linux-kernel - Re: [BUG] While changing the cpufreq governor, kernel hits a bug in workqueue.c

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Date:	Tue, 12 Aug 2008 23:44:51 +0200
From:	Johannes Weiner <hannes@...urebad.de>
To:	Nageswara R Sastry <rnsastry@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, balbir@...ux.vnet.ibm.com,
	ego@...ux.vnet.ibm.com, svaidy@...ux.vnet.ibm.com,
	davej@...emonkey.org.uk,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Alexey Starikovskiy <astarikovskiy@...e.de>
Subject: Re: [BUG] While changing the cpufreq governor, kernel hits a bug in workqueue.c

Johannes Weiner <hannes@...urebad.de> writes:

> Hi Nageswara,
>
> Nageswara R Sastry <rnsastry@...ux.vnet.ibm.com> writes:
>
>> Any updates on this bug.
>
> Sorry.  I was just looking into it again and still see no resolution.
> Added Venkatesh and Alexey to CC.

Sorry, wrong address for Alexey.  Hope this one works.

> To summarize again:
>
> The problem we have is that cpufreq_ondemand has a self-rearming worker
> function which we should but can not cancel synchroneously with the
> current code.  The locking order is like this:
>
> store()
>   policy_rwsem write
>     cpufreq_governor_dbs()
>     dbs_mutex
>
> work lock
>   do_dbs_timer()
>     policy_rwsem write
>
> which gives a locking hierarchy of:
>
> work lock
>   policy_rwsem
>     dbs_mutex
>
> Now, we _should_ cancel the worker synchroneously, but that won't fly
> since when cpufreq_governor_dbs() is called, we already hold
> policy_rwsem and grabbing the work lock is illegal.
>
> I will continue to read code and see if I can come up with a solution,
> but any help is really appreciated.
>
> I have not yet looked at cpufreq_conservative...
>
>> Thanks and Regards
>> R.Nageswara Sastry
>
> Thanks for your patience and insistence,
>
> 	Hannes
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