scheduler (sched_4bsd) questions

[Stephan Uphoff]

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 :-(

This was found using Peter Holm's test and a slight modification of this
giant hog detector. (kern_clock.diff)

I definitely won't have time to fix kern_mutex.c for the next few days
so please add the line:

options		MUTEX_WAKE_ALL		# Needed do not remove

to your configuration files.

I also had overlooked 
	 	http://www.holm.cc/stress/log/cons80.html
Showing that my patch for kern_switch.c (switch_patch) has a bug.
I will send an updated patch later today.

	Stephan

PS: I love the firewire debugging speed!