diff --git a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
index 4e9a9b66a63a7..ad675c4358ad4 100644
--- a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
+++ b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
@@ -116,6 +116,10 @@ function(_get_compile_options_from_config output_var)
     list(APPEND config_options "-DLIBC_THREAD_MODE=${LIBC_CONF_THREAD_MODE}")
   endif()
 
+  if(LIBC_CONF_RAW_MUTEX_DEFAULT_SPIN_COUNT)
+    list(APPEND config_options "-DLIBC_COPT_RAW_MUTEX_DEFAULT_SPIN_COUNT=${LIBC_CONF_RAW_MUTEX_DEFAULT_SPIN_COUNT}")
+  endif()
+
   set(${output_var} ${config_options} PARENT_SCOPE)
 endfunction(_get_compile_options_from_config)
 
diff --git a/libc/hdr/errno_macros.h b/libc/hdr/errno_macros.h
index 27ea49977d8c8..e2913f45a8ad9 100644
--- a/libc/hdr/errno_macros.h
+++ b/libc/hdr/errno_macros.h
@@ -15,7 +15,9 @@
 #include <linux/errno.h>
 
 #include "include/llvm-libc-macros/error-number-macros.h"
-#else // __linux__
+#elif defined(__APPLE__)
+#include <sys/errno.h>
+#else // __APPLE__
 #include "include/llvm-libc-macros/generic-error-number-macros.h"
 #endif
 
diff --git a/libc/include/errno.h.def b/libc/include/errno.h.def
index aa1f6c9e48444..0ac8e57f7a158 100644
--- a/libc/include/errno.h.def
+++ b/libc/include/errno.h.def
@@ -21,8 +21,14 @@
 
 #include "llvm-libc-macros/linux/error-number-macros.h"
 
-#else // __linux__
+#elif defined(__APPLE__)
+
+#include <sys/errno.h>
+
+#else // __APPLE__
+
 #include "llvm-libc-macros/generic-error-number-macros.h"
+
 #endif
 
 __BEGIN_C_DECLS
diff --git a/libc/src/__support/threads/CMakeLists.txt b/libc/src/__support/threads/CMakeLists.txt
index f8a44937721b4..dc1445634da2b 100644
--- a/libc/src/__support/threads/CMakeLists.txt
+++ b/libc/src/__support/threads/CMakeLists.txt
@@ -23,31 +23,54 @@ if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
   add_subdirectory(${LIBC_TARGET_OS})
 endif()
 
-if(TARGET libc.src.__support.threads.${LIBC_TARGET_OS}.mutex)
+if(TARGET libc.src.__support.threads.${LIBC_TARGET_OS}.futex_utils)
+  add_header_library(
+    raw_mutex
+    HDRS
+    raw_mutex.h
+    COMPILE_OPTIONS
+    ${monotonicity_flags}
+    DEPENDS
+    .${LIBC_TARGET_OS}.futex_utils
+    libc.src.__support.threads.sleep
+    libc.src.__support.time.abs_timeout
+    libc.src.__support.time.monotonicity
+    libc.src.__support.CPP.optional
+    libc.hdr.types.pid_t
+  )
+
+  add_header_library(
+    unix_mutex
+    HDRS
+    unix_mutex.h
+    DEPENDS
+    .raw_mutex
+  )
+
   add_header_library(
     mutex
     HDRS
-      mutex.h
+    mutex.h
     DEPENDS
-      .${LIBC_TARGET_OS}.mutex
+    .unix_mutex
   )
 
