RE: [RFC/PATCH] FUSYN Realtime & robust mutexes for Linux, v2.3.1

[Perez-Gonzalez, Inaky]

> From: Linh Dang
> 
> Ulrich Drepper <drepper@xxxxxxxxxx> wrote:
> > The fast path for all locking primitives etc in nptl today is
> > entirely at userlevel.  Normally just a single atomic operation with
> > a dozen other instructions.  With the fusyn stuff each and every
> > locking operation needs a system call to register/unregister the
> > thread as it locks/unlocks mutex/rwlocks/etc.  Go figure how well
> > this works.  We are talking about making the fast path of the
> > locking primitives two/three/four orders of magnitude more
> > expensive.  And this for absolutely no benefit for 99.999% of all
> > the code which uses threads.
> >
> 
> Is there an EFFICIENT way to add priority-inheritance to futex? the
> lack of priority-inheritance is biggest headache for RT applications
> running on top of NPTL/kernel-2.6. And there's is a LOT more of us (RT
> users who want to use NPTL/kernel-2.6) than you might think. I guess
> we're just not vocal.

No.

You need the concept of ownership (who do I have to boost?). You need 
spinlocks to be able to traverse the who-is-waiting-for-whom trees
(you might have three guys trying to lock from three different CPUs
plus other guys preempting in the middle and you need to protect those
trees.

Iñaky Pérez-González -- Not speaking for Intel -- all opinions are my own (and my fault)
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