* [PATCH] serial: 8250_port: Remove dangerous pr_debug()
@ 2017-01-10 15:11 Alexey Brodkin
2017-01-10 15:56 ` Andy Shevchenko
0 siblings, 1 reply; 2+ messages in thread
From: Alexey Brodkin @ 2017-01-10 15:11 UTC (permalink / raw)
To: linux-serial
Cc: linux-kernel, Alexey Brodkin, Greg Kroah-Hartman, Jiri Slaby,
Peter Hurley, Andy Shevchenko, Phillip Raffeck, Anton Wuerfel,
Matwey V. Kornilov, Yegor Yefremov, Thor Thayer
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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.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>
---
drivers/tty/serial/8250/8250_port.c | 4 ----
1 file changed, 4 deletions(-)
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index fe4399b41df6..3cfdd745a97a 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1753,8 +1753,6 @@ void serial8250_tx_chars(struct uart_8250_port *up)
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
@@ -1819,8 +1817,6 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
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);
--
2.7.4
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] serial: 8250_port: Remove dangerous pr_debug()
2017-01-10 15:11 [PATCH] serial: 8250_port: Remove dangerous pr_debug() Alexey Brodkin
@ 2017-01-10 15:56 ` Andy Shevchenko
0 siblings, 0 replies; 2+ messages in thread
From: Andy Shevchenko @ 2017-01-10 15:56 UTC (permalink / raw)
To: Alexey Brodkin, linux-serial
Cc: linux-kernel, Greg Kroah-Hartman, Jiri Slaby, Peter Hurley,
Phillip Raffeck, Anton Wuerfel, Matwey V. Kornilov,
Yegor Yefremov, Thor Thayer
On Tue, 2017-01-10 at 18:11 +0300, Alexey Brodkin wrote:
> 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------------------------
Before it was disabled globally for the driver, but someone converted
macro to pr_debug() and problem appears.
Since it's known issue I remember discussion where someone proposed to
schedule printk() in such cases. Don't know if the idea made upstream or
on its way.
FWIW:
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
>
> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Jiri Slaby <jslaby@suse.com>
> Cc: Peter Hurley <peter@hurleysoftware.com>
> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.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>
> ---
> drivers/tty/serial/8250/8250_port.c | 4 ----
> 1 file changed, 4 deletions(-)
>
> diff --git a/drivers/tty/serial/8250/8250_port.c
> b/drivers/tty/serial/8250/8250_port.c
> index fe4399b41df6..3cfdd745a97a 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -1753,8 +1753,6 @@ void serial8250_tx_chars(struct uart_8250_port
> *up)
> 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
> @@ -1819,8 +1817,6 @@ int serial8250_handle_irq(struct uart_port
> *port, unsigned int iir)
>
> 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);
--
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Intel Finland Oy
^ permalink raw reply [flat|nested] 2+ messages in thread
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