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    Subjecttty vs workqueue oddities
    Hi Alan !

    Current upstream (but that's been around for at least 2 or 3 days) seems
    to have a strange behaviour on one of my powerbooks. Something like
    "dmesg" or "cat" of a large file in an X terminal "hangs" the machine
    litterally for minutes. It generally recovers, so not always.

    Network is unresponsive as well.

    My attempts at stopping it into xmon always landed in process_one_work()
    or flush_to_ldisc() from what I can tell, and a simple ftrace run shows
    something that looks like an -enormous- lot of:

    kworker/0:1-258 [000] 412.105871: flush_to_ldisc <-process_one_work
    kworker/0:1-258 [000] 412.105871: tty_ldisc_ref <-flush_to_ldisc
    kworker/0:1-258 [000] 412.105872: n_tty_receive_buf <-flush_to_ldisc
    kworker/0:1-258 [000] 412.105872: kill_fasync <-n_tty_receive_buf
    kworker/0:1-258 [000] 412.105873: __wake_up <-n_tty_receive_buf
    kworker/0:1-258 [000] 412.105873: __wake_up_common <-__wake_up
    kworker/0:1-258 [000] 412.105874: default_wake_function <-__wake_up_common
    kworker/0:1-258 [000] 412.105874: try_to_wake_up <-default_wake_function
    kworker/0:1-258 [000] 412.105874: tty_throttle <-n_tty_receive_buf
    kworker/0:1-258 [000] 412.105875: mutex_lock <-tty_throttle
    kworker/0:1-258 [000] 412.105875: mutex_unlock <-tty_throttle
    kworker/0:1-258 [000] 412.105876: schedule_work <-flush_to_ldisc
    kworker/0:1-258 [000] 412.105876: queue_work <-schedule_work
    kworker/0:1-258 [000] 412.105877: queue_work_on <-queue_work
    kworker/0:1-258 [000] 412.105877: __queue_work <-queue_work_on
    kworker/0:1-258 [000] 412.105878: insert_work <-__queue_work
    kworker/0:1-258 [000] 412.105878: tty_ldisc_deref <-flush_to_ldisc
    kworker/0:1-258 [000] 412.105879: put_ldisc <-tty_ldisc_deref
    kworker/0:1-258 [000] 412.105879: __wake_up <-put_ldisc
    kworker/0:1-258 [000] 412.105880: __wake_up_common <-__wake_up
    kworker/0:1-258 [000] 412.105880: cwq_dec_nr_in_flight <-process_one_work
    kworker/0:1-258 [000] 412.105880: process_one_work <-worker_thread

    and repeat that sequence more/less identical ad nauseum

    Sometimes it breaks out and makes progress, usually after a few mn.

    2.6.39 is fine. I'm going to attempt a bisection but it's a bit slow on
    those machines and I'm running out of time today, so I wanted to shoot
    that to you in case it rings a bell.


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