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

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Date: Tue, 16 Jan 2024 04:32:33 -0300
From: Leonardo Bras <leobras@...hat.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	John Ogness <john.ogness@...utronix.de>,
	Tony Lindgren <tony@...mide.com>,
	Marcelo Tosatti <mtosatti@...hat.com>
Cc: Leonardo Bras <leobras@...hat.com>,
	linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: [RFC PATCH v1 2/2] serial/8250: Avoid getting lock in RT atomic context

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@...hat.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