[Vm-dev] OwnedLock primitives request for review

Ben Coman btc at openinworld.com
Wed Mar 23 18:07:38 UTC 2016

I have the OwnedLock primitives and image side code suggested by
Dennis for Pharo ready for review...
   (207 #primitiveOwnedLockRelease)
   (208 primitiveOwnedLockTryAcquire)
   (209 primitiveOwnedLockWaitAcquire)

Dennis identified an interesting corner case codified in

1) low priority process B waits on lock because it is owned by another process A
2) process A release lock but process B not resumed since higher
priority process C is running
3) process C terminates process B
4) process B should (but does not) release acquired lock during termination.

This is because B is given the lock by A's call to primitivexxRelease,
and the ensure in #critical doesn't wrap B's call to
primitivexxWaitAcquire.  However wrapping an ensure around the whole
#critical means if the process is terminated while waiting, you need
to be careful not to releasing someone else's lock.   All a little bit

Now I read recently something about primitive handling of forwarders
being done by side-effect-free failing of primitives and retying, and
I wonder if the same might be feasible here.  Rather than A's call to
primitivexxRelease setting B as the lock owner, it just sets the owner
to nil, and when B is woken up, it retries primitivexxWaitAcquire.

How does that sound?

cheers -ben
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