imon_probe(struct usb_interface *interface,
 	dev_dbg(dev, "%s: found iMON device (%04x:%04x, intf%d)\n",
 		__func__, vendor, product, ifnum);
 
-	/* prevent races probing devices w/multiple interfaces */
-	mutex_lock(&driver_lock);
-
 	first_if = usb_ifnum_to_if(usbdev, 0);
 	if (!first_if) {
 		ret = -ENODEV;
 		goto fail;
 	}
 
-	first_if_ctx = usb_get_intfdata(first_if);
-
 	if (ifnum == 0) {
 		ictx = imon_init_intf0(interface, id);
 		if (!ictx) {
@@ -2434,9 +2436,11 @@  static int imon_probe(struct usb_interface *interface,
 			ret = -ENODEV;
 			goto fail;
 		}
+		refcount_set(&ictx->users, 1);
 
 	} else {
 		/* this is the secondary interface on the device */
+		struct imon_context *first_if_ctx = usb_get_intfdata(first_if);
 
 		/* fail early if first intf failed to register */
 		if (!first_if_ctx) {
@@ -2450,14 +2454,13 @@  static int imon_probe(struct usb_interface *interface,
 			ret = -ENODEV;
 			goto fail;
 		}
+		refcount_inc(&ictx->users);
 
 	}
 
 	usb_set_intfdata(interface, ictx);
 
 	if (ifnum == 0) {
-		mutex_lock(&ictx->lock);
-
 		if (product == 0xffdc && ictx->rf_device) {
 			sysfs_err = sysfs_create_group(&interface->dev.kobj,
 						       &imon_rf_attr_group);
@@ -2468,21 +2471,17 @@  static int imon_probe(struct usb_interface *interface,
 
 		if (ictx->display_supported)
 			imon_init_display(ictx, interface);
-
-		mutex_unlock(&ictx->lock);
 	}
 
 	dev_info(dev, "iMON device (%04x:%04x, intf%d) on usb<%d:%d> initialized\n",
 		 vendor, product, ifnum,
 		 usbdev->bus->busnum, usbdev->devnum);
 
-	mutex_unlock(&driver_lock);
 	usb_put_dev(usbdev);
 
 	return 0;
 
 fail:
-	mutex_unlock(&driver_lock);
 	usb_put_dev(usbdev);
 	dev_err(dev, "unable to register, err %d\n", ret);
 
@@ -2498,10 +2497,8 @@  static void imon_disconnect(struct usb_interface *interface)
 	struct device *dev;
 	int ifnum;
 
-	/* prevent races with multi-interface device probing and display_open */
-	mutex_lock(&driver_lock);
-
 	ictx = usb_get_intfdata(interface);
+	ictx->disconnected = true;
 	dev = ictx->dev;
 	ifnum = interface->cur_altsetting->desc.bInterfaceNumber;
 
@@ -2542,11 +2539,9 @@  static void imon_disconnect(struct usb_interface *interface)
 		}
 	}
 
-	if (!ictx->dev_present_intf0 && !ictx->dev_present_intf1)
+	if (refcount_dec_and_test(&ictx->users))
 		free_imon_context(ictx);
 
-	mutex_unlock(&driver_lock);
-
 	dev_dbg(dev, "%s: iMON device (intf%d) disconnected\n",
 		__func__, ifnum);
 }