serial: 8250_dw: Fix deadlock in LCR workaround

commit 7fd6f640f2dd17dac6ddd6702c378cb0bb9cfa11 upstream.

Trying to write console output from within the serial console driver
while the port->lock is held causes recursive deadlock:

  CPU 0
spin_lock_irqsave(&port->lock)
printk()
  console_unlock()
    call_console_drivers()
      serial8250_console_write()
        spin_lock_irqsave(&port->lock)
** DEADLOCK **

The 8250_dw i/o accessors try to write a console error message if the
LCR workaround was unsuccessful. When the port->lock is already held
(eg., when called from serial8250_set_termios()), this deadlocks.

Make the error message a FIXME until a general solution is devised.

Cc: Tim Kryger <tim.kryger@gmail.com>
Reported-by: Zhang Zhen <zhenzhang.zhang@huawei.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[ luis: backported to 3.16:
  - dropped changes to dw8250_serial_outq() ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 51b307aab75e919b0bace8e3b5604f2268ae0b7a..f4e14f9baae5a475fec0f689c362b943e8756f91 100644 (file)
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -165,7 +165,10 @@ static void dw8250_serial_out(struct uart_port *p, int offset, int value)
                        dw8250_force_idle(p);
                        writeb(value, p->membase + (UART_LCR << p->regshift));
                }
-               dev_err(p->dev, "Couldn't set LCR to %d\n", value);
+               /*
+                * FIXME: this deadlocks if port->lock is already held
+                * dev_err(p->dev, "Couldn't set LCR to %d\n", value);
+                */
        }
 }
 
@@ -202,7 +205,10 @@ static void dw8250_serial_out32(struct uart_port *p, int offset, int value)
                        dw8250_force_idle(p);
                        writel(value, p->membase + (UART_LCR << p->regshift));
                }
-               dev_err(p->dev, "Couldn't set LCR to %d\n", value);
+               /*
+                * FIXME: this deadlocks if port->lock is already held
+                * dev_err(p->dev, "Couldn't set LCR to %d\n", value);
+                */
        }
 }