[ORC] Lock ThreadSafeContext during Module destructing in ThreadSafeModule.

Failure to lock the context can lead to data races if other threads are
operating on other ThreadSafeModules that share the same context.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343261 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ExecutionEngine/Orc/ThreadSafeModule.h

@@ -102,15 +102,29 @@ public:
   /// unique_ptr<LLVMContext>. This creates a new ThreadSafeContext from the
   /// given context.
   ThreadSafeModule(std::unique_ptr<Module> M, std::unique_ptr<LLVMContext> Ctx)
-      : TSCtx(std::move(Ctx)), M(std::move(M)) {}
+      : M(std::move(M)), TSCtx(std::move(Ctx)) {}
 
+  /// Construct a ThreadSafeModule from a unique_ptr<Module> and an
+  /// existing ThreadSafeContext.
   ThreadSafeModule(std::unique_ptr<Module> M, ThreadSafeContext TSCtx)
-      : TSCtx(std::move(TSCtx)), M(std::move(M)) {}
+      : M(std::move(M)), TSCtx(std::move(TSCtx)) {}
 
+  ~ThreadSafeModule() {
+    // We need to lock the context while we destruct the module.
+    if (M) {
+      auto L = getContextLock();
+      M = nullptr;
+    }
+  }
+
+  /// Get the module wrapped by this ThreadSafeModule.
   Module* getModule() { return M.get(); }
 
+  /// Take out a lock on the ThreadSafeContext for this module.
   ThreadSafeContext::Lock getContextLock() { return TSCtx.getLock(); }
 
+  /// Boolean conversion: This ThreadSafeModule will evaluate to true if it
+  /// wraps a non-null module.
   explicit operator bool() {
     if (M) {
       assert(TSCtx.getContext() && "Non-null module must have non-null context");
@@ -120,8 +134,8 @@ public:
   }
 
 private:
-  ThreadSafeContext TSCtx;
   std::unique_ptr<Module> M;
+  ThreadSafeContext TSCtx;
 };
 
 using GVPredicate = std::function<bool(const GlobalValue&)>;