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