In some email I received from Jason R Thorpe, sie wrote:
> On Sat, May 25, 2002 at 08:59:20PM +0100, David Laight wrote:
> 
>  > I found some problems with the 'solaris' semantics of saving the spl
>  > level within the mutex, the 'SVR4' semantics where the spl level
>  > is returned to the user are more flexible.
> 
> Some say "flexible", others say "error-prone".
> 
>  > 	pl1 = read_lock( table_lock )
>  > 	entry = search_table( key );
>  > 	pl2 = lock( entry->lock );
>  > 	unlock( table_lock, pl2 );
>  > 	.....
>  > 	unlock( entry->lock, pl1 );
> 
> You can already do this with Solaris-style mutexes if they are adaptive
> mutexes.  Interrupt-blocking mutexes are meant to not be used that often
> in the Solaris kernel (and I happen to agree with their approach).

It'd be very cool if we could have adaptive mutexes in NetBSD.

Darren