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

[Jiri Slaby]

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@xxxxxxxxxxxxx>
> > ---
> >   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


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