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    Subject[PATCH 4.9 033/104] serial: 8250_port: Remove dangerous pr_debug()
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Alexey Brodkin <Alexey.Brodkin@synopsys.com>


    [ Upstream commit 699a11ba7ec869b006623182881f2f1f5b4aea53 ]

    With CONFIG_DYNAMIC_DEBUG if dyndbg enables debug output in
    8250_port.c deadlock happens inevitably on UART IRQ handling.

    That's the problematic execution path:
    ---------------------------->8------------------------
    UART IRQ:
    serial8250_interrupt() ->
    serial8250_handle_irq(): lock "port->lock" ->
    pr_debug() ->
    serial8250_console_write(): bump in locked "port->lock".

    OR (if above pr_debug() gets removed):
    serial8250_tx_chars() ->
    pr_debug() ->
    serial8250_console_write(): bump in locked "port->lock".
    ---------------------------->8------------------------

    So let's get rid of those not that much useful debug entries.

    Discussed problem could be easily reproduced with QEMU for x86_64.
    As well as this fix could be mimicked with muting of dynamic debug for
    the problematic lines as simple as:
    ---------------------------->8------------------------
    dyndbg="+p; file 8250_port.c line 1756 -p; file 8250_port.c line 1822 -p"
    ---------------------------->8------------------------

    Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
    Cc: Jiri Slaby <jslaby@suse.com>
    Cc: Peter Hurley <peter@hurleysoftware.com>
    Cc: Phillip Raffeck <phillip.raffeck@fau.de>
    Cc: Anton Wuerfel <anton.wuerfel@fau.de>
    Cc: "Matwey V. Kornilov" <matwey@sai.msu.ru>
    Cc: Yegor Yefremov <yegorslists@googlemail.com>
    Cc: Thor Thayer <tthayer@opensource.altera.com>
    Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/tty/serial/8250/8250_port.c | 4 ----
    1 file changed, 4 deletions(-)

    --- a/drivers/tty/serial/8250/8250_port.c
    +++ b/drivers/tty/serial/8250/8250_port.c
    @@ -1751,8 +1751,6 @@ void serial8250_tx_chars(struct uart_825
    if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
    uart_write_wakeup(port);

    - pr_debug("%s: THRE\n", __func__);
    -
    /*
    * With RPM enabled, we have to wait until the FIFO is empty before the
    * HW can go idle. So we get here once again with empty FIFO and disable
    @@ -1817,8 +1815,6 @@ int serial8250_handle_irq(struct uart_po

    status = serial_port_in(port, UART_LSR);

    - pr_debug("%s: status = %x\n", __func__, status);
    -
    if (status & (UART_LSR_DR | UART_LSR_BI)) {
    if (!up->dma || handle_rx_dma(up, iir))
    status = serial8250_rx_chars(up, status);

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