serial: 8250: fix handle_irq locking

The 8250 handle_irq callback is not just called from the interrupt
handler but also from a timer callback when polling (e.g. for ports
without an interrupt line). Consequently the callback must explicitly
disable interrupts to avoid a potential deadlock with another interrupt
in polled mode.

Add back an irqrestore-version of the sysrq port-unlock helper and use
it in the 8250 callbacks that need it.

Fixes: 75f4e830 ("serial: do not restore interrupt state in sysrq helper")
Cc: stable@vger.kernel.org	# 5.13
Cc: Joel Stanley <joel@jms.id.au>
Cc: Andrew Jeffery <andrew@aj.id.au>
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210714080427.28164-1-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_aspeed_vuart.c

@@ -329,6 +329,7 @@ static int aspeed_vuart_handle_irq(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
 	unsigned int iir, lsr;
+	unsigned long flags;
 	unsigned int space, count;
 
 	iir = serial_port_in(port, UART_IIR);
@@ -336,7 +337,7 @@ static int aspeed_vuart_handle_irq(struct uart_port *port)
 	if (iir & UART_IIR_NO_INT)
 		return 0;
 
-	spin_lock(&port->lock);
+	spin_lock_irqsave(&port->lock, flags);
 
 	lsr = serial_port_in(port, UART_LSR);
 
@@ -370,7 +371,7 @@ static int aspeed_vuart_handle_irq(struct uart_port *port)
 	if (lsr & UART_LSR_THRE)
 		serial8250_tx_chars(up);
 
-	uart_unlock_and_check_sysrq(port);
+	uart_unlock_and_check_sysrq_irqrestore(port, flags);
 
 	return 1;
 }

drivers/tty/serial/8250/8250_fsl.c

@@ -30,10 +30,11 @@ struct fsl8250_data {
 int fsl8250_handle_irq(struct uart_port *port)
 {
 	unsigned char lsr, orig_lsr;
+	unsigned long flags;
 	unsigned int iir;
 	struct uart_8250_port *up = up_to_u8250p(port);
 
-	spin_lock(&up->port.lock);
+	spin_lock_irqsave(&up->port.lock, flags);
 
 	iir = port->serial_in(port, UART_IIR);
 	if (iir & UART_IIR_NO_INT) {
@@ -82,7 +83,7 @@ int fsl8250_handle_irq(struct uart_port *port)
 
 	up->lsr_saved_flags = orig_lsr;
 
-	uart_unlock_and_check_sysrq(&up->port);
+	uart_unlock_and_check_sysrq_irqrestore(&up->port, flags);
 
 	return 1;
 }

drivers/tty/serial/8250/8250_port.c

@@ -1899,11 +1899,12 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
 	unsigned char status;
 	struct uart_8250_port *up = up_to_u8250p(port);
 	bool skip_rx = false;
+	unsigned long flags;
 
 	if (iir & UART_IIR_NO_INT)
 		return 0;
 
-	spin_lock(&port->lock);
+	spin_lock_irqsave(&port->lock, flags);
 
 	status = serial_port_in(port, UART_LSR);
 
@@ -1929,7 +1930,7 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
 		(up->ier & UART_IER_THRI))
 		serial8250_tx_chars(up);
 
-	uart_unlock_and_check_sysrq(port);
+	uart_unlock_and_check_sysrq_irqrestore(port, flags);
 
 	return 1;
 }

include/linux/serial_core.h

@@ -518,6 +518,25 @@ static inline void uart_unlock_and_check_sysrq(struct uart_port *port)
 	if (sysrq_ch)
 		handle_sysrq(sysrq_ch);
 }
+
+static inline void uart_unlock_and_check_sysrq_irqrestore(struct uart_port *port,
+		unsigned long flags)
+{
+	int sysrq_ch;
+
+	if (!port->has_sysrq) {
+		spin_unlock_irqrestore(&port->lock, flags);
+		return;
+	}
+
+	sysrq_ch = port->sysrq_ch;
+	port->sysrq_ch = 0;
+
+	spin_unlock_irqrestore(&port->lock, flags);
+
+	if (sysrq_ch)
+		handle_sysrq(sysrq_ch);
+}
 #else	/* CONFIG_MAGIC_SYSRQ_SERIAL */
 static inline int uart_handle_sysrq_char(struct uart_port *port, unsigned int ch)
 {
@@ -531,6 +550,11 @@ static inline void uart_unlock_and_check_sysrq(struct uart_port *port)
 {
 	spin_unlock(&port->lock);
 }
+static inline void uart_unlock_and_check_sysrq_irqrestore(struct uart_port *port,
+		unsigned long flags)
+{
+	spin_unlock_irqrestore(&port->lock, flags);
+}
 #endif	/* CONFIG_MAGIC_SYSRQ_SERIAL */
 
 /*