scheduler (sched_4bsd) questions

[Stephan Uphoff]

On Mon, 2004-10-04 at 11:31, John Baldwin wrote:
> On Friday 01 October 2004 12:13 am, Stephan Uphoff wrote:
> > On Wed, 2004-09-29 at 18:14, Stephan Uphoff wrote:
> > > I was looking at the MUTEX_WAKE_ALL undefined case when I used the
> > > critical section for turnstile_claim().
> > > However there are bigger problems with MUTEX_WAKE_ALL undefined
> > > so you are right - the critical section for turnstile_claim is pretty
> > > useless.
> >
> > Arghhh !!!
> >
> > MUTEX_WAKE_ALL is NOT an option in GENERIC.
> > I recall verifying that it is defined twice. Guess I must have looked at
> > the wrong source tree :-(
> > This means yes - we have bigger problems!
> >
> > Example:
> >
> > Thread A holds a mutex x contested by Thread B and C and has priority
> > pri(A).
> >
> > Thread C holds a mutex y and pri(B) < pri(C)
> >
> > Thread A releases the lock wakes thread B but lets C on the turnstile
> > wait queue.
> >
> > An interrupt thread I tries to lock mutex y owned by C.
> >
> > However priority inheritance does not work since B needs to run first to
> > take ownership of the lock.
> >
> > I is blocked :-(
> 
> Ermm, if the interrupt happens after x is released then I's priority should 
> propagate from I to C to B.  

There is a hole after the mutex x is released by A - but before B can
claim the mutex. The turnstile for mutex x is unowned and interrupt
thread I when trying to donate its priority will run into:

	if (td == NULL) {
			/*
			 * This really isn't quite right. Really
			 * ought to bump priority of thread that
			 * next acquires the lock.
			 */
			return;
		}

So B needs to run and acquire the mutex before priority inheritance
works again and does not get a priority boost to do so. 

This is easy to fix and MUTEX_WAKE_ALL can be removed again at that time
- but my time budget is limited and Peter has an interesting bug left
that has priority.

> If the interrupt happens before x is released, 
> then the final bit of propagate_priority() should handle it since it resorts 
> the turnstile's thread queue so that C will be awakened rather than B.

Agreed.

	Stephan