Index: include/llvm/ExecutionEngine/Orc/Core.h
===================================================================
--- include/llvm/ExecutionEngine/Orc/Core.h
+++ include/llvm/ExecutionEngine/Orc/Core.h
@@ -634,6 +634,15 @@
     return ES.runSessionLocked([&]() { return F(SearchOrder); });
   }
 
+  /// Visit related dylibs in search order. This locks a mutex, so SearchOrder
+  /// can't be modified during visitation. Session lock is NOT acquired.
+  template <typename Func>
+  auto visitInSearchOrder(Func &&F) const
+      -> decltype(F(std::declval<const JITDylibList &>())) {
+    std::lock_guard<std::mutex> Lock(SearchOrderMutex);
+    return F(SearchOrder);
+  }
+
   /// Define all symbols provided by the materialization unit to be part
   ///        of the given JITDylib.
   template <typename UniquePtrToMaterializationUnit>
@@ -753,6 +762,7 @@
   MaterializingInfosMap MaterializingInfos;
   FallbackDefinitionGeneratorFunction FallbackDefinitionGenerator;
   JITDylibList SearchOrder;
+  mutable std::mutex SearchOrderMutex;
 };
 
 /// An ExecutionSession represents a running JIT program.
Index: lib/ExecutionEngine/Orc/Core.cpp
===================================================================
--- lib/ExecutionEngine/Orc/Core.cpp
+++ lib/ExecutionEngine/Orc/Core.cpp
@@ -1239,14 +1239,17 @@
   if (SearchThisJITDylibFirst && NewSearchOrder.front() != this)
     NewSearchOrder.insert(NewSearchOrder.begin(), this);
 
+  std::lock_guard<std::mutex> Lock(SearchOrderMutex);
   ES.runSessionLocked([&]() { SearchOrder = std::move(NewSearchOrder); });
 }
 
 void JITDylib::addToSearchOrder(JITDylib &JD) {
+  std::lock_guard<std::mutex> Lock(SearchOrderMutex);
   ES.runSessionLocked([&]() { SearchOrder.push_back(&JD); });
 }
 
 void JITDylib::replaceInSearchOrder(JITDylib &OldJD, JITDylib &NewJD) {
+  std::lock_guard<std::mutex> Lock(SearchOrderMutex);
   ES.runSessionLocked([&]() {
     auto I = std::find(SearchOrder.begin(), SearchOrder.end(), &OldJD);
 
@@ -1256,6 +1259,7 @@
 }
 
 void JITDylib::removeFromSearchOrder(JITDylib &JD) {
+  std::lock_guard<std::mutex> Lock(SearchOrderMutex);
   ES.runSessionLocked([&]() {
     auto I = std::find(SearchOrder.begin(), SearchOrder.end(), &JD);
     if (I != SearchOrder.end())
Index: lib/ExecutionEngine/Orc/RTDyldObjectLinkingLayer.cpp
===================================================================
--- lib/ExecutionEngine/Orc/RTDyldObjectLinkingLayer.cpp
+++ lib/ExecutionEngine/Orc/RTDyldObjectLinkingLayer.cpp
@@ -30,7 +30,7 @@
     };
 
     auto InternedResult =
-        MR.getTargetJITDylib().withSearchOrderDo([&](const JITDylibList &JDs) {
+        MR.getTargetJITDylib().visitInSearchOrder([&](const JITDylibList &JDs) {
           return ES.lookup(JDs, InternedSymbols, RegisterDependencies, false);
         });