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SubjectRe: [patch] Real-Time Preemption, -RT-2.6.9-rc4-mm1-U8
On Thu, 21 Oct 2004, john cooper wrote:

> Ingo Molnar wrote:
>
> >* Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> >
> >>Yeah, for a semaphore it is, but not for a mutex.
> >>
> >
> >but mutexes dont exist in upstream Linux as a separate entity. (they
> >exist in my tree but that's another ballgame.)
> >
> Mutexes layered on existing semaphores seems convenient
> at the moment. However a more native mutex mechanism
> which tracks ownership would provide a basis for PI as
> well as further instrumentation. This may not be an
> issue at the present but I don't think it is too far
> off.
>
> -john
>

Actually you need to have another kind of semaphore based on a new kind of
wait-queue: Priority based. I.e. the task with the highest priority get
woken up first. Then on top of that you build your mutex.

I was planning to start to look at it and try to see if I could get
anything to work, but I must admit I haven't got much further than
just getting Igno's -U8.1 up running.

An idea I had was to make a macro in list.h called
list_insert_sorted(list, element, condition_statement)
and use that in this kind of wait_queue...

To get a mutex with priority inheritance add an element pointing to the
current owner and a field where you store the owners original priority
which it has to be set back to when it relases the mutex (I am not sure
how this will work out if someone holds several mutexes!)

Regards,
Esben


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