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SubjectRe: [patch 1/9] Fix HPET init race
On Tue, 6 Feb 2007 17:44:59 +0100
Jiri Bohac <jbohac@suse.cz> wrote:

> On Thu, Feb 01, 2007 at 06:34:50PM -0800, Andrew Morton wrote:
> > On Thu, 01 Feb 2007 10:59:53 +0100 jbohac@suse.cz wrote:
> >
> > > Fix a race in the initialization of HPET, which might result in a
> > > 5 minute lockup on boot.
> > >
> >
> > What race? Please always describe bugs when fixing them.
>
> If the value of the HPET_T0_CMP register is reached and exceeded
> by the value of the HPET_COUNTER register after HPET_T0_CMP is
> read into trigger, but before the first iteration of the while,
> the while loop will iterate "endlessly" until the HPET overlaps
> eventually (in as much as 5 minutes).
>
> > >
> > > Index: linux-2.6.20-rc5/arch/x86_64/kernel/apic.c
> > > ===================================================================
> > > --- linux-2.6.20-rc5.orig/arch/x86_64/kernel/apic.c
> > > +++ linux-2.6.20-rc5/arch/x86_64/kernel/apic.c
> > > @@ -764,10 +767,12 @@ static void setup_APIC_timer(unsigned in
> > >
> > > /* wait for irq slice */
> > > if (vxtime.hpet_address && hpet_use_timer) {
> > > - int trigger = hpet_readl(HPET_T0_CMP);
> > > - while (hpet_readl(HPET_COUNTER) >= trigger)
> > > - /* do nothing */ ;
> > > - while (hpet_readl(HPET_COUNTER) < trigger)
> > > + int trigger;
> > > + do
> > > + trigger = hpet_readl(HPET_T0_CMP);
> > > + while (hpet_readl(HPET_COUNTER) >= trigger);
> > > +
> >
> > Is this signedness-safe and wraparound-safe? It might be better to make
> > `trigger' unsigned and do
> >
> > while (hpet_readl(HPET_COUNTER) - trigger >= 0)
>
>
> Yes, making trigger unsigned is a good idea (although having it
> signed would probably never cause any problem, because this is
> called during boot and it takes ~2 minutes for HPET to overflow
> the s32)
>
> But no, looping while the unsigned result is >= 0 does not seem
> that good an idea to me ;-)
>
> An updated patch follows. It is still not wraparound safe (a
> lockup would still happen if it's called ~5 minutes after boot, but
> this should never happen -- it's called early during boot)
>
> --- linux-2.6.20-rc5.orig/arch/x86_64/kernel/apic.c 2007-02-06 16:56:00.000000000 +0100
> +++ linux-2.6.20-rc5/arch/x86_64/kernel/apic.c 2007-02-06 17:26:42.000000000 +0100
> @@ -764,10 +764,12 @@ static void setup_APIC_timer(unsigned in
>
> /* wait for irq slice */
> if (vxtime.hpet_address && hpet_use_timer) {
> - int trigger = hpet_readl(HPET_T0_CMP);
> - while (hpet_readl(HPET_COUNTER) >= trigger)
> - /* do nothing */ ;
> - while (hpet_readl(HPET_COUNTER) < trigger)
> + u32 trigger;
> + do
> + trigger = hpet_readl(HPET_T0_CMP);
> + while (hpet_readl(HPET_COUNTER) >= trigger);
> +
> + while (hpet_readl(HPET_COUNTER) < trigger)
> /* do nothing */ ;
> } else {
> int c1, c2;
>

Well it still seems a bit dodgy to me - I'd have thought it'd be possible
(and nicer) to come up with a version which is safe to call at any time.

Are you sure this won't cause a kexec'ed kernel to lock up for five
minutes, for example?

Anyway, I'll let Andi worry about this one. Please send him a signed-off
and fully changelogged patch and still cc myself, thanks.

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