   add_object_library(
     fork_callbacks
     SRCS
-      fork_callbacks.cpp
+    fork_callbacks.cpp
     HDRS
-      fork_callbacks.h
+    fork_callbacks.h
     DEPENDS
-      .mutex
-      libc.src.__support.CPP.mutex
+    .mutex
+    libc.src.__support.CPP.mutex
   )
 elseif(NOT (LIBC_CONF_THREAD_MODE STREQUAL LIBC_THREAD_MODE_PLATFORM))
   add_header_library(
     mutex
-  HDRS
+    HDRS
     mutex.h
-  DEPENDS
+    DEPENDS
     .mutex_common
   )
 endif()
diff --git a/libc/src/__support/threads/darwin/CMakeLists.txt b/libc/src/__support/threads/darwin/CMakeLists.txt
new file mode 100644
index 0000000000000..9c651d8c3b0f5
--- /dev/null
+++ b/libc/src/__support/threads/darwin/CMakeLists.txt
@@ -0,0 +1,16 @@
+if(NOT TARGET libc.src.__support.OSUtil.osutil)
+  return()
+endif()
+
+add_header_library(
+  futex_utils
+  HDRS
+    futex_utils.h
+  DEPENDS
+    libc.include.sys_syscall
+    libc.src.__support.OSUtil.osutil
+    libc.src.__support.CPP.atomic
+    libc.src.__support.CPP.limits
+    libc.src.__support.CPP.optional
+    libc.src.__support.threads.mutex_common
+)
diff --git a/libc/src/__support/threads/darwin/futex_utils.h b/libc/src/__support/threads/darwin/futex_utils.h
new file mode 100644
index 0000000000000..f880d09f621fc
--- /dev/null
+++ b/libc/src/__support/threads/darwin/futex_utils.h
@@ -0,0 +1,77 @@
+//===--- Futex utils for Darwin ----------------------------------*- C++-*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_DARWIN_FUTEX_UTILS_H
+#define LLVM_LIBC_SRC___SUPPORT_THREADS_DARWIN_FUTEX_UTILS_H
+
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/CPP/optional.h"
+#include "src/__support/time/abs_timeout.h"
+#include "src/__support/time/clock_conversion.h"
+
+#include <os/os_sync_wait_on_address.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+using FutexWordType = uint32_t;
+
+struct Futex : public cpp::Atomic<FutexWordType> {
+  using cpp::Atomic<FutexWordType>::Atomic;
+  using Timeout = internal::AbsTimeout;
+
+  LIBC_INLINE long wait(FutexWordType val, cpp::optional<Timeout> timeout,
+                        bool /* is_shared */) {
+    // TODO(bojle): consider using OS_SYNC_WAIT_ON_ADDRESS_SHARED to sync
+    // betweeen processes. Catch: it is recommended to only be used by shared
+    // processes, not threads of a same process.
+
+    for (;;) {
+      if (this->load(cpp::MemoryOrder::RELAXED) != val)
+        return 0;
+      long ret = 0;
+      if (timeout) {
+        // Assuming, OS_CLOCK_MACH_ABSOLUTE_TIME is equivalent to CLOCK_REALTIME
+        uint64_t tnsec = timeout->get_timespec().tv_sec * 1000000000 +
+                         timeout->get_timespec().tv_nsec;
+        ret = os_sync_wait_on_address_with_timeout(
+            reinterpret_cast<void *>(this), static_cast<uint64_t>(val),
+            sizeof(FutexWordType), OS_SYNC_WAIT_ON_ADDRESS_NONE,
+            OS_CLOCK_MACH_ABSOLUTE_TIME, tnsec);
+      } else {
+        ret = os_sync_wait_on_address(
+            reinterpret_cast<void *>(this), static_cast<uint64_t>(val),
+            sizeof(FutexWordType), OS_SYNC_WAIT_ON_ADDRESS_NONE);
+      }
+      if ((ret < 0) && (errno == ETIMEDOUT))
+        return -ETIMEDOUT;
+      // case when os_sync returns early with an error. retry.
+      if ((ret < 0) && ((errno == EINTR) || (errno == EFAULT))) {
+        continue;
+      }
+      return ret;
+    }
+  }
+
+  LIBC_INLINE long notify_one(bool /* is_shared */) {
+    // TODO(bojle): deal with is_shared
+    return os_sync_wake_by_address_any(reinterpret_cast<void *>(this),
+                                       sizeof(FutexWordType),
+                                       OS_SYNC_WAKE_BY_ADDRESS_NONE);
+  }
+
+  LIBC_INLINE long notify_all(bool /* is_shared */) {
+    // TODO(bojle): deal with is_shared
+    return os_sync_wake_by_address_all(reinterpret_cast<void *>(this),
+                                       sizeof(FutexWordType),
+                                       OS_SYNC_WAKE_BY_ADDRESS_NONE);
+  }
+};
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_THREADS_DARWIN_FUTEX_UTILS_H
diff --git a/libc/src/__support/threads/linux/CMakeLists.txt b/libc/src/__support/threads/linux/CMakeLists.txt
index 39d2c6fef0fed..cc596d217d7d2 100644
--- a/libc/src/__support/threads/linux/CMakeLists.txt
+++ b/libc/src/__support/threads/linux/CMakeLists.txt
@@ -21,7 +21,7 @@ add_header_library(
     libc.src.__support.CPP.atomic
     libc.src.__support.CPP.limits
     libc.src.__support.CPP.optional
-    libc.src.__support.time.linux.abs_timeout
+    libc.src.__support.time.abs_timeout
 )
 
