[Live-devel] Possible issue in DelayQueue

[Hansen Jan Rørgård]

Hi,

In the liveMedia FAQ section: "Is this code 'thread safe'? I.e., can it be accessed by more than one thread at the same time?", it is stated that:
"...Another possible way to access the code from multiple threads is to have each thread use its own "UsageEnvironment" and "TaskScheduler" objects, and thus its own event loop."

I know that it is not recommended to use this approach, but I am working with an application that has been implemented that way and it has been working fine. However, I have stumbled upon an issue when investigating a crash - see part of the back trace below.

I suspect that the:
class DelayQueueEntry {
...
private:
...
  static intptr_t tokenCounter;    <-----
};
might cause the crash due to something resembling the following sequence:

Thread1->...->new DelayQueueEntry(...)
Thread1-> DelayQueueEntry - read tokenCounter = 0
Thread1-> DelayQueueEntry - increment tokenCounter (1)
--- thread switch
Thread2->...-> new DelayQueueEntry(...)
Thread2-> DelayQueueEntry - read tokenCounter = 0
Thread2-> DelayQueueEntry - increment tokenCounter (1)
Thread2-> DelayQueueEntry - write tokenCounter = 1
Thread2->...-> new DelayQueueEntry(...)
Thread2-> DelayQueueEntry - read tokenCounter = 1
Thread2-> DelayQueueEntry - increment tokenCounter (2)
Thread2-> DelayQueueEntry - write tokenCounter = 2
--- thread switch
Thread1-> DelayQueueEntry - write tokenCounter = 1
--- thread switch
Thread2->...-> new DelayQueueEntry(...)
Thread2-> DelayQueueEntry - read tokenCounter = 1
Thread2-> DelayQueueEntry - increment tokenCounter (2)
Thread2-> DelayQueueEntry - write tokenCounter = 2

Such a sequence would result in reuse of a token within a single thread, which I suspect might be an issue that could lead to a crash.
Do you also see an issue here?

#0  0x00007ffdbc34b8c0 in ucrtbase!exit () ...
...
#6  0x00007ffdc4b3921d in ntdll!.chkstk () from C:\Windows\SYSTEM32\ntdll.dll
#7  0x00007ffdc4af5b5b in ntdll!RtlAppendUnicodeToString () from C:\Windows\SYSTEM32\ntdll.dll
#8  0x00007ffdc4b383de in ntdll!KiUserExceptionDispatcher () from C:\Windows\SYSTEM32\ntdll.dll
#9  0x0000000028e9f89c in DelayQueue::addEntry (this=0x2fc03fa8, newEntry=0x3b0097f0) at DelayQueue.cpp:139
...

Best Regards

Jan Rørgaard Hansen
Lead Software Developer
Communication Solutions
Business Area Surveillance

Tel.:  + 45 3638 3000
Call me on: Skype<skype:jan.r.hansen at dk.saabgroup.com?call>
E-mail: jan.r.hansen at dk.saabgroup.com<mailto:jan.r.hansen at dk.saabgroup.com>


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