[PATCH] 2.5: console close race fix

[Robert Love <rml@tech9.net>]

Linus,

There is a race between con_flush_chars and con_close.  I first
discovered it with the preemptive kernel patch, but it is a general SMP
race.  The idea for the fix is originally by Andrew Morton.

This fix is in 2.4.  Please, apply.

	Robert Love

diff -urN linux-2.5.3-pre5/drivers/char/console.c linux/drivers/char/console.c
--- linux-2.5.3-pre5/drivers/char/console.c	Thu Jan 24 17:02:57 2002
+++ linux/drivers/char/console.c	Fri Jan 25 18:12:36 2002
@@ -2349,14 +2349,18 @@
 
 static void con_flush_chars(struct tty_struct *tty)
 {
-	struct vt_struct *vt = (struct vt_struct *)tty->driver_data;
+	struct vt_struct *vt;
 
 	if (in_interrupt())	/* from flush_to_ldisc */
 		return;
 
 	pm_access(pm_con);
+	
+	/* if we race with con_close(), vt may be null */
 	acquire_console_sem();
-	set_cursor(vt->vc_num);
+	vt = (struct vt_struct *)tty->driver_data;
+	if (vt)
+		set_cursor(vt->vc_num);
 	release_console_sem();
 }