Re: tickle nmi watchdog whilst doing serial writes.

Dave Jones
2005-05-14T06:58:02Z

On Fri, May 13, 2005 at 11:43:31PM -0700, Andrew Morton wrote:
> Dave Jones <davej@redhat.com> wrote:
> >
> > This was fun. I inserted a music CD with some obnoxious copy-protection
> > on it into the drive, and lots of SCSI errors went zipping over to
> > the serial console. Unfortunatly, the box was also compiling a kernel,
> > playing oggs, and doing a number of other things at the same time,
> > so this happened..
> >
> > NMI Watchdog detected LOCKUP on CPU2CPU 2
>
> OK.. But calling touch_nmi_watchdog() at 1MHz seems a bit excessive, and
> might perturb the finely-tuned timing in there.
>
> How's about this?

Umm.. Despite it being past my bedtime, I'm pretty sure I'm
missing something here...

> + while (!(serial_in(up, UART_MSR) & UART_MSR_CTS) && --tmout)
> udelay(1);

I don't see how this is any better than the current code.
We're doing 1000000 udelays. Whilst we're doing that,
the nmi watchdog goes bonkers.

> + if (tmout < 1000000)
> + touch_nmi_watchdog();

So by the time we do this, its already triggered.

How about..

--- linux-2.6.11/drivers/serial/8250.c~ 2005-05-14 02:49:02.000000000 -0400
+++ linux-2.6.11/drivers/serial/8250.c 2005-05-14 02:54:30.000000000 -0400
@@ -40,6 +40,7 @@
#include <linux/serial_core.h>
#include <linux/serial.h>
#include <linux/serial_8250.h>
+#include <linux/nmi.h>

#include <asm/io.h>
#include <asm/irq.h>
@@ -2099,8 +2100,15 @@ static inline void wait_for_xmitr(struct
if (up->port.flags & UPF_CONS_FLOW) {
tmout = 1000000;
while (--tmout &&
- ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0))
+ ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0)) {
+ int cnt=0;
udelay(1);
+ cnt++;
+ if (cnt==100) {
+ touch_nmi_watchdog();
+ cnt=0;
+ }
+ }
}
}

Signed-off-by: Dave Jones <davej@redhat.com>

totally uncompiled & untested. I'm not even sure about
that magic '100'. I'll let someone less sleep-deprived
than myself choose a better number.

Dave