linux-kernel - Re: kernel deadlock

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Date:	Tue, 10 Sep 2013 09:38:44 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Lin Ming <minggr@...il.com>
CC:	Gerlando Falauto <gerlando.falauto@...mile.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Cochran <richardcochran@...il.com>,
	Prarit Bhargava <prarit@...hat.com>,
	"Brunck, Holger" <Holger.Brunck@...mile.com>,
	"Longchamp, Valentin" <Valentin.Longchamp@...mile.com>,
	"Bigler, Stefan" <Stefan.Bigler@...mile.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: kernel deadlock

On 09/10/2013 01:59 AM, Lin Ming wrote:
>  On Tue, Sep 10, 2013 at 4:29 AM, John Stultz <john.stultz@...aro.org> wrote:
>
> [snip]
>
>> So I think I've managed to finally reproduce this and hunt it down.
>>
>> With Peter's "sched: Fix HRTICK" patch and HRTICK enabled, I found I
>> could trigger a hard hang at boot on my x86_64 kvm system. sysrq didn't
>> function, so I checked out info cpus and that pointed to both cpus being
> Hi,
>
> Is "info cpus" a command of kvm/qemu? That's very helpful.
Yes. If you switch into the qemu monitor, you can use "info cpus" to
show the current instruction pointers

> I can reproduce this bug, but there is no any output.
> How did you find out that both cpus being in ktime_get() and
> ktime_get_update_offsets().

Once I had the instruction pointers, I ran gdb on the vmlinux and used
"list *<address>" to show where it was stuck.

>> in ktime_get() and ktime_get_update_offsets(), which suggested a
>> seqcount deadlock (basically calling something that reads the seqlock
>> while we hold the write on it).
> HRTICK enabled, then I can reproduce this simply with,
>
> while [ 1 ] ;
>     adjtimex -t 9999
> done
>
> And your patch fixed it.

Great! Can I add a Tested-by: from you on the patch?

thanks
-john

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