[libcxx-commits] [libcxx] 6e22b53 - [libc++] Fix `std::atomic::wait` ulock wait UL_COMPARE_AND_WAIT64 (#92783)

[via libcxx-commits]

Author: Hui
Date: 2024-06-01T21:12:04+01:00
New Revision: 6e22b538da4b09efb10a59582a3f43d8128ae7d1

URL: https://github.com/llvm/llvm-project/commit/6e22b538da4b09efb10a59582a3f43d8128ae7d1
DIFF: https://github.com/llvm/llvm-project/commit/6e22b538da4b09efb10a59582a3f43d8128ae7d1.diff

LOG: [libc++] Fix `std::atomic::wait` ulock wait UL_COMPARE_AND_WAIT64 (#92783)

in `atomic::wait`, when we call the platform wait ulock_wait , we are
using UL_COMPARE_AND_WAIT. But we should use UL_COMPARE_AND_WAIT64
instead as the address we are waiting for is a 64 bit integer.

fixes https://github.com/llvm/llvm-project/issues/85107

It is rather hard to test directly because in `atomic::wait`, before
calling into the platform wait, our c++ code has some poll logic which
checks the value not changing. Thus in this patch, the test is using the
internal function.

Added: 
    libcxx/test/libcxx/atomics/atomics.syn/wait.pass.cpp

Modified: 
    libcxx/src/atomic.cpp

Removed: 
    


################################################################################
diff  --git a/libcxx/src/atomic.cpp b/libcxx/src/atomic.cpp
index 44100d4414e0d..7142f38c96090 100644
--- a/libcxx/src/atomic.cpp
+++ b/libcxx/src/atomic.cpp
@@ -69,17 +69,20 @@ extern "C" int __ulock_wait(
     uint32_t operation, void* addr, uint64_t value, uint32_t timeout); /* timeout is specified in microseconds */
 extern "C" int __ulock_wake(uint32_t operation, void* addr, uint64_t wake_value);
 
-#  define UL_COMPARE_AND_WAIT 1
+// https://github.com/apple/darwin-xnu/blob/2ff845c2e033bd0ff64b5b6aa6063a1f8f65aa32/bsd/sys/ulock.h#L82
+#  define UL_COMPARE_AND_WAIT64 5
 #  define ULF_WAKE_ALL 0x00000100
 
 static void
 __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, __cxx_contention_t __val) {
-  __ulock_wait(UL_COMPARE_AND_WAIT, const_cast<__cxx_atomic_contention_t*>(__ptr), __val, 0);
+  static_assert(sizeof(__cxx_atomic_contention_t) == 8, "Waiting on 8 bytes value");
+  __ulock_wait(UL_COMPARE_AND_WAIT64, const_cast<__cxx_atomic_contention_t*>(__ptr), __val, 0);
 }
 
 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, bool __notify_one) {
+  static_assert(sizeof(__cxx_atomic_contention_t) == 8, "Waking up on 8 bytes value");
   __ulock_wake(
-      UL_COMPARE_AND_WAIT | (__notify_one ? 0 : ULF_WAKE_ALL), const_cast<__cxx_atomic_contention_t*>(__ptr), 0);
+      UL_COMPARE_AND_WAIT64 | (__notify_one ? 0 : ULF_WAKE_ALL), const_cast<__cxx_atomic_contention_t*>(__ptr), 0);
 }
 
 #elif defined(__FreeBSD__) && __SIZEOF_LONG__ == 8

diff  --git a/libcxx/test/libcxx/atomics/atomics.syn/wait.pass.cpp b/libcxx/test/libcxx/atomics/atomics.syn/wait.pass.cpp
new file mode 100644
index 0000000000000..f29dc6eb463fe
--- /dev/null
+++ b/libcxx/test/libcxx/atomics/atomics.syn/wait.pass.cpp
@@ -0,0 +1,32 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-threads
+// This bug was first fixed in LLVM 19
+// UNSUPPORTED: using-built-library-before-llvm-19
+// XFAIL: availability-synchronization_library-missing
+
+#include <atomic>
+#include <cassert>
+
+void test_85107() {
+  if constexpr (sizeof(std::__cxx_contention_t) == 8 && sizeof(long) > 4) {
+    // https://github.com/llvm/llvm-project/issues/85107
+    // [libc++] atomic_wait uses UL_COMPARE_AND_WAIT when it should use UL_COMPARE_AND_WAIT64 on Darwin
+    constexpr std::__cxx_contention_t old_val = 0;
+    constexpr std::__cxx_contention_t new_val = old_val + (1ll << 32);
+    std::__cxx_atomic_contention_t ct(new_val);
+    std::__libcpp_atomic_wait(&ct, old_val); // this will hang forever if the bug is present
+  }
+}
+
+int main(int, char**) {
+  test_85107();
+
+  return 0;
+}