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    Subject[34-longterm 053/209] TTY: restore tty_ldisc_wait_idle
    Date
    From: Jiri Slaby <jslaby@suse.cz>

    =====================================================================
    | This is a commit scheduled for the next v2.6.34 longterm release. |
    | If you see a problem with using this for longterm, please comment.|
    =====================================================================

    commit 100eeae2c5ce23b4db93ff320ee330ef1d740151 upstream.

    It was removed in 65b770468e98 (tty-ldisc: turn ldisc user count into
    a proper refcount), but we need to wait for last user to quit the
    ldisc before we close it in tty_set_ldisc.

    Otherwise weird things start to happen. There might be processes
    waiting in tty_read->n_tty_read on tty->read_wait for input to appear
    and at that moment, a change of ldisc is fatal. n_tty_close is called,
    it frees read_buf and the waiting process is still in the middle of
    reading and goes nuts after it is woken.

    Previously we prevented close to happen when others are in ldisc ops
    by tty_ldisc_wait_idle in tty_set_ldisc. But the commit above removed
    that. So revoke the change and test whether there is 1 user (=we), and
    allow the close then.

    We can do that without ldisc/tty locks, because nobody else can open
    the device due to TTY_LDISC_CHANGING bit set, so we in fact wait for
    everybody to leave.

    I don't understand why tty_ldisc_lock would be needed either when the
    counter is an atomic variable, so this is a lockless
    tty_ldisc_wait_idle.

    On the other hand, if we fail to wait (timeout or signal), we have to
    reenable the halted ldiscs, so we take ldisc lock and reuse the setup
    path at the end of tty_set_ldisc.

    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
    Tested-by: Sebastian Andrzej Siewior <bigeasy@breakpoint.cc>
    Signed-off-by: Andi Kleen <ak@linux.intel.com>
    LKML-Reference: <20101031104136.GA511@Chamillionaire.breakpoint.cc>
    LKML-Reference: <1287669539-22644-1-git-send-email-jslaby@suse.cz>
    Cc: Alan Cox <alan@linux.intel.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
    Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
    ---
    drivers/char/tty_ldisc.c | 29 +++++++++++++++++++++++++++++
    1 files changed, 29 insertions(+), 0 deletions(-)

    diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
    index 500e740..df5e7db 100644
    --- a/drivers/char/tty_ldisc.c
    +++ b/drivers/char/tty_ldisc.c
    @@ -47,6 +47,7 @@

    static DEFINE_SPINLOCK(tty_ldisc_lock);
    static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait);
    +static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_idle);
    /* Line disc dispatch table */
    static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];

    @@ -83,6 +84,7 @@ static void put_ldisc(struct tty_ldisc *ld)
    return;
    }
    local_irq_restore(flags);
    + wake_up(&tty_ldisc_idle);
    }

    /**
    @@ -530,6 +532,23 @@ static int tty_ldisc_halt(struct tty_struct *tty)
    }

    /**
    + * tty_ldisc_wait_idle - wait for the ldisc to become idle
    + * @tty: tty to wait for
    + *
    + * Wait for the line discipline to become idle. The discipline must
    + * have been halted for this to guarantee it remains idle.
    + */
    +static int tty_ldisc_wait_idle(struct tty_struct *tty)
    +{
    + int ret;
    + ret = wait_event_interruptible_timeout(tty_ldisc_idle,
    + atomic_read(&tty->ldisc->users) == 1, 5 * HZ);
    + if (ret < 0)
    + return ret;
    + return ret > 0 ? 0 : -EBUSY;
    +}
    +
    +/**
    * tty_set_ldisc - set line discipline
    * @tty: the terminal to set
    * @ldisc: the line discipline
    @@ -632,8 +651,17 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)

    flush_scheduled_work();

    + retval = tty_ldisc_wait_idle(tty);
    +
    mutex_lock(&tty->ldisc_mutex);
    lock_kernel();
    +
    + /* handle wait idle failure locked */
    + if (retval) {
    + tty_ldisc_put(new_ldisc);
    + goto enable;
    + }
    +
    if (test_bit(TTY_HUPPED, &tty->flags)) {
    /* We were raced by the hangup method. It will have stomped
    the ldisc data and closed the ldisc down */
    @@ -667,6 +695,7 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)

    tty_ldisc_put(o_ldisc);

    +enable:
    /*
    * Allow ldisc referencing to occur again
    */
    --
    1.7.4.4


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