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

[Matthew Wilcox <matthew@wil.cx>]

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.
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