 set(monotonicity_flags)
@@ -38,8 +38,8 @@ add_header_library(
   DEPENDS
     .futex_utils
     libc.src.__support.threads.sleep
-    libc.src.__support.time.linux.abs_timeout
-    libc.src.__support.time.linux.monotonicity
+    libc.src.__support.time.abs_timeout
+    libc.src.__support.time.monotonicity
     libc.src.__support.CPP.optional
     libc.hdr.types.pid_t
   COMPILE_OPTIONS
diff --git a/libc/src/__support/threads/linux/futex_utils.h b/libc/src/__support/threads/linux/futex_utils.h
index 943a99ab38c8c..e1cfa36566555 100644
--- a/libc/src/__support/threads/linux/futex_utils.h
+++ b/libc/src/__support/threads/linux/futex_utils.h
@@ -16,7 +16,7 @@
 #include "src/__support/macros/attributes.h"
 #include "src/__support/macros/config.h"
 #include "src/__support/threads/linux/futex_word.h"
-#include "src/__support/time/linux/abs_timeout.h"
+#include "src/__support/time/abs_timeout.h"
 #include <linux/errno.h>
 #include <linux/futex.h>
 
diff --git a/libc/src/__support/threads/linux/rwlock.h b/libc/src/__support/threads/linux/rwlock.h
index f7aeb5b709aa3..165e17239bbd5 100644
--- a/libc/src/__support/threads/linux/rwlock.h
+++ b/libc/src/__support/threads/linux/rwlock.h
@@ -35,7 +35,7 @@
 #endif
 
 #if LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY
-#include "src/__support/time/linux/monotonicity.h"
+#include "src/__support/time/monotonicity.h"
 #endif
 
 namespace LIBC_NAMESPACE_DECL {
diff --git a/libc/src/__support/threads/mutex.h b/libc/src/__support/threads/mutex.h
index f64f7e7b40082..25feea891e429 100644
--- a/libc/src/__support/threads/mutex.h
+++ b/libc/src/__support/threads/mutex.h
@@ -40,9 +40,9 @@
 // few global locks. So, to avoid static initialization order fiasco, we
 // want the constructors of the Mutex classes to be constexprs.
 
-#if defined(__linux__)
-#include "src/__support/threads/linux/mutex.h"
-#endif // __linux__
+#if defined(__linux__) || defined(__APPLE__)
+#include "src/__support/threads/unix_mutex.h"
+#endif
 
 #elif LIBC_THREAD_MODE == LIBC_THREAD_MODE_SINGLE
 
diff --git a/libc/src/__support/threads/linux/raw_mutex.h b/libc/src/__support/threads/raw_mutex.h
similarity index 87%
rename from libc/src/__support/threads/linux/raw_mutex.h
rename to libc/src/__support/threads/raw_mutex.h
index 47f0aa70f1c46..c8f35a5785fd7 100644
--- a/libc/src/__support/threads/linux/raw_mutex.h
+++ b/libc/src/__support/threads/raw_mutex.h
@@ -1,12 +1,12 @@
-//===--- Implementation of a Linux RawMutex class ---------------*- C++ -*-===//
+//===--- Implementation of the RawMutex class ---------------*- C++ -*-===//
 //
 // 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
 //
-//===----------------------------------------------------------------------===//
-#ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_RAW_MUTEX_H
-#define LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_RAW_MUTEX_H
+//===------------------------------------------------------------------===//
+#ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_RAW_MUTEX_H
+#define LLVM_LIBC_SRC___SUPPORT_THREADS_RAW_MUTEX_H
 
 #include "src/__support/CPP/optional.h"
 #include "src/__support/common.h"
@@ -14,17 +14,25 @@
 #include "src/__support/macros/attributes.h"
 #include "src/__support/macros/config.h"
 #include "src/__support/macros/optimization.h"
-#include "src/__support/threads/linux/futex_utils.h"
-#include "src/__support/threads/linux/futex_word.h"
 #include "src/__support/threads/sleep.h"
-#include "src/__support/time/linux/abs_timeout.h"
+#include "src/__support/time/abs_timeout.h"
+#include "sys/errno.h"
+
+#include <stdio.h>
+
+#if defined(__linux__)
+#include "src/__support/threads/linux/futex_utils.h"
+#elif defined(__APPLE__)
+#include "src/__support/threads/darwin/futex_utils.h"
+#endif
 
