[PATCH 6.19 061/220] serial: 8250: always disable IRQ during THRE test

Greg Kroah-Hartman
2026-03-23T13:52:35Z

From: Peng Zhang <zhangpeng.00@bytedance.com>

commit 24b98e8664e157aff0814a0f49895ee8223f382f upstream.

commit 039d4926379b ("serial: 8250: Toggle IER bits on only after irq
has been set up") moved IRQ setup before the THRE test, in combination
with commit 205d300aea75 ("serial: 8250: change lock order in
serial8250_do_startup()") the interrupt handler can run during the
test and race with its IIR reads. This can produce wrong THRE test
results and cause spurious registration of the
serial8250_backup_timeout timer. Unconditionally disable the IRQ for
the short duration of the test and re-enable it afterwards to avoid
the race.

Fixes: 039d4926379b ("serial: 8250: Toggle IER bits on only after irq has been set up")
Depends-on: 205d300aea75 ("serial: 8250: change lock order in serial8250_do_startup()")
Cc: stable <stable@kernel.org>
Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Alban Bedel <alban.bedel@lht.dlh.de>
Tested-by: Maximilian Lueer <maximilian.lueer@lht.dlh.de>
Link: https://patch.msgid.link/20260224121639.579404-1-alban.bedel@lht.dlh.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/tty/serial/8250/8250_port.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)

--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2147,8 +2147,7 @@ static void serial8250_THRE_test(struct
if (up->port.flags & UPF_NO_THRE_TEST)
return;

- if (port->irqflags & IRQF_SHARED)
- disable_irq_nosync(port->irq);
+ disable_irq(port->irq);

/*
* Test for UARTs that do not reassert THRE when the transmitter is idle and the interrupt
@@ -2170,8 +2169,7 @@ static void serial8250_THRE_test(struct
serial_port_out(port, UART_IER, 0);
}

- if (port->irqflags & IRQF_SHARED)
- enable_irq(port->irq);
+ enable_irq(port->irq);

/*
* If the interrupt is not reasserted, or we otherwise don't trust the iir, setup a timer to