[PATCH] Console blanking locking fix

I've had WARN_CONSOLE_UNLOCKED warnings when calling TIOCLINUX
TIOCL_BLANKSCREEN and TIOCL_UNBLANKSCREEN.

(I'm blind and I use a braille display.  I use those functions to blank my
laptop's screen so people don't read it, and hopefully to conserve power.)

The warnings are from these places:
do_blank_screen at drivers/char/vt.c:2754 (Not tainted)
save_screen at drivers/char/vt.c:575 (Not tainted)
do_unblank_screen at drivers/char/vt.c:2822 (Not tainted)
set_palette at drivers/char/vt.c:2908 (Not tainted)

At a glance I would think the following patch ought to fix that.  Tested on
one machine.  Could you please tell me if this is correct and/or forward
the patch where appropriate...
Signed-off-by: NStephane Doyon <s.doyon@videotron.ca>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

drivers/char/vt.c

@@ -2272,7 +2272,9 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
 			ret = paste_selection(tty);
 			break;
 		case TIOCL_UNBLANKSCREEN:
+			acquire_console_sem();
 			unblank_screen();
+			release_console_sem();
 			break;
 		case TIOCL_SELLOADLUT:
 			ret = sel_loadlut(p);
@@ -2317,8 +2319,10 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
 			}
 			break;
 		case TIOCL_BLANKSCREEN:	/* until explicitly unblanked, not only poked */
+			acquire_console_sem();
 			ignore_poke = 1;
 			do_blank_screen(0);
+			release_console_sem();
 			break;
 		case TIOCL_BLANKEDSCREEN:
 			ret = console_blanked;