linux-kernel - Re: kernel deadlock

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Message-ID: <522F4797.1050509@linaro.org>
Date:	Tue, 10 Sep 2013 09:23:51 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Ingo Molnar <mingo@...nel.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>
Subject: Re: kernel deadlock

On 09/10/2013 12:29 AM, Ingo Molnar wrote:
> * John Stultz <john.stultz@...aro.org> wrote:
>
>> Now, I'm still in the dark as to why HRTICK exposes this, but seems like 
>> the following patch should resolve the issue (and quiets the lockdep 
>> warnings in my testing). Let me know how it works for you!
>>
>> Ingo: This makes me think we really should have some lockdep smarts 
>> added to seqlock/seqcount structures. Is there something already 
>> discovered thats preventing this, or has this just not yet been tried?
> There's no fundamental reason to not do it, it's a very good idea!
>
>> From 09885df9734aa678ccb61dbef356fea4faeff16c Mon Sep 17 00:00:00 2001
>> From: John Stultz <john.stultz@...aro.org>
>> Date: Mon, 9 Sep 2013 13:07:18 -0700
>> Subject: [PATCH] timekeepeing: Fix HRTICK related deadlock from ntp lock changes
> Cool fix! I think I've seen this deadlock in various incarnations but was 
> never able to track it down. Do you have a guesstimate about roughly when 
> it was introduced? Is it an ancient bug?

3.10, I believe. So no not ancient, but we've seen and resolved similar
issues in the past (the old printk while holding xtime_lock deadlock).

Hopefully the lockdep enablement helps sort these out for good.

thanks
-john

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