[ORC] Avoid Race in Assertions

[pree-jackie]

Summary

Prevents when two threads execute the assertions and two threads passed the check before any one is acquiring the SessionLock.

[dblaikie]

Does the original bug show up with TSan on any existing/checked in tests, or could TSan catch it in some new test that isn't checked in yet/should be checked in?

[pree-jackie]

I think it's more of an race condition than a data race. I will write a test case and check whether TSan can catch it. But the idea here is createJITDylib and getJITdylibByName must be executed by the same thread,

[lhames]

Tentatively LGTM. Was this is generating a TSan error, or a crash? I would expect this to pass TSan, so writing a test case is probably not practical.

llvm/lib/ExecutionEngine/Orc/Core.cpp

 
 JITDylib &ExecutionSession::createJITDylib(std::string Name,
                                            bool AddToMainDylibSearchOrder) {
-  assert(!getJITDylibByName(Name) && "JITDylib with that name already exists");
   return runSessionLocked([&, this]() -> JITDylib & {
+    assert(!getJITDylibByName(Name) &&
+           "JITDylib with that name already exists");
     JDs.push_back(
         std::unique_ptr<JITDylib>(new JITDylib(*this, std::move(Name))));
     if (AddToMainDylibSearchOrder)