[Vm-dev] Where to get Monitor implementation based on primitives?

[Denis Kudriashov]

Eliot I found that there are no methods for simulation code (if it right
name for it). So stepping over new primitives failed (which is infinite
recursion for Pharo case)

Context>>doPrimitive: primitiveIndex method: meth receiver: aReceiver args:
arguments
...
"Mutex>>primitiveEnterCriticalSection
Mutex>>primitiveTestAndSetOwnershipOfCriticalSection"
(primitiveIndex = 186 or: [primitiveIndex = 187]) ifTrue:
[| active effective |
active := Processor activeProcess.
effective := active effectiveProcess.
"active == effective"
value := primitiveIndex = 186
ifTrue: [aReceiver *primitiveEnterCriticalSectionOnBehalfOf*: effective]
ifFalse: [aReceiver
*primitiveTestAndSetOwnershipOfCriticalSectionOnBehalfOf*: effective].
^(self isPrimFailToken: value)
ifTrue: [value]
ifFalse: [self push: value]].

How this methods should be implemented?

2016-01-11 13:59 GMT+01:00 Denis Kudriashov <dionisiydk at gmail.com>:

> Hello.
> I publish slice 17373
> <https://pharo.fogbugz.com/f/cases/17373/Mutex-should-be-based-on-VM-primitives-and-implement-critical-methods-from-Semaphore>
> .
>
> I introduce LockOwnership class which implements VM primitives as:
> - acquire
> - tryAcquire
> - release
> (It comment saves copyright from CriticalSection)
>
> Mutext uses it as ownership instance variable to implement critical
> methods correctly:
> - critical:
> - critical:ifLocked:
> - critical:ifError:
> - critical:ifCurtailed:
>
> For the integration process old Mutex instance variables are not removed. So
> loading this code should not broke current Mutex instances. But when we
> integrate it Mutex will have only variable ownership.
>
> If you look critical implementation you will understand why I not like
> semantic of primitives. it should be inverted in future. #acquire should
> return true when ownership is acquired right now.
>
> Process>>terminate now detects waiting on LockOwnership and ask it to
> handle wait termination.Then LockOwnership inject right value into lock
> state variable. Such variables should be marked with special pragma
>     <lockAt: #ownershipVariableName tracksStateAt: 1> "index of local
> variable"
> Method can contain mulpible pragmas to reference all ownerships in method.
> ReadWriteLock for example needs this.
>
> 2016-01-11 13:30 GMT+01:00 Henrik Johansen <henrik.s.johansen at veloxit.no>:
>
>>
>>
>> On 08 Jan 2016, at 4:25 , Ben Coman <btc at openInWorld.com
>> <btc at openinworld.com>> wrote:
>>
>>
>> On Fri, Jan 8, 2016 at 9:39 PM, Ben Coman <btc at openinworld.com> wrote:
>>
>> On Fri, Jan 8, 2016 at 5:42 PM, stephane ducasse
>> <stephane.ducasse at gmail.com> wrote:
>>
>>
>> I have a (stupid) question.
>> Is the code running without the primitives?
>> Are the code below the primitives correct?
>> I asked that because we can have 100 eyes and brains on the smalltalk
>> level and far less on the VM primitive level.
>>
>>
>> Because:
>> 1. Concurrency bugs can be subtle and the *exact* conditions can be
>> hard to reproduce for debugging.  For example, the solution to a
>> couple of problems with Delay [1] [2] were solved by moving away from
>> Semaphore>>critical: to use signalling.
>>
>> 2. The in-image atomicity of determining whether a signal was actually
>> consumed or not during process suspension/termination is awkward.  Its
>> seems hard to *really* know for sure it right (but I haven't looked in
>> depth into Denis' latest proposals.)
>>
>> 3. The existing in-image implementation of Semaphore>>critical messes
>> around in  Process>>terminate in a *special* way that is not easy for
>> those 100 eyes to understand. For example, I personally am not
>> comfortable with understanding how the special Semaphore handling in
>> Process>>terminate works, but I can easily follow how
>> primitiveEnterCriticalSection just looking at the code [3].
>>
>>
>> Points 2 & 3 might possibly be addressed by having new
>> primitiveWaitReturned
>> *always* return true, so if the process is terminated while waiting,
>> the assignment to signalConsumed doesn't occur...
>>
>>  critical: mutuallyExcludedBlock
>>    signalConsumed := false.
>>    [
>>        signalConsumed := self primitiveWaitReturned.
>>        blockValue := mutuallyExcludedBlock value
>>    ] ensure: [ signalConsumed ifTrue: [self signal] ].
>>    ^blockValue
>>
>> where primitiveWait (https://git.io/vuDjd) is copied
>> and (just guessing) the marked line added...
>>
>>  primitiveWaitReturned
>>      | sema excessSignals activeProc inInterpreter |
>>      sema := self stackTop. "rcvr"
>> "==>>" self pop: argumentCount + 1 thenPush: objectMemory trueObject.
>> "<<=="
>>     excessSignals := self fetchInteger: ExcessSignalsIndex ofObject: sema.
>>      excessSignals > 0
>>          ifTrue:
>>            [self storeInteger: ExcessSignalsIndex
>>                        ofObject: sema
>>                        withValue: excessSignals - 1]
>>          ifFalse:
>>            inInterpreter := instructionPointer >= objectMemory
>> startOfMemory.
>>            activeProc := self activeProcess.
>>            self addLastLink: activeProc toList: sema.
>>            self transferTo: self wakeHighestPriority from: CSWait.
>>            self
>> forProcessPrimitiveReturnToExecutivePostContextSwitch: inInterpreter]
>>
>> which I guess could be added quickly if Esteban could compile the
>> latest pharo-spur-vm ;)
>>
>> cheers -ben
>>
>>
>> Won't work, there's no guarantee thread has actually ran and
>> signalConsumed been assigned the primitive result after Semaphore resumed
>> the waiting thread, before  a higher priority thread runs and terminates
>> it. (which is exactly the case handled by special code in #terminate)
>>
>> Cheers,
>> Henry
>>
>>
>
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