linux-kernel - Re: [PATCH] arch: x86: init hpet event

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Date:	Mon, 7 May 2012 22:26:34 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Vladimir Davydov <vdavydov@...allels.com>
cc:	"Venkatesh Pallipadi (Venki)" <venki@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arch: x86: init hpet event_handler to noop

On Tue, 24 Apr 2012, Vladimir Davydov wrote:

> If hpet is enabled by hpet_late_init() - this usually occurs on systems
> with buggy BIOS, which does not report about hpet presence through ACPI,
> hpet_clockevent's event_handler can be left uninitialized by
> clockevents_register_device() because of hpet_clockevent low rating (by
> the time hpet_late_init() is called, high prio apic timers have already
> been setup). The event_handler is then initialized a bit later by the
> clocksource_done_booting() procedure.
> 

This explanation is worse than an oracle and aside of that, it's
patently wrong.

How the hell is clocksource_done_booting() related to the HPET
clockevent mechanism?

> Normally, timer interrupts should not be delivered between these two
> calls, but if e.g. the kernel is booted using kexec, there might be some
> pending interrupts from the previous kernel's context, which can lead to
> a NULL pointer dereference in timer_interrupt().

How is kexec related to this?

And how should pending interrupts be not handled by the always first
initialized PIT ?

> Avoid this by initializing hpet's event_handler to noop in its definition.

"Avoid" is the correct term: You're avoiding to track down the root
cause of the problem. 

This is fairy tale mode. I really love fairy tales, just not in the
context of kernel code.

Please provide proper proof why this can happen instead of some
handwavy explanations.

Thanks,

	tglx
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