[Vm-dev] Why isn't signalSemaphoreWithIndex() thread-safe?

[Igor Stasenko]

2009/9/21 Joshua Gargus <schwa at fastmail.us>:
>
> Igor Stasenko wrote:
>>
>> 2009/9/21 Joshua Gargus <schwa at fastmail.us>:
>>
>>> Eliot Miranda wrote:
>>>
>>>> ------------------------------------------------------------------------
>>>>
>>>>
>>>>
>>>> On Sun, Sep 20, 2009 at 1:00 PM, Eliot Miranda
>>>> <eliot.miranda at gmail.com <mailto:eliot.miranda at gmail.com>> wrote:
>>>>
>>>>      Instead we can use three variables per index to implement an
>>>>     exclusion-free solution, thusly:
>>>>
>>>>     typedef struct { int lock; // accessed via e.g. XCHG to protect
>>>>     signalRequest
>>>>                            int requests;
>>>>                            int responses;
>>>>                } ExternalSignalRequest;
>>>>
>>>>     ExternalSignalRequest *externalSignalRequests;
>>>>     int checkExternalSignalRequests;
>>>>
>>>>     void
>>>>     requestSignal(int i)
>>>>     {
>>>>            while (!lock(&externalSignalRequests[i].lock))
>>>>                  usleep(1);
>>>>
>>>>            ++externalSignalRequests[i].requests;
>>>>
>>>>             unlock(&externalSignalRequests[i].lock);
>>>>
>>>>             checkExternalSignalRequests = 1;
>>>>             forceInterruptCheck();  // set a flag to cause the VM to
>>>>     check for interrupts; in the stack VM this is stackLimit :=
>>>>     (usqInt)-1;
>>>>     }
>>>>
>>>>
>>>>     The VM responds in checkEvents:
>>>>
>>>>         if (checkExternalSignalRequests)
>>>>             for (i = 0; i < numExternalSemaphores; i++)
>>>>                 while (externalSignalRequests[i]. responses
>>>>     != externalSignalRequests[i]. requests) {
>>>>                     signalSemaphoreWithIndex(i);
>>>>                     externalSignalRequests[i]. responses;
>>>>                 }
>>>>
>>>>
>>> Am I correct that ExternalSignalRequest.reponses is never reset to zero,
>>> instead eventually wrapping around?  That worried me for a minute, but I
>>> see that it doesn't matter.  Neeto.
>>>
>>>
>>
>> Nope, it does resets the counters.. see signalExternalSemaphores  method.
>>
>
> I was responding to Eliot's proposal, not the existing code.  If I
> understand properly, he's proposing to maintain a per-semaphore count of
> signal requests/responses, so there would be no
> "semaphoresToSignalCountA/B".
>
Ah.. sorry for not reading carefully..
In proposed scenario i don't like 1 (one) thing, that checkForInterrupts will
spend a linear amount of time to check what semaphores need to be signaled
instead of spending linear amount of time to signal semaphores which
is to be signaled:

     for (i = 0; i < numExternalSemaphores; i++)

means, that if you having 1000 external semaphores, and only 1 of them
signaled currently, you have to
loop through all of them, inevitably :(

> Cheers,
> Josh
>
>



-- 
Best regards,
Igor Stasenko AKA sig.