Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -10839,6 +10839,10 @@
   "redefinition of module '%0'">;
 def note_prev_module_definition : Note<"previously defined here">;
 def note_prev_module_definition_from_ast_file : Note<"module loaded from '%0'">;
+def err_module_langugae_linkage_no_global : Error <
+  "The declaration %0 appears within a linkage-specification should be "
+  "attached to global module. But the compiler failed to search the global "
+  "module.">;
 def err_module_not_defined : Error<
   "definition of module '%0' is not available; use -fmodule-file= to specify "
   "path to precompiled module interface">;
Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -2188,6 +2188,7 @@
   };
   /// The modules we're currently parsing.
   llvm::SmallVector<ModuleScope, 16> ModuleScopes;
+  Module *GlobalModule = nullptr;
 
   /// Namespace definitions that we will export when they finish.
   llvm::SmallPtrSet<const NamespaceDecl*, 8> DeferredExportedNamespaces;
@@ -2197,6 +2198,8 @@
     return ModuleScopes.empty() ? nullptr : ModuleScopes.back().Module;
   }
 
+  Module *getGlobalModule() const { return GlobalModule; }
+
   VisibleModuleSet VisibleModules;
 
 public:
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -9339,6 +9339,23 @@
       }
     }
 
+    // C++ [module.unit]p7.2
+    // Otherwise, if the declaration
+    //  - ...
+    //  - ...
+    //  - appears within a linkage-specification
+    // it is attached to the global module.
+    if (Module *M = NewFD->getOwningModule())
+      if (M->Kind == Module::ModuleInterfaceUnit &&
+          (NewFD->isExternCContext() || NewFD->isExternCXXContext())) {
+        if (!getGlobalModule())
+          Diag(NewFD->getLocation(),
+               diag::err_module_langugae_linkage_no_global)
+              << NewFD->getName();
+        NewFD->setLocalOwningModule(getGlobalModule());
+        NewFD->setModuleOwnershipKind(Decl::ModuleOwnershipKind::Unowned);
+      }
+
     if (isFriend) {
       if (FunctionTemplate) {
         FunctionTemplate->setObjectOfFriendDecl();
@@ -16407,6 +16424,22 @@
       New->setModulePrivate();
   }
 
+  // C++ [module.unit]p7.2
+  // Otherwise, if the declaration
+  //  - ...
+  //  - ...
+  //  - appears within a linkage-specification
+  // it is attached to the global module.
+  if (Module *M = New->getOwningModule())
+    if (M->Kind == Module::ModuleInterfaceUnit &&
+        (New->isExternCContext() || New->isExternCXXContext())) {
+      if (!getGlobalModule())
+        Diag(New->getLocation(), diag::err_module_langugae_linkage_no_global)
+            << New->getName();
+      New->setLocalOwningModule(getGlobalModule());
+      New->setModuleOwnershipKind(Decl::ModuleOwnershipKind::Unowned);
+    }
+
   // If this is a specialization of a member class (of a class template),
   // check the specialization.
   if (isMemberSpecialization && CheckMemberSpecialization(New, Previous))
Index: clang/lib/Sema/SemaModule.cpp
===================================================================
--- clang/lib/Sema/SemaModule.cpp
+++ clang/lib/Sema/SemaModule.cpp
@@ -66,10 +66,13 @@
     return nullptr;
   }
 
+  assert(
+      !GlobalModule &&
+      "Global module already created but didn't covered by the check above.");
   // We start in the global module; all those declarations are implicitly
   // module-private (though they do not have module linkage).
   auto &Map = PP.getHeaderSearchInfo().getModuleMap();
-  auto *GlobalModule = Map.createGlobalModuleFragmentForModuleUnit(ModuleLoc);
+  GlobalModule = Map.createGlobalModuleFragmentForModuleUnit(ModuleLoc);
   assert(GlobalModule && "module creation should not fail");
 
   // Enter the scope of the global module.
Index: clang/test/CXX/module/module.linkage_specification/Inputs/h1.h
===================================================================
--- /dev/null
+++ clang/test/CXX/module/module.linkage_specification/Inputs/h1.h
@@ -0,0 +1,6 @@
+extern "C" void foo();
+extern "C" {
+    void bar();
+    int baz();
+    double double_func();
+}
\ No newline at end of file
Index: clang/test/CXX/module/module.linkage_specification/Inputs/h2.h
===================================================================
--- /dev/null
+++ clang/test/CXX/module/module.linkage_specification/Inputs/h2.h
@@ -0,0 +1 @@
+extern "C++" class CPP;
\ No newline at end of file
Index: clang/test/CXX/module/module.linkage_specification/Inputs/h4.h
===================================================================
--- /dev/null
+++ clang/test/CXX/module/module.linkage_specification/Inputs/h4.h
@@ -0,0 +1 @@
+extern "C" struct C;
\ No newline at end of file
Index: clang/test/CXX/module/module.linkage_specification/p1.cpp
===================================================================
--- /dev/null
+++ clang/test/CXX/module/module.linkage_specification/p1.cpp
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -std=c++2a %s -verify
+// expected-no-diagnostics
+module;
+
+#include "Inputs/h1.h"
+
+export module x;
+
+extern "C" void foo() {
+  return;
+}
Index: clang/test/CXX/module/module.linkage_specification/p2.cpp
===================================================================
--- /dev/null
+++ clang/test/CXX/module/module.linkage_specification/p2.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -std=c++2a %s -verify
+// expected-no-diagnostics
+module;
+
+#include "Inputs/h2.h"
+
+export module x;
+
+extern "C++" class CPP {};
Index: clang/test/CXX/module/module.linkage_specification/p3.cpp
===================================================================
--- /dev/null
+++ clang/test/CXX/module/module.linkage_specification/p3.cpp
@@ -0,0 +1,5 @@
+// RUN: %clang_cc1 -std=c++2a %s -verify
+export module x;
+
+extern "C" void foo();     // expected-error {{The declaration foo appears within a linkage-specification should be attached to global module. But the compiler failed to search the global module.}}
+extern "C++" class CPP {}; // expected-error {{The declaration CPP appears within a linkage-specification should be attached to global module. But the compiler failed to search the global module.}}
Index: clang/test/CXX/module/module.linkage_specification/p4.cpp
===================================================================
--- /dev/null
+++ clang/test/CXX/module/module.linkage_specification/p4.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -std=c++2a %s -verify
+// expected-no-diagnostics
+module;
+
+#include "Inputs/h4.h"
+
+export module x;
+
+extern "C" struct C {
+  int a;
+  int b;
+  double d;
+};
\ No newline at end of file
Index: clang/test/CodeGenCXX/Inputs/module-extern-C.h
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/Inputs/module-extern-C.h
@@ -0,0 +1 @@
+extern "C" void foo();
\ No newline at end of file
Index: clang/test/CodeGenCXX/module-extern-C.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/module-extern-C.cpp
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -std=c++2a -emit-llvm -triple %itanium_abi_triple -o - %s | FileCheck %s
+
+module;
+
+#include "Inputs/module-extern-C.h"
+
+export module x;
+
+// CHECK: define dso_local void @foo()
+extern "C" void foo() {
+  return;
+}
+
+extern "C" {
+    // CHECK: define dso_local void @bar()
+    void bar() {
+        return;
+    }
+    // CHECK: define dso_local i32 @baz()
+    int baz() {
+        return 3;
+    }
+    // CHECK: define dso_local double @double_func()
+    double double_func() {
+        return 5.0;
+    }
+}
\ No newline at end of file