linux-kernel - [PATCH] serial: 8250_port: Remove dangerous pr

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Date:   Tue, 10 Jan 2017 18:11:29 +0300
From:   Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:     linux-serial@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Hurley <peter@...leysoftware.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Phillip Raffeck <phillip.raffeck@....de>,
        Anton Wuerfel <anton.wuerfel@....de>,
        "Matwey V. Kornilov" <matwey@....msu.ru>,
        Yegor Yefremov <yegorslists@...glemail.com>,
        Thor Thayer <tthayer@...nsource.altera.com>
Subject: [PATCH] serial: 8250_port: Remove dangerous pr_debug()

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@...opsys.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Jiri Slaby <jslaby@...e.com>
Cc: Peter Hurley <peter@...leysoftware.com>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Phillip Raffeck <phillip.raffeck@....de>
Cc: Anton Wuerfel <anton.wuerfel@....de>
Cc: "Matwey V. Kornilov" <matwey@....msu.ru>
Cc: Yegor Yefremov <yegorslists@...glemail.com>
Cc: Thor Thayer <tthayer@...nsource.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