gpio: cdev: wake up linereq poll() on device unbind

Add a notifier block to the linereq structure and register it with the
gpio_device's device notifier. Upon reception of an event, wake up the
wait queue so that the user-space be forced out of poll() and need to go
into a new system call which will then fail due to the chip being gone.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Kent Gibson <warthog618@gmail.com>

Author: Bartosz Golaszewski

drivers/gpio/gpiolib-cdev.c

@@ -555,6 +555,7 @@ struct line {
  * @label: consumer label used to tag GPIO descriptors
  * @num_lines: the number of lines in the lines array
  * @wait: wait queue that handles blocking reads of events
+ * @device_unregistered_nb: notifier block for receiving gdev unregister events
  * @event_buffer_size: the number of elements allocated in @events
  * @events: KFIFO for the GPIO events
  * @seqno: the sequence number for edge events generated on all lines in
@@ -569,6 +570,7 @@ struct linereq {
 	const char *label;
 	u32 num_lines;
 	wait_queue_head_t wait;
+	struct notifier_block device_unregistered_nb;
 	u32 event_buffer_size;
 	DECLARE_KFIFO_PTR(events, struct gpio_v2_line_event);
 	atomic_t seqno;
@@ -610,6 +612,17 @@ struct linereq {
 	 GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE | \
 	 GPIO_V2_LINE_EDGE_FLAGS)
 
+static int linereq_unregistered_notify(struct notifier_block *nb,
+				       unsigned long action, void *data)
+{
+	struct linereq *lr = container_of(nb, struct linereq,
+					  device_unregistered_nb);
+
+	wake_up_poll(&lr->wait, EPOLLIN | EPOLLERR);
+
+	return NOTIFY_OK;
+}
+
 static void linereq_put_event(struct linereq *lr,
 			      struct gpio_v2_line_event *le)
 {
@@ -1567,6 +1580,10 @@ static void linereq_free(struct linereq *lr)
 {
 	unsigned int i;
 
+	if (lr->device_unregistered_nb.notifier_call)
+		blocking_notifier_chain_unregister(&lr->gdev->device_notifier,
+						   &lr->device_unregistered_nb);
+
 	for (i = 0; i < lr->num_lines; i++) {
 		if (lr->lines[i].desc) {
 			edge_detector_stop(&lr->lines[i]);
@@ -1727,6 +1744,12 @@ static int linereq_create(struct gpio_device *gdev, void __user *ip)
 			offset);
 	}
 
+	lr->device_unregistered_nb.notifier_call = linereq_unregistered_notify;
+	ret = blocking_notifier_chain_register(&gdev->device_notifier,
+					       &lr->device_unregistered_nb);
+	if (ret)
+		goto out_free_linereq;
+
 	fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC);
 	if (fd < 0) {
 		ret = fd;