From a5660b41af6a28f8004e70eb261e1202ad55c5e3 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Mon, 4 Apr 2011 14:26:54 -0700 Subject: [PATCH] tty: fix endless work loop when the buffer fills up Commit f23eb2b2b285 ('tty: stop using "delayed_work" in the tty layer') ended up causing hung machines on UP with no preemption, because the work routine to flip the buffer data to the ldisc would endlessly re-arm itself if the destination buffer had filled up. With the delayed work, that only caused a timer-driving polling of the tty state every timer tick, but without the delay we just ended up with basically a busy loop instead. Stop the insane polling, and instead make the code that opens up the receive room re-schedule the buffer flip work. That's what we should have been doing anyway. This same "poll for tty room" issue is almost certainly also the cause of excessive kworker activity when idle reported by Dave Jones, who also reported "flush_to_ldisc executing 2500 times a second" back in Nov 2010: http://lkml.org/lkml/2010/11/30/592 which is that silly flushing done every timer tick. Wasting both power and CPU for no good reason. Reported-and-tested-by: Alexander Beregalov Reported-and-tested-by: Sitsofe Wheeler Cc: Greg KH Cc: Alan Cox Cc: Dave Jones Signed-off-by: Linus Torvalds --- drivers/tty/n_tty.c | 6 ++++++ drivers/tty/tty_buffer.c | 4 +--- 2 files changed, 7 insertions(+), 3 deletions(-) diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c index 428f4fe0b5f..0ad32888091 100644 --- a/drivers/tty/n_tty.c +++ b/drivers/tty/n_tty.c @@ -95,6 +95,7 @@ static void n_tty_set_room(struct tty_struct *tty) { /* tty->read_cnt is not read locked ? */ int left = N_TTY_BUF_SIZE - tty->read_cnt - 1; + int old_left; /* * If we are doing input canonicalization, and there are no @@ -104,7 +105,12 @@ static void n_tty_set_room(struct tty_struct *tty) */ if (left <= 0) left = tty->icanon && !tty->canon_data; + old_left = tty->receive_room; tty->receive_room = left; + + /* Did this open up the receive buffer? We may need to flip */ + if (left && !old_left) + schedule_work(&tty->buf.work); } static void put_tty_queue_nolock(unsigned char c, struct tty_struct *tty) diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c index b9451219528..f1a7918d71a 100644 --- a/drivers/tty/tty_buffer.c +++ b/drivers/tty/tty_buffer.c @@ -442,10 +442,8 @@ static void flush_to_ldisc(struct work_struct *work) line discipline as we want to empty the queue */ if (test_bit(TTY_FLUSHPENDING, &tty->flags)) break; - if (!tty->receive_room || seen_tail) { - schedule_work(&tty->buf.work); + if (!tty->receive_room || seen_tail) break; - } if (count > tty->receive_room) count = tty->receive_room; char_buf = head->char_buf_ptr + head->read; -- 2.20.1