From 33d7136336a94feade73b08b9c4bd1420df4e572 Mon Sep 17 00:00:00 2001 From: Peter Hurley Date: Sat, 9 Jan 2016 21:45:08 -0800 Subject: [PATCH] n_tty: Always wake up read()/poll() if new input A read() in non-canonical mode when VMIN > 0 and VTIME == 0 does not complete until at least VMIN chars have been read (or the user buffer is full). In this infrequent read mode, n_tty_read() attempts to reduce wakeups by computing the amount of data still necessary to complete the read (minimum_to_wake) and only waking the read()/poll() when that much unread data has been processed. This is the only read mode for which new data does not necessarily generate a wakeup. However, this optimization is broken and commonly leads to hung reads even though the necessary amount of data has been received. Since the optimization is of marginal value anyway, just remove the whole thing. This also remedies a race between a concurrent poll() and read() in this mode, where the poll() can reset the minimum_to_wake of the read() (and vice versa). Signed-off-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman --- drivers/tty/n_tty.c | 34 ++-------------------------------- 1 file changed, 2 insertions(+), 32 deletions(-) diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c index 827206914b02..d04f398d5de3 100644 --- a/drivers/tty/n_tty.c +++ b/drivers/tty/n_tty.c @@ -113,8 +113,6 @@ struct n_tty_data { DECLARE_BITMAP(read_flags, N_TTY_BUF_SIZE); unsigned char echo_buf[N_TTY_BUF_SIZE]; - int minimum_to_wake; - /* consumer-published */ size_t read_tail; size_t line_start; @@ -1633,7 +1631,7 @@ static void __receive_buf(struct tty_struct *tty, const unsigned char *cp, /* publish read_head to consumer */ smp_store_release(&ldata->commit_head, ldata->read_head); - if ((read_cnt(ldata) >= ldata->minimum_to_wake) || L_EXTPROC(tty)) { + if (read_cnt(ldata)) { kill_fasync(&tty->fasync, SIGIO, POLL_IN); wake_up_interruptible_poll(&tty->read_wait, POLLIN); } @@ -1900,7 +1898,6 @@ static int n_tty_open(struct tty_struct *tty) reset_buffer_flags(tty->disc_data); ldata->column = 0; ldata->canon_column = 0; - ldata->minimum_to_wake = 1; ldata->num_overrun = 0; ldata->no_room = 0; ldata->lnext = 0; @@ -2163,14 +2160,9 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file, minimum = MIN_CHAR(tty); if (minimum) { time = (HZ / 10) * TIME_CHAR(tty); - if (time) - ldata->minimum_to_wake = 1; - else if (!waitqueue_active(&tty->read_wait) || - (ldata->minimum_to_wake > minimum)) - ldata->minimum_to_wake = minimum; } else { timeout = (HZ / 10) * TIME_CHAR(tty); - ldata->minimum_to_wake = minimum = 1; + minimum = 1; } } @@ -2197,10 +2189,6 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file, break; } - if (((minimum - (b - buf)) < ldata->minimum_to_wake) && - ((minimum - (b - buf)) >= 1)) - ldata->minimum_to_wake = (minimum - (b - buf)); - done = check_other_done(tty); if (!input_available_p(tty, 0)) { @@ -2266,9 +2254,6 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file, up_read(&tty->termios_rwsem); remove_wait_queue(&tty->read_wait, &wait); - if (!waitqueue_active(&tty->read_wait)) - ldata->minimum_to_wake = minimum; - mutex_unlock(&ldata->atomic_read_lock); if (b - buf) @@ -2403,7 +2388,6 @@ break_out: static unsigned int n_tty_poll(struct tty_struct *tty, struct file *file, poll_table *wait) { - struct n_tty_data *ldata = tty->disc_data; unsigned int mask = 0; poll_wait(file, &tty->read_wait, wait); @@ -2416,12 +2400,6 @@ static unsigned int n_tty_poll(struct tty_struct *tty, struct file *file, mask |= POLLPRI | POLLIN | POLLRDNORM; if (tty_hung_up_p(file)) mask |= POLLHUP; - if (!(mask & (POLLHUP | POLLIN | POLLRDNORM))) { - if (MIN_CHAR(tty) && !TIME_CHAR(tty)) - ldata->minimum_to_wake = MIN_CHAR(tty); - else - ldata->minimum_to_wake = 1; - } if (tty->ops->write && !tty_is_writelocked(tty) && tty_chars_in_buffer(tty) < WAKEUP_CHARS && tty_write_room(tty) > 0) @@ -2472,14 +2450,6 @@ static int n_tty_ioctl(struct tty_struct *tty, struct file *file, static void n_tty_fasync(struct tty_struct *tty, int on) { - struct n_tty_data *ldata = tty->disc_data; - - if (!waitqueue_active(&tty->read_wait)) { - if (on) - ldata->minimum_to_wake = 1; - else if (!tty->fasync) - ldata->minimum_to_wake = N_TTY_BUF_SIZE; - } } static struct tty_ldisc_ops n_tty_ops = { -- 2.30.2