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Commit 16759f6c authored by Peter Hurley's avatar Peter Hurley Committed by Greg Kroah-Hartman
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tty: Document unsafe ldisc reference acquire 



Merge get_ldisc() into its only call site.
Note how, after merging, the unsafe acquire of an ldisc reference
is obvious.

   CPU 0 in tty_ldisc_try()         |  CPU 1 in tty_ldisc_halt()
                                    |
test_bit(TTY_LDISC, &tty_flags)     |
if (true)                           |  clear_bit(TTY_LDISC, &tty_flags)
  tty->ldisc != 0?                  |  atomic_read(&tty->ldisc->users)
  if (true)                         |  ret_val == 1?
    atomic_inc(&tty->ldisc->users)  |  if (false)
                                    |    wait
                                    |
<goes on assuming safe ldisc use>   |  <doesn't wait - proceeds w/ close>
                                    |

The spin lock in tty_ldisc_try() does nothing wrt synchronizing
the ldisc halt since it's not acquired as part of halting.

Signed-off-by: default avatarPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent ebc9baed

drivers/tty/tty_ldisc.c

+5 −9
Original line number Diff line number Diff line
@@ -42,13 +42,6 @@ static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait); 
/* Line disc dispatch table */

static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];



static inline struct tty_ldisc *get_ldisc(struct tty_ldisc *ld)

{

	if (ld)

		atomic_inc(&ld->users);

	return ld;

}



/**

 *	tty_register_ldisc	-	install a line discipline

 *	@disc: ldisc number
@@ -269,10 +262,13 @@ static struct tty_ldisc *tty_ldisc_try(struct tty_struct *tty) 
	unsigned long flags;

	struct tty_ldisc *ld;



	/* FIXME: this allows reference acquire after TTY_LDISC is cleared */

	raw_spin_lock_irqsave(&tty_ldisc_lock, flags);

	ld = NULL;

	if (test_bit(TTY_LDISC, &tty->flags))

		ld = get_ldisc(tty->ldisc);

	if (test_bit(TTY_LDISC, &tty->flags) && tty->ldisc) {

		ld = tty->ldisc;

		atomic_inc(&ld->users);

	}

	raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);

	return ld;

}