linux-kernel - Re: [PATCH] tty: serial: replace spin_lock_irqsave by spin

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Date:   Fri, 20 Nov 2020 13:49:57 +0100
From:   Johan Hovold <johan@...nel.org>
To:     "tiantao (H)" <tiantao6@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Johan Hovold <johan@...nel.org>, Tian Tao <tiantao6@...ilicon.com>,
        gregkh@...uxfoundation.org, jirislaby@...nel.org, afaerber@...e.de,
        manivannan.sadhasivam@...aro.org, linux-serial@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: serial: replace spin_lock_irqsave by spin_lock in
 hard IRQ

On Fri, Nov 20, 2020 at 07:25:03PM +0800, tiantao (H) wrote:
> 在 2020/11/20 16:23, Johan Hovold 写道:
> > On Thu, Nov 19, 2020 at 05:01:29PM +0800, Tian Tao wrote:
> >> The code has been in a irq-disabled context since it is hard IRQ. There
> >> is no necessity to do it again.
> >>
> >> Signed-off-by: Tian Tao <tiantao6@...ilicon.com>
> >> ---
> >>   drivers/tty/serial/owl-uart.c | 5 ++---
> >>   1 file changed, 2 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/tty/serial/owl-uart.c b/drivers/tty/serial/owl-uart.c
> >> index c149f8c3..472fdaf 100644
> >> --- a/drivers/tty/serial/owl-uart.c
> >> +++ b/drivers/tty/serial/owl-uart.c
> >> @@ -251,10 +251,9 @@ static void owl_uart_receive_chars(struct uart_port *port)
> >>   static irqreturn_t owl_uart_irq(int irq, void *dev_id)
> >>   {
> >>   	struct uart_port *port = dev_id;
> >> -	unsigned long flags;
> >>   	u32 stat;
> >>   
> >> -	spin_lock_irqsave(&port->lock, flags);
> >> +	spin_lock(&port->lock);
> > 
> > Same thing here; this will break with forced irq threading (i.e.
> > "threadirqs") since the console code can still end up being called from
> > interrupt context.

> As the following code shows, owl_uart_irq does not run in the irq 
> threading context.
>   ret = request_irq(port->irq, owl_uart_irq, IRQF_TRIGGER_HIGH,
>                          "owl-uart", port);
>          if (ret)
>                  return ret;

It still runs in a thread when interrupts are forced to be threaded
using the kernel parameter "threadirqs".

We just had a revert of a change like yours after lockdep reported the
resulting lock inversion with forced interrupt threading.

Whether drivers should have to care about "threadirqs" is a somewhat
different question. Not sure how that's even supposed to work generally
unless we mass-convert drivers to spin_lock_irqsave() (or mark their
interrupts IRQF_NO_THREAD).

Thomas, any comments?

Johan