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

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Date:	Thu, 1 Feb 2007 18:34:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	jbohac@...e.cz
Cc:	Andi Kleen <ak@...e.de>, linux-kernel@...r.kernel.org,
	Vojtech Pavlik <vojtech@...e.cz>, ssouhlal@...ebsd.org,
	arjan@...radead.org, tglx@...utronix.de, johnstul@...ibm.com,
	zippel@...ux-m68k.org, andrea@...e.de
Subject: Re: [patch 1/9] Fix HPET init race

On Thu, 01 Feb 2007 10:59:53 +0100 jbohac@...e.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.

> 
> 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)

> +		while (hpet_readl(HPET_COUNTER) <  trigger)
>   			/* do nothing */ ;

ditto.
-
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