tty: serial: msm_serial: Fix lockup for sysrq and oops

As the commit 677fe555 ("serial: imx: Fix recursive locking bug")
has mentioned the uart driver might cause recursive locking between
normal printing and the kernel debugging facilities (e.g. sysrq and
oops).  In the commit it gave out suggestion for fixing recursive
locking issue: "The solution is to avoid locking in the sysrq case
and trylock in the oops_in_progress case."

This patch follows the suggestion (also used the exactly same code with
other serial drivers, e.g. amba-pl011.c) to fix the recursive locking
issue, this can avoid stuck caused by deadlock and print out log for
sysrq and oops.

Fixes: 04896a77 ("msm_serial: serial driver for MSM7K onboard serial peripheral.")
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: Jeffrey Hugo <jeffrey.l.hugo@gmail.com>
Link: https://lore.kernel.org/r/20191127141544.4277-2-leo.yan@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/msm_serial.c

@@ -1580,6 +1580,7 @@ static void __msm_console_write(struct uart_port *port, const char *s,
 	int num_newlines = 0;
 	bool replaced = false;
 	void __iomem *tf;
+	int locked = 1;
 
 	if (is_uartdm)
 		tf = port->membase + UARTDM_TF;
@@ -1592,7 +1593,13 @@ static void __msm_console_write(struct uart_port *port, const char *s,
 			num_newlines++;
 	count += num_newlines;
 
+	if (port->sysrq)
+		locked = 0;
+	else if (oops_in_progress)
+		locked = spin_trylock(&port->lock);
+	else
 		spin_lock(&port->lock);
+
 	if (is_uartdm)
 		msm_reset_dm_count(port, count);
 
@@ -1628,6 +1635,8 @@ static void __msm_console_write(struct uart_port *port, const char *s,
 		iowrite32_rep(tf, buf, 1);
 		i += num_chars;
 	}
+
+	if (locked)
 		spin_unlock(&port->lock);
 }