forked from Minki/linux
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: Peter Hurley <peter@hurleysoftware.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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@ -42,13 +42,6 @@ static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait);
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/* Line disc dispatch table */
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/* Line disc dispatch table */
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static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];
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static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];
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static inline struct tty_ldisc *get_ldisc(struct tty_ldisc *ld)
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{
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if (ld)
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atomic_inc(&ld->users);
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return ld;
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}
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/**
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/**
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* tty_register_ldisc - install a line discipline
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* tty_register_ldisc - install a line discipline
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* @disc: ldisc number
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* @disc: ldisc number
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@ -269,10 +262,13 @@ static struct tty_ldisc *tty_ldisc_try(struct tty_struct *tty)
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unsigned long flags;
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unsigned long flags;
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struct tty_ldisc *ld;
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struct tty_ldisc *ld;
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/* FIXME: this allows reference acquire after TTY_LDISC is cleared */
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raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
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raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
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ld = NULL;
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ld = NULL;
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if (test_bit(TTY_LDISC, &tty->flags))
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if (test_bit(TTY_LDISC, &tty->flags) && tty->ldisc) {
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ld = get_ldisc(tty->ldisc);
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ld = tty->ldisc;
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atomic_inc(&ld->users);
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}
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raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
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raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
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return ld;
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return ld;
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}
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}
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