Date: Thu, 9 Aug 2007 13:30:11 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Aaron Durbin <adurbin@google.com>
Cc: linux-kernel@vger.kernel.org, ak@suse.de, Robin Holt <holt@sgi.com>
Subject: Re: [PATCH] x86_64: Avoid NMI Watchdog and/or long wait in
 setup_APIC_timer
Message-Id: <20070809133011.2d03c04a.akpm@linux-foundation.org>
In-Reply-To: <20070808231719.GA7101@google.com>
References: <20070808231719.GA7101@google.com>
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On Wed, 8 Aug 2007 16:17:19 -0700
Aaron Durbin <adurbin@google.com> wrote:

> In setup_APIC_timer with the HPET in use, a condition can arise while
> waiting for the next irq slice to expire on the HPET which will either
> cause an NMI watchdog to fire or a 3 minute busy loop if the NMI
> watchdog is disabled.
> 
> The HPET comparator and the counter keep incrementing during its normal
> operation. When a comparison event fires the comparator will increment
> by the designated period. If the HPET trigger occurs right after
> the 'int trigger = hpet_readl(HPET_T0_CMP);' line, we will will spin
> for up to 3 minutes (with a clock of 25MHz) waiting for the HPET
> counter to wrap around. However, when the NMI watchdog is enabled the
> NMI watchdog will detect a lockup and reboot the machine. This
> scenario can be exasperated by the presence of an SMI which will
> increase the window of opportunity for the condition to occur.
> 
> The fix is to wait for the compartor to change which signals the
> end of the tick slice.
> 
> ---
> 
> The last patch had a typo in the diff which really would cause the
> problem state above. Sorry.
> 
> diff --git a/arch/x86_64/kernel/apic.c b/arch/x86_64/kernel/apic.c
> index 900ff38..06797e2 100644
> --- a/arch/x86_64/kernel/apic.c
> +++ b/arch/x86_64/kernel/apic.c
> @@ -791,10 +791,12 @@ static void setup_APIC_timer(unsigned in
>  
>  	/* wait for irq slice */
>  	if (hpet_address && hpet_use_timer) {
> +		/*
> +		 * Wait for the comparator value to change which signals that
> +		 * the tick slice has expired.
> +		 */
>  		int trigger = hpet_readl(HPET_T0_CMP);
> -		while (hpet_readl(HPET_COUNTER) >= trigger)
> -			/* do nothing */ ;
> -		while (hpet_readl(HPET_COUNTER) <  trigger)
> +		while (trigger == hpet_readl(HPET_T0_CMP))
>  			/* do nothing */ ;
>  	} else {
>  		int c1, c2;

but, but.  Didn't this get fixed by
ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.23-rc2/2.6.23-rc2-mm1/broken-out/fix-hpet-init-race.patch
?
-
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