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Subject: Re: hware clock left bad after a system failure
Date: Wed, 16 Nov 2005 10:52:27 -0500
Message-ID: <Pine.LNX.4.61.0511161037130.12055@chaos.analogic.com>
Thread-Topic: hware clock left bad after a system failure
Thread-Index: AcXqxb6LItyNepzLSJO2cAK/tpTr9Q==
From: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
To: "Eyal Lebedinsky" <eyal@eyal.emu.id.au>
Cc: "list linux-kernel" <linux-kernel@vger.kernel.org>
Reply-To: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
Sender: linux-kernel-owner@vger.kernel.org
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On Wed, 16 Nov 2005, Eyal Lebedinsky wrote:

> I recently had two cases where my machine locked up and needed
> a hard reset. The last time magic SysRq did not respond at all.
>
> In these cases I found that the hware clock was set incorrectly
> and the machine comes up with a bad date. It seems that the clock
> is ahead by as much as my TZ (+10 in my case). I may be able
> to understand if it was set 10h behind (kernel set it to UTC)
> but this is the other way. The machine comes up with UTC+20.
>
> Now this is just trouble. The machine comes up and spends 15m
> fscking. I then reset the clock and reboot and it does the whole
> fsck again because it thinks the fs was not checked for eons. It
> does not understand time in the future.
>
> So the points are
>
> - why is the clock mangled in this way?

I am assuming that you have an ix86 kind of machine.

It's probably mangled because you had a hardware-crash.

If you have a driver that accesses the RTC, it needs to leave
the index register at offset 0 so that a hardware crash can
only upset the seconds. Otherwise, even the RTC checksum
can get screwed up, forcing manual reconfiguration of the
BIOS.

During a hardware-crash, the chip enables may go TRUE. This
means that an RTC write can occur with junk that's on the
data-bus.

Now, you need to find out why you had a hardware-crash which
is quite unlike a software-crash. A hardware crash occurs when
you turn OFF the power or the power-good line from the
power-supply goes FALSE. You do not get a hardware-crash from
hitting the reset button. You may have induced the RTC failure
if you hit the power switch instead of the reset button.

> - should e2fsck not allow future check time (maybe within some
>  limits)?

Doesn't the `init` script ask you if you want to fsck the
drive? Most distributions do. Anyways, a time in the future
is one of the ways e2fsck may discover that your file-system
is dorked. You certainly don't want it to ignore it by default.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.51 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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