On Thu, Feb 01, 2007 at 06:34:50PM -0800, Andrew Morton wrote:
> On Thu, 01 Feb 2007 10:59:53 +0100 [email protected] 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;
--
Jiri Bohac <[email protected]>
SUSE Labs, SUSE CZ
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