[PATCH 4.9 033/104] serial: 8250_port: Remove dangerous pr_debug()

From: Greg Kroah-Hartman
Date: Fri Oct 06 2017 - 04:55:05 EST

4.9-stable review patch. If anyone has any objections, please let me know.


From: Alexey Brodkin <Alexey.Brodkin@xxxxxxxxxxxx>

[ 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:
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".

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:
dyndbg="+p; file 8250_port.c line 1756 -p; file 8250_port.c line 1822 -p"

Signed-off-by: Alexey Brodkin <abrodkin@xxxxxxxxxxxx>
Cc: Jiri Slaby <jslaby@xxxxxxxx>
Cc: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Cc: Phillip Raffeck <phillip.raffeck@xxxxxx>
Cc: Anton Wuerfel <anton.wuerfel@xxxxxx>
Cc: "Matwey V. Kornilov" <matwey@xxxxxxxxxx>
Cc: Yegor Yefremov <yegorslists@xxxxxxxxxxxxxx>
Cc: Thor Thayer <tthayer@xxxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
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)

- 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);