Input: evdev - never leave the client buffer empty after write

When the client buffer is very small and wraps around a lot, it may
well be that a write increases the head such that head == tail. If
this happens between the point where a poll is triggered and the
actual data is being read, there will be no data to read. This is
confusing to applications, which might end up closing the file.

This patch solves the problem by making sure the client buffer is
never empty after writing to it.
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/evdev.c

@@ -54,11 +54,15 @@ static void evdev_pass_event(struct evdev_client *client,
 			     struct input_event *event)
 {
 	/*
-	 * Interrupts are disabled, just acquire the lock
+	 * Interrupts are disabled, just acquire the lock.
+	 * Make sure we don't leave with the client buffer
+	 * "empty" by having client->head == client->tail.
 	 */
 	spin_lock(&client->buffer_lock);
-	client->buffer[client->head++] = *event;
-	client->head &= client->bufsize - 1;
+	do {
+		client->buffer[client->head++] = *event;
+		client->head &= client->bufsize - 1;
+	} while (client->head == client->tail);
 	spin_unlock(&client->buffer_lock);
 
 	if (event->type == EV_SYN)