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SubjectRe: [Patch] Fix the possibility of insane return value of hpet_calibrate() against SMI. (take 2)
[Re: [Patch] Fix the possibility of insane return value of hpet_calibrate() against SMI. (take 2)] On 30/03/2009 (Mon 14:06) Andrew Morton wrote:

> On Wed, 18 Mar 2009 11:47:04 +0900
> Yasunori Goto <y-goto@jp.fujitsu.com> wrote:
>
> > hpet_calibrate() has a possibility of miss-calibration due to SMI.
> > If SMI interrupts in the while loop of calibration, then return value
> > will be big. This change calibrates until stabilizing by the return
> > value with a small value.
> >
> >
> > Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>
> >
> >
> > ---
> > drivers/char/hpet.c | 21 ++++++++++++++++++++-
> > 1 file changed, 20 insertions(+), 1 deletion(-)
> >
> > Index: hpet_test/drivers/char/hpet.c
> > ===================================================================
> > --- hpet_test.orig/drivers/char/hpet.c 2009-03-12 15:47:45.000000000 +0900
> > +++ hpet_test/drivers/char/hpet.c 2009-03-18 11:12:42.000000000 +0900
> > @@ -713,7 +713,7 @@
> > */
> > #define TICK_CALIBRATE (1000UL)
> >
> > -static unsigned long hpet_calibrate(struct hpets *hpetp)
> > +static unsigned long __hpet_calibrate(struct hpets *hpetp)
> > {
> > struct hpet_timer __iomem *timer = NULL;
> > unsigned long t, m, count, i, flags, start;
> > @@ -750,6 +750,25 @@
> > return (m - start) / i;
> > }
> >
> > +static unsigned long hpet_calibrate(struct hpets *hpetp)
> > +{
> > + unsigned long ret = ~0UL, tmp;
> > +
> > + /*
> > + * Try to calibrate until return value becomes stable small value.
> > + * If SMI interruption occurs in calibration loop, the return value
> > + * will be big. This avoids its impact.
> > + */
> > + do {
> > + tmp = __hpet_calibrate(hpetp);
> > + if (ret <= tmp)
> > + break;
> > + ret = tmp;
> > + } while (1);
> > +
> > + return ret;
> > +}
>
> Call me paranoid, but I'd like to see a maximum retry count here and an
> error message-and-continue if it is exceeded. To prevent mysterious
> boot-time lockups from misbehaving hpets, perhaps?

I'd mentioned that here:

http://lkml.org/lkml/2009/3/18/180

but the general consensus was that it was impossible and I was just
being paranoid. :-)

Acked-by: Paul Gortmaker <paul.gortmaker@windriver.com>

Paul.

>
> Also, style nit - I find
>
> for ( ; ; ) {
> ...
> }
>
> to be more readable than
>
> do {
> ...
> } while (1);
>
> and I believe the former is more common.
>
>
> And
>
> unsigned long ret = -1;
>
> has the same effect as
>
> unsigned long ret = ~0UL;
>
> but is more maintainable - it doesn't subtly break if someone changes
> the type of `ret'. (This is a bit of an ugly C trick).
>
>
>


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