[PATCH] tty reference count fix

Fix hole where tty structure can be released when reference count is non
zero. Existing code can sleep without tty_sem protection between deciding
to release the tty structure (setting local variables tty_closing and
otty_closing) and setting TTY_CLOSING to prevent further opens. An open
can occur during this interval causing release_dev() to free the tty
structure while it is still referenced.

This should fix bugzilla.kernel.org [Bug 6041] New: Unable to handle kernel
paging request

In Bug 6041, tty_open() oopes on accessing the tty structure it has
successfully claimed. Bug was on SMP machine with the same tty being
opened and closed by multiple processes, and DEBUG_PAGEALLOC enabled.

Signed-off-by: Paul Fulghum <[e-mail]>
Cc: Alan Cox <[e-mail]>
Cc: Jesper Juhl <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/char/tty_io.c

···
 		tty_closing = tty->count <= 1;
 		o_tty_closing = o_tty &&
 			(o_tty->count <= (pty_master ? 1 : 0));
-		up(&tty_sem);
 		do_sleep = 0;
 
 		if (tty_closing) {
···
 
 		printk(KERN_WARNING "release_dev: %s: read/write wait queue "
 				    "active!\n", tty_name(tty, buf));
+		up(&tty_sem);
 		schedule();
 	}	
 
···
 	 * both sides, and we've completed the last operation that could 
 	 * block, so it's safe to proceed with closing.
 	 */
-	 
-	down(&tty_sem);
 	if (pty_master) {
 		if (--o_tty->count < 0) {
 			printk(KERN_WARNING "release_dev: bad pty slave count "
···
 		       tty->count, tty_name(tty, buf));
 		tty->count = 0;
 	}
-	up(&tty_sem);
 	
 	/*
 	 * We've decremented tty->count, so we need to remove this file
···
 			} while_each_task_pid(o_tty->session, PIDTYPE_SID, p);
 		read_unlock(&tasklist_lock);
 	}
+
+	up(&tty_sem);
 
 	/* check whether both sides are closing ... */
 	if (!tty_closing || (o_tty && !o_tty_closing))