Re: [patch 1/9] Fix HPET init race

[Andrew Morton <akpm@linux-foundation.org>]

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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