 #ifndef LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY
 #define LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY 1
 #endif
 
+// TODO(bojle): check this for darwin impl
 #if LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY
-#include "src/__support/time/linux/monotonicity.h"
+#include "src/__support/time/monotonicity.h"
 #endif
 
 #ifndef LIBC_COPT_RAW_MUTEX_DEFAULT_SPIN_COUNT
@@ -93,7 +101,9 @@ class RawMutex {
   LIBC_INLINE void wake(bool is_pshared) { futex.notify_one(is_pshared); }
 
 public:
-  LIBC_INLINE static void init(RawMutex *mutex) { mutex->futex = UNLOCKED; }
+  LIBC_INLINE static void init(RawMutex *mutex) {
+    mutex->futex.store(UNLOCKED);
+  }
   LIBC_INLINE constexpr RawMutex() : futex(UNLOCKED) {}
   [[nodiscard]] LIBC_INLINE bool try_lock() {
     FutexWordType expected = UNLOCKED;
@@ -122,7 +132,7 @@ class RawMutex {
     LIBC_ASSERT(lock->futex == UNLOCKED && "Mutex destroyed while used.");
   }
   LIBC_INLINE Futex &get_raw_futex() { return futex; }
-  LIBC_INLINE void reset() { futex = UNLOCKED; }
+  LIBC_INLINE void reset() { futex.store(UNLOCKED); }
 };
 } // namespace LIBC_NAMESPACE_DECL
 
diff --git a/libc/src/__support/threads/linux/mutex.h b/libc/src/__support/threads/unix_mutex.h
similarity index 90%
rename from libc/src/__support/threads/linux/mutex.h
rename to libc/src/__support/threads/unix_mutex.h
index 0c4b1ae09af6f..31acd582d4c6c 100644
--- a/libc/src/__support/threads/linux/mutex.h
+++ b/libc/src/__support/threads/unix_mutex.h
@@ -1,4 +1,4 @@
-//===--- Implementation of a Linux mutex class ------------------*- C++ -*-===//
+//===--- Implementation of a Unix mutex class ------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -6,16 +6,15 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_MUTEX_H
-#define LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_MUTEX_H
+#ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_UNIX_MUTEX_H
+#define LLVM_LIBC_SRC___SUPPORT_THREADS_UNIX_MUTEX_H
 
 #include "hdr/types/pid_t.h"
 #include "src/__support/CPP/optional.h"
 #include "src/__support/libc_assert.h"
 #include "src/__support/macros/config.h"
-#include "src/__support/threads/linux/futex_utils.h"
-#include "src/__support/threads/linux/raw_mutex.h"
 #include "src/__support/threads/mutex_common.h"
+#include "src/__support/threads/raw_mutex.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
diff --git a/libc/src/__support/time/CMakeLists.txt b/libc/src/__support/time/CMakeLists.txt
index 3851037e4161f..b90f977b00fe5 100644
--- a/libc/src/__support/time/CMakeLists.txt
+++ b/libc/src/__support/time/CMakeLists.txt
@@ -28,3 +28,32 @@ if(TARGET libc.src.__support.time.${LIBC_TARGET_OS}.clock_settime)
       libc.src.__support.time.${LIBC_TARGET_OS}.clock_settime
   )
 endif()
+
+add_header_library(
+  abs_timeout
+  HDRS
+    abs_timeout.h
+  DEPENDS
+    libc.hdr.types.struct_timespec
+    libc.src.__support.time.units
+    libc.src.__support.CPP.expected
+)
+
+add_header_library(
+  clock_conversion
+  HDRS
+    clock_conversion.h
+  DEPENDS
+    .clock_gettime
+    libc.src.__support.time.units
+)
+
+add_header_library(
+  monotonicity
+  HDRS
+    monotonicity.h
+  DEPENDS
+    .clock_conversion
+    .abs_timeout
+    libc.hdr.time_macros
+)
diff --git a/libc/src/__support/time/linux/abs_timeout.h b/libc/src/__support/time/abs_timeout.h
similarity index 85%
rename from libc/src/__support/time/linux/abs_timeout.h
rename to libc/src/__support/time/abs_timeout.h
index 37e602672208f..23ad52f530545 100644
--- a/libc/src/__support/time/linux/abs_timeout.h
+++ b/libc/src/__support/time/abs_timeout.h
@@ -1,13 +1,13 @@
-//===--- Linux absolute timeout ---------------------------------*- C++ -*-===//
+//===--- absolute timeout ---------------------------------*- C++ -*-===//
 //
 // 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
 //
-//===----------------------------------------------------------------------===//
+//===----------------------------------------------------------------===//
 
