Commit 24b98e86 authored Feb 24, 2026 by Peng Zhang Committed by Greg Kroah-Hartman Mar 12, 2026

serial: 8250: always disable IRQ during THRE test



commit 039d4926 ("serial: 8250: Toggle IER bits on only after irq
has been set up") moved IRQ setup before the THRE test, in combination
with commit 205d300a ("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: 039d4926 ("serial: 8250: Toggle IER bits on only after irq has been set up")
Depends-on: 205d300a ("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>

parent a424a34b

drivers/tty/serial/8250/8250_port.c

+2 −4

Original line number	Diff line number	Diff line
		@@ -2147,8 +2147,7 @@ static void serial8250_THRE_test(struct uart_port *port)
		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
		@@ -2170,7 +2169,6 @@ static void serial8250_THRE_test(struct uart_port *port)
		serial_port_out(port, UART_IER, 0);
		}

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

		/*