Re: [PATCH 1/1] serial: mcf: Don't take spinlocks in already protected functions

Yury Georgievskiy
2010-06-14T20:20:35Z

Andrew Morton wrote:
> On Wed, 9 Jun 2010 09:56:26 +0200
> ygeorgie@gmail.com wrote:
>
> > From: Yury Georgievskiy <ygeorgie@gmail.com>
> >
> > Don't take the port spinlock in uart functions where the serial core
> > already takes care of locking/unlocking them.
> >
> > The code would actually lock up on architectures where spinlocks are
> > implemented.
> >
> > Also protect calling mcf_rx_chars/mcf_tx_chars in the
> > interrupt handler by the port spinlock and use IRQ_RETVAL
> > to return from isr.
> >
>
> Thanks. Did you runtime test this?

Unfortunately not.

I spotted it as I am now writing the UART driver for Philips SCC2698B integrated circuit
and was looking for serial drivers examples.

And also came across d8d721f4c005f9a69bd1b5d5c6ba99b7e1d464de commit that has patched a
similar problem.

>
> > @@ -368,11 +354,15 @@ static irqreturn_t mcf_interrupt(int irq, void *data)
> > unsigned int isr;
> >
> > isr = readb(port->membase + MCFUART_UISR) & pp->imr;
> > +
> > + spin_lock(&port->lock);
> > if (isr & MCFUART_UIR_RXREADY)
> > mcf_rx_chars(pp);
> > if (isr & MCFUART_UIR_TXREADY)
> > mcf_tx_chars(pp);
> > - return IRQ_HANDLED;
> > + spin_unlock(&port->lock);
> > +
> > + return IRQ_RETVAL(isr);
> > }
>
> I think this is a little abusive of IRQ_RETVAL. If there are some bits
> set in `isr' other than MCFUART_UIR_RXREADY and MCFUART_UIR_TXREADY, we
> claim we handled it, only we didn't.
>
> Probably the code works OK, but it all seems a bit uncomfortable.
> Perhaps make it more explicit?
>

Fair enough.
The code looks more relevant.

>
> --- a/drivers/serial/mcf.c~serial-mcf-dont-take-spinlocks-in-already-protected-functions-fix
> +++ a/drivers/serial/mcf.c
> @@ -352,17 +352,22 @@ static irqreturn_t mcf_interrupt(int irq
> struct uart_port *port = data;
> struct mcf_uart *pp = container_of(port, struct mcf_uart, port);
> unsigned int isr;
> + irqreturn_t ret = IRQ_NONE;
>
> isr = readb(port->membase + MCFUART_UISR) & pp->imr;
>
> spin_lock(&port->lock);
> - if (isr & MCFUART_UIR_RXREADY)
> + if (isr & MCFUART_UIR_RXREADY) {
> mcf_rx_chars(pp);
> - if (isr & MCFUART_UIR_TXREADY)
> + ret = IRQ_HANDLED;
> + }
> + if (isr & MCFUART_UIR_TXREADY) {
> mcf_tx_chars(pp);
> + ret = IRQ_HANDLED;
> + }
> spin_unlock(&port->lock);
>
> - return IRQ_RETVAL(isr);
> + return ret;
> }
>
> /****************************************************************************/
> _
>