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    Subject[PATCH 01/13] tty_ldisc: add more limits to the @write_wakeup
    Date
    From: Huang Shijie <b32955@freescale.com>

    In the uart_handle_cts_change(), uart_write_wakeup() is called after
    we call @uart_port->ops->start_tx().

    The Documentation/serial/driver tells us:
    -----------------------------------------------
    start_tx(port)
    Start transmitting characters.

    Locking: port->lock taken.
    Interrupts: locally disabled.
    -----------------------------------------------

    So when the uart_write_wakeup() is called, the port->lock is taken by
    the upper. See the following callstack:

    |_ uart_write_wakeup
    |_ tty_wakeup
    |_ ld->ops->write_wakeup

    With the port->lock held, we call the @write_wakeup. Some implemetation of
    the @write_wakeup does not notice that the port->lock is held, and it still
    tries to send data with uart_write() which will try to grab the prot->lock.
    A dead lock occurs, see the following log caught in the Bluetooth by uart:

    --------------------------------------------------------------------
    BUG: spinlock lockup suspected on CPU#0, swapper/0/0
    lock: 0xdc3f4410, .magic: dead4ead, .owner: swapper/0/0, .owner_cpu: 0
    CPU: 0 PID: 0 Comm: swapper/0 Tainted: G W 3.10.17-16839-ge4a1bef #1320
    [<80014cbc>] (unwind_backtrace+0x0/0x138) from [<8001251c>] (show_stack+0x10/0x14)
    [<8001251c>] (show_stack+0x10/0x14) from [<802816ac>] (do_raw_spin_lock+0x108/0x184)
    [<802816ac>] (do_raw_spin_lock+0x108/0x184) from [<806a22b0>] (_raw_spin_lock_irqsave+0x54/0x60)
    [<806a22b0>] (_raw_spin_lock_irqsave+0x54/0x60) from [<802f5754>] (uart_write+0x38/0xe0)
    [<802f5754>] (uart_write+0x38/0xe0) from [<80455270>] (hci_uart_tx_wakeup+0xa4/0x168)
    [<80455270>] (hci_uart_tx_wakeup+0xa4/0x168) from [<802dab18>] (tty_wakeup+0x50/0x5c)
    [<802dab18>] (tty_wakeup+0x50/0x5c) from [<802f81a4>] (imx_rtsint+0x50/0x80)
    [<802f81a4>] (imx_rtsint+0x50/0x80) from [<802f88f4>] (imx_int+0x158/0x17c)
    [<802f88f4>] (imx_int+0x158/0x17c) from [<8007abe0>] (handle_irq_event_percpu+0x50/0x194)
    [<8007abe0>] (handle_irq_event_percpu+0x50/0x194) from [<8007ad60>] (handle_irq_event+0x3c/0x5c)
    --------------------------------------------------------------------

    This patch adds more limits to the @write_wakeup, the one who wants to
    implemet the @write_wakeup should follow the limits which avoid the deadlock.

    Signed-off-by: Huang Shijie <b32955@freescale.com>
    Signed-off-by: Felipe Balbi <balbi@ti.com>
    ---
    include/linux/tty_ldisc.h | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
    index add26da..00c9d68 100644
    --- a/include/linux/tty_ldisc.h
    +++ b/include/linux/tty_ldisc.h
    @@ -92,7 +92,10 @@
    * This function is called by the low-level tty driver to signal
    * that line discpline should try to send more characters to the
    * low-level driver for transmission. If the line discpline does
    - * not have any more data to send, it can just return.
    + * not have any more data to send, it can just return. If the line
    + * discipline does have some data to send, please arise a tasklet
    + * or workqueue to do the real data transfer. Do not send data in
    + * this hook, it may leads to a deadlock.
    *
    * int (*hangup)(struct tty_struct *)
    *
    --
    1.9.2.459.g68773ac


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