lockdep splat ("possible circular locking dependency detected") with PL011 on 5.8

[peterz at infradead.org]

On Tue, Aug 11, 2020 at 11:13:13AM +0100, Will Deacon wrote:
> Hi,
> 
> Using magic-sysrq via a keyboard interrupt over the serial console results in
> the following lockdep splat with the PL011 UART driver on v5.8. I can reproduce
> the issue under QEMU with arm64 defconfig + PROVE_LOCKING.
> 
> Any chance somebody could take a look, please? It's a little annoying,
> because it means when I uses magic-sysrq to increase the loglevel prior
> to testing something else, lockdep gets disabled as a result.
> 

Going by msm_serial, the thing to do is something like this:


diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 8efd7c2a34fe..1717790ece2b 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -308,8 +308,9 @@ static void pl011_write(unsigned int val, const struct uart_amba_port *uap,
  */
 static int pl011_fifo_to_tty(struct uart_amba_port *uap)
 {
-	u16 status;
 	unsigned int ch, flag, fifotaken;
+	int sysrq;
+	u16 status;
 
 	for (fifotaken = 0; fifotaken != 256; fifotaken++) {
 		status = pl011_read(uap, REG_FR);
@@ -344,10 +345,12 @@ static int pl011_fifo_to_tty(struct uart_amba_port *uap)
 				flag = TTY_FRAME;
 		}
 
-		if (uart_handle_sysrq_char(&uap->port, ch & 255))
-			continue;
+		spin_unlock(&uap->port.lock);
+		sysrq = uart_handle_sysrq_char(&uap->port, ch & 255);
+		spin_lock(&uap->port.lock);
 
-		uart_insert_char(&uap->port, ch, UART011_DR_OE, ch, flag);
+		if (!sysrq)
+			uart_insert_char(&uap->port, ch, UART011_DR_OE, ch, flag);
 	}
 
 	return fifotaken;