Re: 2.6.18-rt1

Daniel Walker
2006-09-22T14:42:56Z

On Thu, 2006-09-21 at 21:02 +0200, Ingo Molnar wrote:
> * Daniel Walker <dwalker@mvista.com> wrote:
>
> > On Wed, 2006-09-20 at 22:14 +0200, Ingo Molnar wrote:
> > > > if (up->port.sysrq) {
> > > > /* serial8250_handle_port() already took the lock */
> > > > locked = 0;
> >
> >
> > In this case it had interrupts off in the !PREEMPT_RT case, but your
> > change leaves them on here.. _irqsave only runs in two of the three
> > cases..
>
> doh, right you are. I updated to the patch below.
>
> Ingo

On closer inspection I still think this is wrong. (Although it looks
really nice..) find below speaking only in term of !PREEMPT_RT ,

> Index: linux/drivers/serial/8250.c
> ===================================================================
> --- linux.orig/drivers/serial/8250.c
> +++ linux/drivers/serial/8250.c
> @@ -2252,14 +2252,10 @@ serial8250_console_write(struct console
>
> touch_nmi_watchdog();
>
> - local_irq_save(flags);
> - if (up->port.sysrq) {
> - /* serial8250_handle_port() already took the lock */
> - locked = 0;

in the old version interrupts are off, and stay off until the function
returns in all cases. Even "locked = 0" .

> - } else if (oops_in_progress) {
> - locked = spin_trylock(&up->port.lock);
> - } else
> - spin_lock(&up->port.lock);
> + if (up->port.sysrq || oops_in_progress)
> + locked = spin_trylock_irqsave(&up->port.lock, flags);

Now in the new version interrupts are only off if you _get the lock_.
Presumably the lock is taken in the calling function, but interrupts
aren't disabled.

I'm assuming the code is disabling interrupts for a good reason, I don't
know enough about the code to say it isn't.

> + else
> + spin_lock_irqsave(&up->port.lock, flags);
>
> /*
> * First save the IER then disable the interrupts
> @@ -2281,8 +2277,7 @@ serial8250_console_write(struct console
> serial_out(up, UART_IER, ier);
>
> if (locked)
> - spin_unlock(&up->port.lock);
> - local_irq_restore(flags);
> + spin_unlock_irqrestore(&up->port.lock, flags);
> }
>
> static int serial8250_console_setup(struct console *co, char *options)
> -
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