Re: lockdep splat ("possible circular locking dependency detected") with PL011 on 5.8
From: peterz@infradead.org
Date: 2020-08-11 10:39:14
Also in:
lkml
Subsystem:
arm primecell uart pl010 and pl011 drivers, the rest, tty layer and serial drivers · Maintainers:
Russell King, Linus Torvalds, Greg Kroah-Hartman, Jiri Slaby
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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