Hi!

Johan Hovold [off-list ref] writes:

On Wed, Dec 09, 2020 at 10:17:27AM +0100, Steffen Trumtrar wrote:

quoted

In most cases serial8250_tx_chars is called with spinlock held.
Fix the remaining location, too.

Please explain where __start_tx() is called without holding the port
lock and consider fixing that up.

quoted

Signed-off-by: Steffen Trumtrar <redacted>
---
 drivers/tty/serial/8250/8250_port.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index b0af13074cd3..3310c2b70138 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c

@@ -1559,6 +1559,7 @@ static void serial8250_stop_tx(struct uart_port *port)
 static inline void __start_tx(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
+	unsigned long flags;

 	if (up->dma && !up->dma->tx_dma(up))
 		return;

@@ -1569,8 +1570,11 @@ static inline void __start_tx(struct uart_port *port)

 			lsr = serial_in(up, UART_LSR);
 			up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
-			if (lsr & UART_LSR_THRE)
+			if (lsr & UART_LSR_THRE) {
+				spin_lock_irqsave(&port->lock, flags);
 				serial8250_tx_chars(up);
+				spin_unlock_irqrestore(&port->lock, flags);
+			}

Since several callers of __start_tx() do hold the lock, this change will
introduce a deadlock.

Meh, yeah :(

Seem like the correct solution would be to fix start_tx_rs485 and
serial8250_start_tx instead.


Best regards,
Steffen Trumtrar

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