vt: fix race in vt_waitactive()

pm_restore_console() is called from the suspend/resume path, and this
calls vt_move_to_console(), which calls vt_waitactive().

There's a race in this path which causes the process which requests the
suspend to sleep indefinitely waiting for an event which already
happened:

P1                                      P2
 vt_move_to_console()
  set_console()
    schedule_console_callback()
  vt_waitactive()
    check n == fg_console +1
                                       console_callback()
                                         switch_screen()
                                         vt_event_post() // no waiters

    vt_event_wait() // forever

Fix the race by ensuring we're registered for the event before we check
if it's already completed.
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt_ioctl.c

@@ -110,16 +110,7 @@ void vt_event_post(unsigned int event, unsigned int old, unsigned int new)
 		wake_up_interruptible(&vt_event_waitqueue);
 }
 
-/**
- *	vt_event_wait		-	wait for an event
- *	@vw: our event
- *
- *	Waits for an event to occur which completes our vt_event_wait
- *	structure. On return the structure has wv->done set to 1 for success
- *	or 0 if some event such as a signal ended the wait.
- */
-
-static void vt_event_wait(struct vt_event_wait *vw)
+static void __vt_event_queue(struct vt_event_wait *vw)
 {
 	unsigned long flags;
 	/* Prepare the event */
@@ -129,14 +120,40 @@ static void vt_event_wait(struct vt_event_wait *vw)
 	spin_lock_irqsave(&vt_event_lock, flags);
 	list_add(&vw->list, &vt_events);
 	spin_unlock_irqrestore(&vt_event_lock, flags);
+}
+
+static void __vt_event_wait(struct vt_event_wait *vw)
+{
 	/* Wait for it to pass */
 	wait_event_interruptible(vt_event_waitqueue, vw->done);
+}
+
+static void __vt_event_dequeue(struct vt_event_wait *vw)
+{
+	unsigned long flags;
+
 	/* Dequeue it */
 	spin_lock_irqsave(&vt_event_lock, flags);
 	list_del(&vw->list);
 	spin_unlock_irqrestore(&vt_event_lock, flags);
 }
 
+/**
+ *	vt_event_wait		-	wait for an event
+ *	@vw: our event
+ *
+ *	Waits for an event to occur which completes our vt_event_wait
+ *	structure. On return the structure has wv->done set to 1 for success
+ *	or 0 if some event such as a signal ended the wait.
+ */
+
+static void vt_event_wait(struct vt_event_wait *vw)
+{
+	__vt_event_queue(vw);
+	__vt_event_wait(vw);
+	__vt_event_dequeue(vw);
+}
+
 /**
  *	vt_event_wait_ioctl	-	event ioctl handler
  *	@arg: argument to ioctl
@@ -177,10 +194,14 @@ int vt_waitactive(int n)
 {
 	struct vt_event_wait vw;
 	do {
-		if (n == fg_console + 1)
-			break;
 		vw.event.event = VT_EVENT_SWITCH;
-		vt_event_wait(&vw);
+		__vt_event_queue(&vw);
+		if (n == fg_console + 1) {
+			__vt_event_dequeue(&vw);
+			break;
+		}
+		__vt_event_wait(&vw);
+		__vt_event_dequeue(&vw);
 		if (vw.done == 0)
 			return -EINTR;
 	} while (vw.event.newev != n);