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

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Message-ID: <8763q53ot0.fsf@skyscraper.fehenstaub.lan>
Date:	Tue, 12 Aug 2008 23:29:15 +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 <alexey.y.starikovskiy@...el.com>
Subject: Re: [BUG] While changing the cpufreq governor, kernel hits a bug in workqueue.c

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.

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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