linux-kernel - [PATCH 07/13] serial: stm32: fix a deadlock in set

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Date:   Fri, 19 Feb 2021 18:47:30 +0100
From:   Erwan Le Ray <erwan.leray@...s.st.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>
CC:     <linux-serial@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        Erwan Le Ray <erwan.leray@...s.st.com>,
        Fabrice Gasnier <fabrice.gasnier@...s.st.com>,
        Valentin Caron <valentin.caron@...s.st.com>
Subject: [PATCH 07/13] serial: stm32: fix a deadlock in set_termios

CTS/RTS GPIOs support that has been added recently to STM32 UART driver has
introduced scheduled code in a set_termios part protected by a spin lock.
This generates a potential deadlock scenario:

Chain exists of:
&irq_desc_lock_class --> console_owner --> &port_lock_key

Possible unsafe locking scenario:

     CPU0                    CPU1
     ----                    ----
lock(&port_lock_key);
                           lock(console_owner);
                           lock(&port_lock_key);
lock(&irq_desc_lock_class);

*** DEADLOCK ***
4 locks held by stty/766:

Move the scheduled code after the spinlock.

Fixes: 6cf61b9bd7cc ("tty: serial: Add modem control gpio support for STM32 UART")
Signed-off-by: Erwan Le Ray <erwan.leray@...s.st.com>

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index 14011183edfe..4ba164820904 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -829,12 +829,6 @@ static void stm32_usart_set_termios(struct uart_port *port,
 		cr3 |= USART_CR3_CTSE | USART_CR3_RTSE;
 	}
 
-	/* Handle modem control interrupts */
-	if (UART_ENABLE_MS(port, termios->c_cflag))
-		stm32_usart_enable_ms(port);
-	else
-		stm32_usart_disable_ms(port);
-
 	usartdiv = DIV_ROUND_CLOSEST(port->uartclk, baud);
 
 	/*
@@ -916,6 +910,12 @@ static void stm32_usart_set_termios(struct uart_port *port,
 
 	stm32_usart_set_bits(port, ofs->cr1, BIT(cfg->uart_enable_bit));
 	spin_unlock_irqrestore(&port->lock, flags);
+
+	/* Handle modem control interrupts */
+	if (UART_ENABLE_MS(port, termios->c_cflag))
+		stm32_usart_enable_ms(port);
+	else
+		stm32_usart_disable_ms(port);
 }
 
 static const char *stm32_usart_type(struct uart_port *port)
-- 
2.17.1