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

[Eliot Miranda]

On Sun, Sep 20, 2009 at 1:00 PM, Eliot Miranda <eliot.miranda at gmail.com>wrote:

> Hi Josh,
>
> On Sun, Sep 20, 2009 at 12:43 PM, Joshua Gargus <schwa at fastmail.us> wrote:
>
>>
>> Hi,
>>
>> I've always assumed that signalSemaphoreWithIndex() must be thread-safe;
>> after all, it's the "official" mechanism for notifying the image that an
>> asynchronous event has occurred.  However, a pang of paranoia prompted
>> me to actually look at the code, and it seems clearly unsafe.  This is
>> bad, because I've been using it to signal events from separate native
>> threads.
>>
>> What should we do about this?  It seems to me that it should be wrapped
>> in a critical section, using the appropriate platform-specific
>> synchronization primitives.
>>
>
> There's no need for this heavyweight approach because the VM can only
> respond to these signals synchronously.
>

On second reading I think what I wrote below is exactly what you implied.
 Sorry.



>  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;
>             }
>
>
>
>> Thanks,
>> Josh
>>
>
>
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