Traditional and still prevalent concurrency model is based on threads and shared mutable state. Smalltalk is no different in that respect, having processes that can run concurrently virtually "ever since". The traditional solution to prevent race condition is to use locks for synchronization. Many actions must be properly synchronized, even the simplest things like a Transcript or an access to a method source code. In this presentation I will talk about motivation, use and actual implementation of thinlocks - a fast locking mechanism invented for Java - in scope of Smalltalk.

1. Thin locks:
featherweight
synchronization
also for Smalltalk
Jan Vraný

2. Benchmark # Enter/Exit # Contentions % Contentions
JVM boot 6 808 72 1.0%
Groovy 3 + 4 102 835 5 484 5.3%
Tomcat startup 703 267 12992 1.8%

3. close
self closed
ifTrue: [ ^ self ].
File primClose: handle.
self unregister.
handle := nil

4. close
self closed
ifTrue: [ ^ self ].
File primClose: handle.
self unregister.
handle := nil close
self closed
ifTrue: [ ^ self ].
File primClose: handle.
self unregister.
handle := nil
Thread A
Thread B

5. close
self closed
ifTrue: [ ^ self ].
File primClose: handle.
self unregister.
handle := nil close
self closed
ifTrue: [ ^ self ].
File primClose: handle.
self unregister.
handle := nil
Thread A
Thread B
open
handle := ...
Thread C

6. The Deadlock Empire attacks!
The war has begun.
https://deadlockempire.github.io/

7. Thin locks: featherweight synchronization for Java
David F. Bacon, Ravi Konuru, Chet Murthy, Mauricio Serrano
IBM T.J. Watson Research Center

8. unlocked → locked locked twice contention
0
1000
2000
3000
4000
5000
6000
St/X 8.x.x (w/ thin locks)
St/X 6.x.x
Pharo 6.1

9. 10 0
031/63
unlocked

10. 1123 0
031/63
locked once by thread 123

11. 1123 1
031/63
locked twice by thread 123

12. 00xCAFEAFFE
031/63
locked, contended

13. acquireWithTimeoutMs: timeout
%{ /* NOCONTEXT */
if ( stxThinLock( &(__INST(process)) ) {
return true;
}
%}.
process class == SmallInteger
ifTrue:[ self inflate ].
^ super acquireWithTimeoutMs: timeout

14. static inline
bool stxThinLock(UINT* lockword) {
if (CAS(lockword, UNLOCKED, LOCKED_ONCE))
{
return 1;
}
if (CAS(lockword, LOCKED_ONCE, LOCKED_TWICE))
{
return 1;
}
…
#define UNLOCKED 0x1
#define LOCKED_ONCE (activeProcId)
#define LOCKED_TWICE (activeProcId + 2)

15. …
UINT expected = *lockword;
if ((expected & PID_MASK) == activeProcId)&&
(expected & CNT_MASK) < CNT_MAX))
{
if (CAS(lockword, expected, expected + 2))
{
return 1;
}
}
return 0;
}

16. Thin locks: featherweight synchronization for Java
David F. Bacon, Ravi Konuru, Chet Murthy
& Mauricio Serrano