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From: Leonardo Bras <leobras@redhat.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jirislaby@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	=?UTF-8?q?Ilpo=20J=C3=A4rvinen?= <ilpo.jarvinen@linux.intel.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Florian Fainelli <f.fainelli@gmail.com>,
	John Ogness <john.ogness@linutronix.de>,
	Tony Lindgren <tony@atomide.com>,
	Marcelo Tosatti <mtosatti@redhat.com>
Cc: Leonardo Bras <leobras@redhat.com>,
	linux-kernel@vger.kernel.org,
	linux-serial@vger.kernel.org
Subject: [RFC PATCH v1 2/2] serial/8250: Avoid getting lock in RT atomic context
Date: Tue, 16 Jan 2024 04:32:33 -0300
Message-ID: <20240116073234.2355850-4-leobras@redhat.com>
In-Reply-To: <20240116073234.2355850-2-leobras@redhat.com>
References: <20240116073234.2355850-2-leobras@redhat.com>
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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