Levente Uzonyi uploaded a new version of Kernel to project The Trunk:
http://source.squeak.org/trunk/Kernel-ul.520.mcz

==================== Summary ====================

Name: Kernel-ul.520
Author: ul
Time: 8 December 2010, 3:19:51.682 am
UUID: da185163-27f6-6a47-8ecb-e00f9fd8dd3a
Ancestors: Kernel-ul.519

A few changes to Semaphore:
- save a block creation + activation + a possible context switch in #critical:ifCurtailed:
- save a block creation + activation in #critical:ifError:, this changes the behavior when mutuallyExcludedBlock doesn't understand #ifError:
- #critical:ifLocked: is guaranteed to not get locked. The earlier implementation sent #critical: which could cause a context switch, so another process could enter the critical section before the current. Also fixed the comments of this method.

=============== Diff against Kernel-ul.519 ===============

Item was changed:
  ----- Method: Semaphore>>critical:ifCurtailed: (in category 'mutual exclusion') -----
  critical: mutuallyExcludedBlock ifCurtailed: terminationBlock
  	"Evaluate mutuallyExcludedBlock only if the receiver is not currently in 
  	the process of running the critical: message. If the receiver is, evaluate 
  	mutuallyExcludedBlock after the other critical: message is finished."
+ 	^self critical: [ mutuallyExcludedBlock ifCurtailed: terminationBlock ]
- 	^self critical:[[mutuallyExcludedBlock value] ifCurtailed: terminationBlock]
  !

Item was changed:
  ----- Method: Semaphore>>critical:ifError: (in category 'mutual exclusion') -----
  critical: mutuallyExcludedBlock ifError: errorBlock
  	"Evaluate mutuallyExcludedBlock only if the receiver is not currently in 
  	the process of running the critical: message. If the receiver is, evaluate 
  	mutuallyExcludedBlock after the other critical: message is finished."
  	| blockValue hasError errMsg errRcvr |
  	hasError := false.
  	blockValue := self critical:[
+ 		mutuallyExcludedBlock ifError: [ :msg :rcvr |
- 		[mutuallyExcludedBlock value] ifError:[:msg :rcvr|
  			hasError := true.
  			errMsg := msg.
  			errRcvr := rcvr
  		].
  	].
  	hasError ifTrue:[ ^errorBlock value: errMsg value: errRcvr].
  	^blockValue!

Item was changed:
  ----- Method: Semaphore>>critical:ifLocked: (in category 'mutual exclusion') -----
  critical: mutuallyExcludedBlock ifLocked: alternativeBlock
  	"Evaluate mutuallyExcludedBlock only if the receiver is not currently in 
+ 	the process of running the critical: message. If the receiver is, then evaluate 
+ 	alternativeBlock and return."
+ 	"See the comment of #critical: for the explanation how this pattern works
+ 	before changing the code."
+ 
+ 	| caught |
+ 	caught := false.
+ 	^[
+ 		"We're using #== here instead of #=, because it won't introduce a
+ 		suspension point, while #= may do that."
+ 		excessSignals == 0
+ 			ifTrue: [ alternativeBlock value ]
+ 			ifFalse: [
+ 				excessSignals := excessSignals - 1.
+ 				caught := true.
+ 				mutuallyExcludedBlock value ] ]
+ 		ensure: [ caught ifTrue: [ self signal ] ]!
- 	the process of running the critical: message. If the receiver is, evaluate 
- 	mutuallyExcludedBlock after the other critical: message is finished."
- 	
- 	"Note: The following is tricky and depends on the fact that the VM will not switch between processes while executing byte codes (process switches happen only in real sends). The following test is written carefully so that it will result in bytecodes only.
- 	Do not change the following #== for #=, as #== is not a real message send, just a bytecode."
- 	excessSignals == 0 ifTrue: [
- 		"If we come here, then the semaphore was locked when the test executed. 
- 		Evaluate the alternative block and answer its result."
- 		^alternativeBlock value 
- 	].
- 	^self critical: mutuallyExcludedBlock!