From bae6a2a76b632fa5fa4f103bb7aaff067c19c949 Mon Sep 17 00:00:00 2001
From: Wenju He <wenju.he@intel.com>
Date: Sat, 21 Mar 2026 07:33:21 +0800
Subject: [PATCH] [Runtimes] Fix Unix Makefiles race between runtimes-build and
 EXTRA_TARGETS (#187634)

In our downstream we have a non-runtime target depending on libclc
EXTRA_TARGET and then observe a race condition in parallel build: both
runtimes-build (full build, no lock) and libclc EXTRA_TARGET (triggered
by non-runtime target, FileLock) build concurrently, leading to corrupt
libclc library.

This exposes an limitation in ExternalProject EXTRA_TARGET design:
EXTRA_TARGETS in llvm_ExternalProject_Add only depend on
${name}-configure, not ${name}-build. This makes EXTRA_TARGETS unsafe as
dependencies of a non-runtime target..

Fix: Add a locked BUILD_COMMAND to ExternalProject_Add for Unix
Makefiles generator, using the same cmake.lock as EXTRA_TARGETS. This
serializes runtimes-build with all EXTRA_TARGETS under one lock.

With this PR, a non-runtime target can depend on a specific
EXTRA_TARGET, rather than needing to depend on the umbrella runtimes
target. This is an improvement. A non-runtime target can start as soon
as its dependent specific EXTRA_TARGET build finishes, without waiting
for other runtimes. In addition, the non-runtime target may only need a
specific EXTRA_TARGET and minimal dependency on it is accurate.

Assisted-by: Claude Sonnet 4.6

---------

Co-authored-by: Petr Hosek <phosek@google.com>
---
 .../cmake/modules/LLVMExternalProjectUtils.cmake | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/llvm/cmake/modules/LLVMExternalProjectUtils.cmake b/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
index 975798a8c263..61f3bc83a83a 100644
--- a/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
+++ b/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
@@ -9,7 +9,12 @@ function(llvm_ExternalProject_BuildCmd out_var target bin_dir stamp_dir)
   endif()
   if (CMAKE_GENERATOR MATCHES "Make")
     # Use special command for Makefiles to support parallelism.
-    string(JOIN "@" make_cmd "$(MAKE)" "-C" "${bin_dir}" "${target}")
+    # Handle empty target (full build, no target argument).
+    if(target)
+      string(JOIN "@" make_cmd "$(MAKE)" "-C" "${bin_dir}" "${target}")
+    else()
+      string(JOIN "@" make_cmd "$(MAKE)" "-C" "${bin_dir}")
+    endif()
     set(file_lock_script "${LLVM_CMAKE_DIR}/FileLock.cmake")
     set(${out_var} ${CMAKE_COMMAND} "-DLOCK_FILE_PATH=${stamp_dir}/cmake.lock"
                                     "-DCOMMAND=${make_cmd}"
@@ -378,6 +383,14 @@ function(llvm_ExternalProject_Add name source_dir)
     set(verbose -DCMAKE_VERBOSE_MAKEFILE=ON)
   endif()
 
+  if(CMAKE_GENERATOR MATCHES "Make")
+    # Use the same FileLock for Unix Makefiles to serialize the main build with
+    # EXTRA_TARGETS. This prevents concurrent make invocations from corrupting
+    # shared artifacts in BINARY_DIR.
+    llvm_ExternalProject_BuildCmd(build_cmd "" ${BINARY_DIR} ${STAMP_DIR})
+    set(build_command_arg BUILD_COMMAND ${build_cmd})
+  endif()
+
   ExternalProject_Add(${name}
     DEPENDS ${ARG_DEPENDS} llvm-config
     ${name}-clobber
@@ -408,6 +421,7 @@ function(llvm_ExternalProject_Add name source_dir)
                ${cmake_args}
                ${PASSTHROUGH_VARIABLES}
     CMAKE_CACHE_DEFAULT_ARGS ${CMAKE_CACHE_DEFAULT_ARGS}
+    ${build_command_arg}
     INSTALL_COMMAND ""
     STEP_TARGETS configure build
     BUILD_ALWAYS 1