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    Subject[PATCH v4 21/32] tty: Document unsafe ldisc reference acquire
    Date
    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: Peter Hurley <peter@hurleysoftware.com>
    ---
    drivers/tty/tty_ldisc.c | 14 +++++---------
    1 file changed, 5 insertions(+), 9 deletions(-)

    diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
    index 9362a10..5ee0b2b 100644
    --- a/drivers/tty/tty_ldisc.c
    +++ b/drivers/tty/tty_ldisc.c
    @@ -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;
    }
    --
    1.8.1.2


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