[PATCH] vt: fix a potential race in the VT_WAITACTIVE handler

[Michal Januszewski <spock@gentoo.org>]

On a multiprocessor machine the VT_WAITACTIVE ioctl call may return 0
if fg_console has already been updated in redraw_screen(), but the 
console switch itself hasn't been completed. Fix this by checking
fg_console in vt_waitactive() with the console sem held.

Signed-off-by: Michal Januszewski <spock@gentoo.org>

---
diff --git a/drivers/char/vt_ioctl.c b/drivers/char/vt_ioctl.c
index 3a5d301..00b5b34 100644
--- a/drivers/char/vt_ioctl.c
+++ b/drivers/char/vt_ioctl.c
@@ -1041,8 +1041,12 @@ int vt_waitactive(int vt)
 	for (;;) {
 		set_current_state(TASK_INTERRUPTIBLE);
 		retval = 0;
-		if (vt == fg_console)
+		acquire_console_sem();
+		if (vt == fg_console) {
+			release_console_sem();
 			break;
+		}
+		release_console_sem();
 		retval = -EINTR;
 		if (signal_pending(current))
 			break;