linux-kernel - Re: [PATCH] x86: fix hpet timer reinit for x86

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Date:	Thu, 5 Feb 2009 11:01:07 +0300
From:	Kirill Korotaev <dev@...allels.com>
To:	Ingo Molnar <mingo@...e.hu>, Pavel Emelyanov <xemul@...nvz.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kirill Korotaev <dev@...nvz.org>
Subject: Re: [PATCH] x86: fix hpet timer reinit for x86_64




On 2/4/09 4:49 PM, "Ingo Molnar" <mingo@...e.hu> wrote:

> 
> 
> * Pavel Emelyanov <xemul@...nvz.org> wrote:
> 
>> There's a small problem with hpet_rtc_reinit function - it checks
>> for the
>>       hpet_readl(HPET_COUNTER) - hpet_t1_cmp > 0
>> to continue increasing both the HPET_T1_CMP (register) and the
>> hpet_t1_cmp (variable).
>> 
>> But since the HPET_COUNTER is always 32-bit, if the hpet_t1_cmp
>> is 64-bit this condition will always be FALSE once the latter hits
>> the 32-bit boundary, and we can have a situation, when we don't
>> increase the HPET_T1_CMP register high enough.
>> 
>> The result - timer stops ticking, since HPET_T1_CMP becomes less,
>> than the COUNTER and never increased again.
>> 
>> The solution is to cut the upper 32-bit from the hpet_t1_cmp
>> variable to make the comparison to HPET_COUNTER correct.
>> 
>> Reported-by: Kirill Korotaev <dev@...nvz.org>
>> Signed-off-by: Pavel Emelyanov <xemul@...nvz.org>
> 
> applied to tip/timers/urgent, thanks Pavel!
> 
> I'm wondering, what symptoms were seen due to the bug? Did system time stop,
> or did it become erratic, or were there outright lockups?

Symptoms were observed in user space. An application doing read() on
/dev/rtc blocked sometimes for a long periods of time (as turned out - until
counter gets wrap around).

Regards,
Kirill

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