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Date: Wed, 5 Nov 2014 15:04:29 +0100
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Subject: Re: [ANNOUNCE] 3.14.23-rt20
From: Juerg Haefliger <juergh@gmail.com>
To: LKML <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>
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Resending to the list due to mailer/html issues.


On Sun, Nov 2, 2014 at 8:30 AM, Mike Galbraith <umgwanakikbuti@gmail.com> wrote:
>
> On Fri, 2014-10-31 at 17:03 -0400, Steven Rostedt wrote:
> > Dear RT Folks,
> >
> > I'm pleased to announce the 3.14.23-rt20 stable release.
> >
> > This is the first 3.14-rt release in the stable-rt series. Normally I
> > wait till the next development release is out before I pull in a new
> > one. That is, I would pull in 3.14-rt when 3.16-rt or later was
> > released. But because development is now moving at a "hobbyist rate"
> > (read http://lwn.net/Articles/617140/ for details)
> > and 3.14-rt is no longer being developed against, I figured it was
> > time
> > to put it under the "stable-rt" umbrella.
>
> I piddled about with it yesterday, found that you can't change cpufreq
> governor IFF the tree is configured as rt, but works fine as voluntary
> preempt.

The problem seems to be this patch: https://lkml.org/lkml/2014/4/8/584

The cpufreq code does nested down_read_trylocks and only the first one succeeds:

drivers/cpufreq/cpufreq.c:
store
  down_read_trylock(cpufreq_rwsem)  <- succeeds
  store_scaling_governor
    cpufreq_get_policy
      cpufreq_cpu_get
        down_read_trylock(cpufreq_rwsem)  <-- fails

Reverting the above patch 'fixes' the problem. I don't understand
Steven's commit comment that readers of rwsem are not allowed to nest
in mainline since this works just fine in mainline.

...Juerg


>  I'll poke about for the entertainment value.  Having no
> personal need/use for rt detracts from its hobby value somewhat, but rt
> problems do have a tendency to be 'entertaining'.
>
> I'll follow up with a few patches that folks can apply to their trees if
> they so desire.  There being no devel tree to submit against, I can't do
> a proper submission (rules), and some of them you surely don't want :)
>
>         -Mike
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