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

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Message-ID: <20090205145311.GB23903@elte.hu>
Date:	Thu, 5 Feb 2009 15:53:11 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Kirill Korotaev <dev@...allels.com>
Cc:	Pavel Emelyanov <xemul@...nvz.org>,
	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


* Kirill Korotaev <dev@...allels.com> wrote:

> 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).

okay. Could you guys please put this info in the commit log on the 
resubmitted patch? Thanks,

	Ingo
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