Date: Wed, 5 Nov 2014 09:27:07 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Juerg Haefliger <juergh@gmail.com>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Carsten Emde <C.Emde@osadl.org>,
        John Kacur <jkacur@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Clark Williams <williams@redhat.com>
Subject: Re: [ANNOUNCE] 3.14.23-rt20
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On Wed, 5 Nov 2014 14:50:41 +0100
Juerg Haefliger <juergh@gmail.com> wrote:

> 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.

When we allow multiple readers, this will be allowed. But even in
mainline, if a writer were to come in and block between those two
down_read_trylocks(), the second trylock would fail.

PREEMPT_RT just has that fail all the time as we only allow an rwsem to
be held by a single reader.


-- Steve


> 
> ...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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