Re: [ANNOUNCE] 3.14.23-rt20

[Juerg Haefliger]

On Wed, Nov 5, 2014 at 3:27 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Wed, 5 Nov 2014 14:50:41 +0100
> Juerg Haefliger <juergh@xxxxxxxxx> wrote:
>
>> On Sun, Nov 2, 2014 at 8:30 AM, Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
>> 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.

Thanks for the explanation. So is this considered a temporary failure
until multiple readers are allowed or does cpufreq need fixing or
something else? Just trying to figure out what to do next.

...Juerg


> 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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