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    Subject[PATCH 5/9] n_tty: Eliminate echo_commit memory barrier
    Date
    Use output_lock mutex as a memory barrier when storing echo_commit.

    Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
    ---
    drivers/tty/n_tty.c | 31 ++++++++++++++++++++-----------
    1 file changed, 20 insertions(+), 11 deletions(-)

    diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
    index 49ef008..c101492 100644
    --- a/drivers/tty/n_tty.c
    +++ b/drivers/tty/n_tty.c
    @@ -637,21 +637,16 @@ break_out:
    * are prioritized. Also, when control characters are echoed with a
    * prefixed "^", the pair is treated atomically and thus not separated.
    *
    - * Locking: output_lock to protect column state and space left
    + * Locking: callers must hold output_lock
    */

    -static void process_echoes(struct tty_struct *tty)
    +static void __process_echoes(struct tty_struct *tty)
    {
    struct n_tty_data *ldata = tty->disc_data;
    int space, nr;
    size_t tail;
    unsigned char c;

    - if (ldata->echo_commit == ldata->echo_tail)
    - return;
    -
    - mutex_lock(&ldata->output_lock);
    -
    space = tty_write_room(tty);

    tail = ldata->echo_tail;
    @@ -772,20 +767,34 @@ static void process_echoes(struct tty_struct *tty)
    }

    ldata->echo_tail = tail;
    +}
    +
    +static void commit_echoes(struct tty_struct *tty)
    +{
    + struct n_tty_data *ldata = tty->disc_data;

    + mutex_lock(&ldata->output_lock);
    + ldata->echo_commit = ldata->echo_head;
    + __process_echoes(tty);
    mutex_unlock(&ldata->output_lock);

    if (tty->ops->flush_chars)
    tty->ops->flush_chars(tty);
    }

    -static void commit_echoes(struct tty_struct *tty)
    +static void process_echoes(struct tty_struct *tty)
    {
    struct n_tty_data *ldata = tty->disc_data;

    - smp_mb();
    - ldata->echo_commit = ldata->echo_head;
    - process_echoes(tty);
    + if (!L_ECHO(tty) || ldata->echo_commit == ldata->echo_tail)
    + return;
    +
    + mutex_lock(&ldata->output_lock);
    + __process_echoes(tty);
    + mutex_unlock(&ldata->output_lock);
    +
    + if (tty->ops->flush_chars)
    + tty->ops->flush_chars(tty);
    }

    /**
    --
    1.8.1.2


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