Re: [PATCH] serial: 8250_port: Remove dangerous pr_debug()

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

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