linux-kernel - Re: kernel deadlock

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Message-ID: <CAF1ivSaZKsXsPjMP30j9YYZDZPUe6Hs9zXyfqrG7GuGm4rLjew@mail.gmail.com>
Date:	Wed, 11 Sep 2013 08:37:54 +0800
From:	Lin Ming <minggr@...il.com>
To:	John Stultz <john.stultz@...aro.org>
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 Wed, Sep 11, 2013 at 12:38 AM, John Stultz <john.stultz@...aro.org> wrote:
> 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?

Sure.
Tested-by: Lin Ming <minggr@...il.com>

Thanks.
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