Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation

Lee Revell
2005-12-18T01:22:12Z

On Sat, 2005-12-17 at 17:21 -0700, Matthew Wilcox wrote:
> On Sat, Dec 17, 2005 at 07:05:21PM -0500, Lee Revell wrote:
> > On Sat, 2005-12-17 at 16:43 -0700, Matthew Wilcox wrote:
> > > I have a better example of something we currently get wrong that I
> > > haven't heard any RT person worry about yet. If two tasks are sleeping
> > > on the same semaphore, the one to be woken up will be the first one to
> > > wait for it, not the highest-priority task.
> > >
> > > Obviously, this was introduced by the wake-one semantics. But how to
> > > fix it? Should we scan the entire queue looking for the best task to
> > > wake? Should we try to maintain the wait list in priority order? Or
> > > should we just not care? Should we document that we don't care? ;-)
> >
> > It's well known that this is a problem:
> >
> > http://developer.osdl.org/dev/robustmutexes/src/fusyn.hg/Documentation/fusyn/fusyn-why.txt
>
> Erm. That paper is talking about user-space semaphores based on futexes.
> I'm talking about kernel semaphores. At a first glance, fixing futexes
> would be a very different job from fixing semaphores.
>
> BTW, fuqueues? HAHAHAHA.
>

Hmm, interesting, so in fact the scheduler does not always run the
highest priority runnable process? Do you have a test case where
userspace would experience priority inversion due to this?

Maybe it has not been a problem as all the PI cases would involve two RT
processes that make system calls which end up blocking on a semaphore in
the kernel, which is bad RT design anyway - normally you would separate
the RT parts of the app which carefully avoid possibly blocking system
calls from the non RT parts and communicate via lock free ringbuffers or
a similar mechanism.

Lee