linux-kernel - Re: Re: [PATCH] [BUGFIX] crash/ioapic: Prevent crash_kexec() from deadlocking of ioapic

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Date:	Tue, 03 Sep 2013 20:02:55 +0900
From:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Don Zickus <dzickus@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Gleb Natapov <gleb@...hat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Joerg Roedel <joro@...tes.org>, x86@...nel.org,
	stable@...r.kernel.org, Marcelo Tosatti <mtosatti@...hat.com>,
	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>,
	Ingo Molnar <mingo@...hat.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	yrl.pp-manager.tt@...achi.com,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Seiji Aguchi <seiji.aguchi@....com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Re: [PATCH] [BUGFIX] crash/ioapic: Prevent crash_kexec() from
 deadlocking of ioapic_lock

(2013/09/03 9:12), Eric W. Biederman wrote:
>>>> Then again looking at the output of the latest dmesg, it seems the IO APIC
>>>> is initialized way before the tsc is calibrated.  So I am not sure what
>>>> needed to get done or what interrupts are needed before the IO APIC gets
>>>> initialized.
>>>
>>> The practical issue is that jiffies was calibrated off of the PIT timer
>>> if I recall.  But that is all old news.
>>
>> Are the jiffies calibration codes calibrate_delay()?
>> It seems that the jiffies calibration have not used PIT in 2005
>> according to 8a9e1b0.
> 
> Exactly.  That was the original reason why we put in the code to
> disable the IOAPIC and the local apic.  There might have been other
> reasons but that was the primary.

Thanks, but I have still a question for jiffies calibration.

When kernel boots, calibrate_delay_direct() will be called in
calibrate_delay() for calculating loops_per_jiffy.  Then,
calibrate_delay_direct() waits until jiffies is incremented.
I think this means PIT or HPET is still used for the calibration.
Is there something wrong with my understanding?
If wrong, how is jiffies incremented?

>>> The kernel startup path has been fixed for years, and disable_IO_APIC in
>>> crash_kexec has always been a bug work-around for deficiencies in the
>>> kernel's start up path (not part of the guaranteed interface).
>>> Furthermore every real system configuration I have encountered used the
>>> same kernel version for the crashdump kernel and the production kernel.
>>> So we should be good.
>>
>> We also will be use the kdump(crashdump) kernel as the production
>> kernel. Should I only care for the current kernel?
> 
> For this particular issue yes.
> 
> In general it is important for there to be a stable interface between
> the two kernels just so you are not required to use the same kernel
> version, and so there is the possibility of using something besides a
> linux kernel.

OK. In order to judge whether a kernel version as crashdump kernel is
usable or not, I want to understand why we can remove disable_IO_APIC
in detail.

> At the same time it has always been the targets kernel's responsibility
> to sort out the hardware devices unless it can't possibily do it.  And
> apics for the longest time were very very hard to reset in the target
> kernel, but now that they are not.  It makes sense for time permitting
> to remove the now unnecessary code in the crashing kernel.  Because
> ultimately the less code we have the fewer possible ways we can fail
> in a known broken kernel.

Yes, I agree with you.

Thanks,
Yoshihiro YUNOMAE

-- 
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae.ez@...achi.com


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