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

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Message-ID: <0eb4c357-aafd-42bd-a4fd-687a91831bf4@kernel.org>
Date: Wed, 15 Oct 2025 17:49:34 +0200
From: Jiri Slaby <jirislaby@...nel.org>
To: Peng Zhang <zhangpeng.00@...edance.com>, gregkh@...uxfoundation.org
Cc: linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
 songmuchun@...edance.com, Ilpo Järvinen
 <ilpo.jarvinen@...ux.intel.com>, buytenh@...sta.com,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH] serial: 8250: always disable IRQ during THRE test

On 14. 10. 25, 14:19, Peng Zhang wrote:
> 
> 
> 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.

Make sense to me. Care to Cc the people from 039d4926379b next time, so 
that they can comment?

>>       /*
>>        * 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
> 


-- 
js
suse labs