[RFC PATCH v1 2/2] serial/8250: Avoid getting lock in RT atomic context

[Leonardo Bras <leobras@redhat.com>]

With PREEMPT_RT enabled, a spin_lock_irqsave() becomes a possibly sleeping
spin_lock(), without preempt_disable() or irq_disable().

This allows a task T1 to get preempted or interrupted while holding the
port->lock. If the preempting task T2 need the lock, spin_lock() code
will schedule T1 back until it finishes using the lock, and then go back to
T2.

There is an issue if a T1 holding port->lock is interrupted by an
IRQ, and this IRQ handler needs to get port->lock for writting (printk):
spin_lock() code will try to reschedule the interrupt handler, which is in
atomic context, causing a BUG() for trying to reschedule/sleep in atomic
context.

So for the case (PREEMPT_RT && in_atomic()) try to get the lock, and if it
fails proceed anyway, just like it's done in oops_in_progress case.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
---
 drivers/tty/serial/8250/8250_port.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 8ca061d3bbb92..8480832846319 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -3397,7 +3397,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
 
 	touch_nmi_watchdog();
 
-	if (oops_in_progress)
+	if (oops_in_progress || (IS_ENABLED(CONFIG_PREEMPT_RT) && in_atomic())
 		locked = uart_port_trylock_irqsave(port, &flags);
 	else
 		uart_port_lock_irqsave(port, &flags);
-- 
2.43.0