#302 Deadlock in Persistent Streams

[Vidar Hasfjord]

After fixes in 6.40 for multi-threading issues, there is a deadlock in the persistent streams, more specifically in __TStreamableCont::CreateTypes, defined in "objstrm.cpp".

This issue was reported by Igor Epatko (wavesimsoft). See "object stream (objstrm) issue" [discussion:25092ed5] in the Open Discussion forum.

[Vidar Hasfjord]

Hi Igor, are you working on this bug?

There is a problem with your proposed fix:

:::C++
if (!Types)
{
  LOCKTYPES(false);
  Types = new TStreamableTypes;
}

This code sequence allows multiple threads to enter the if-clause at once, leading to multiple initialisation of Types. Naively, we may handle this issue using double-checking:

:::C++
if (!Types)
{
  LOCKTYPES(false);
  if (!Types)
    Types = new TStreamableTypes;
}

However, this may still not be 100% safe due to the way the compiler and CPU might rearrange instructions and memory accesses. See Double-checked locking at Wikipedia for an explanation of the issues.

That said, I think this solution is good enough. It is already unsafe to multi-thread OWLNext (see Multi-threading and OWLNext), and we advice against it, so the slight possibility of a problem here does not matter much.

[Vidar Hasfjord]

By the way, the existing preceding code is not thread-safe either:

:::C++
#if defined(BI_MULTI_THREAD_RTL)
  if(!Lock)
  {
    Lock = new TMRSWSection;
  }
#endif

Here, multiple threads may enter the if-clause at the same time. Also, the compiler may rearrange instructions so that Lock is assigned before completing the constructor, after which a secondary thread may skip the if-clause and proceed to use Lock pointing to an uninitialised (or partially initialised) object.