linux-kernel - Re: [PATCH] serial: 8250: always disable IRQ during THRE test

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Message-ID: <94d19d6d-5bb5-4156-8499-0be3d227f478@bytedance.com>
Date: Tue, 14 Oct 2025 20:19:40 +0800
From: Peng Zhang <zhangpeng.00@...edance.com>
To: gregkh@...uxfoundation.org, jirislaby@...nel.org
Cc: linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
 songmuchun@...edance.com
Subject: Re: [PATCH] serial: 8250: always disable IRQ during THRE test



在 2025/10/14 19:47, Peng Zhang 写道:
> commit 039d4926379b ("serial: 8250: Toggle IER bits on only after irq
> has been set up") moved IRQ setup before the THRE test, so 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")
> Signed-off-by: Peng Zhang <zhangpeng.00@...edance.com>
> ---
>   drivers/tty/serial/8250/8250_port.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 719faf92aa8a..addeef7a0d59 100644
> --- 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 uart_port *port)
>   	if (up->port.flags & UPF_NO_THRE_TEST)
>   		return;
>   
> -	if (port->irqflags & IRQF_SHARED)
> -		disable_irq_nosync(port->irq);
> +	disable_irq_nosync(port->irq);
disable_irq_nosync() may need to be changed to disable_irq() to prevent interrupts
that are currently being handled.>   
>   	/*
>   	 * 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 uart_port *port)
>   		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