On Sun, 14 Jul 1996, Terry Lambert wrote:

> > Solaris kernel also has a debugging feature, it's called "deadman".
> > Every so often a timer based callout runs which runs down all the
> > mutex/semaphore/spinlock holder lists in the kernel and panics if any
> > circular and/or deadlock cases are found.
> 
> The IBM database design manual indicates several methods of fast
> computation of transitive closure over acyclic directed  graphs
> which make this kind of kludge unnecessary.  IMO, deadlock
> avoidance beats deadlock detection any day of the week.  Consider
> the issue of a two context deadly embrace for a resource: how do
> you unwind state so you don't lose already processed context?  In
> the deadlock detection case, the anser is "you probably don't".  Or
> you go to some overly complicated cruft, like the system call
> interrupt code in the trap, using a longjmp to unwind the stack.

How is this different from implementors carefully following a set of rules
in terms of performance?  Are there just too many permutations that it
just isn't reasonable to expect to be able to avoid deadlocks without
computing transitive closure across directed graphs?

-mike hancock