-#ifndef LLVM_LIBC_SRC___SUPPORT_TIME_LINUX_ABS_TIMEOUT_H
-#define LLVM_LIBC_SRC___SUPPORT_TIME_LINUX_ABS_TIMEOUT_H
+#ifndef LLVM_LIBC_SRC___SUPPORT_TIME_ABS_TIMEOUT_H
+#define LLVM_LIBC_SRC___SUPPORT_TIME_ABS_TIMEOUT_H
 
 #include "hdr/time_macros.h"
 #include "hdr/types/struct_timespec.h"
@@ -47,4 +47,4 @@ class AbsTimeout {
 } // namespace internal
 } // namespace LIBC_NAMESPACE_DECL
 
-#endif // LLVM_LIBC_SRC___SUPPORT_TIME_LINUX_ABS_TIMEOUT_H
+#endif // LLVM_LIBC_SRC___SUPPORT_TIME_ABS_TIMEOUT_H
diff --git a/libc/src/__support/time/linux/CMakeLists.txt b/libc/src/__support/time/linux/CMakeLists.txt
index 478529502b403..a7bdb7b36be49 100644
--- a/libc/src/__support/time/linux/CMakeLists.txt
+++ b/libc/src/__support/time/linux/CMakeLists.txt
@@ -28,32 +28,3 @@ add_object_library(
     libc.src.__support.error_or
     libc.src.__support.OSUtil.osutil
 )
-
-add_header_library(
-  clock_conversion
-  HDRS
-    clock_conversion.h
-  DEPENDS
-    .clock_gettime
-    libc.src.__support.time.units
-)
-
-add_header_library(
-  abs_timeout
-  HDRS
-    abs_timeout.h
-  DEPENDS
-    libc.hdr.types.struct_timespec
-    libc.src.__support.time.units
-    libc.src.__support.CPP.expected
-)
-
-add_header_library(
-  monotonicity
-  HDRS
-    monotonicity.h
-  DEPENDS
-    .clock_conversion
-    .abs_timeout
-    libc.hdr.time_macros
-)
diff --git a/libc/src/__support/time/linux/monotonicity.h b/libc/src/__support/time/monotonicity.h
similarity index 79%
rename from libc/src/__support/time/linux/monotonicity.h
rename to libc/src/__support/time/monotonicity.h
index c7234db2e64c4..4fee76da2a62e 100644
--- a/libc/src/__support/time/linux/monotonicity.h
+++ b/libc/src/__support/time/monotonicity.h
@@ -1,19 +1,19 @@
-//===--- timeout linux implementation ---------------------------*- C++ -*-===//
+//===--- timeout implementation ---------------------------*- C++ -*-===//
 //
 // 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
 //
-//===----------------------------------------------------------------------===//
+//===----------------------------------------------------------------===//
 
-#ifndef LLVM_LIBC_SRC___SUPPORT_TIME_LINUX_MONOTONICITY_H
-#define LLVM_LIBC_SRC___SUPPORT_TIME_LINUX_MONOTONICITY_H
+#ifndef LLVM_LIBC_SRC___SUPPORT_TIME_MONOTONICITY_H
+#define LLVM_LIBC_SRC___SUPPORT_TIME_MONOTONICITY_H
 
 #include "hdr/time_macros.h"
 #include "src/__support/libc_assert.h"
 #include "src/__support/macros/config.h"
-#include "src/__support/time/linux/abs_timeout.h"
-#include "src/__support/time/linux/clock_conversion.h"
+#include "src/__support/time/abs_timeout.h"
+#include "src/__support/time/clock_conversion.h"
 namespace LIBC_NAMESPACE_DECL {
 namespace internal {
 // This function is separated from abs_timeout.
@@ -41,4 +41,4 @@ LIBC_INLINE void ensure_monotonicity(AbsTimeout &timeout) {
 } // namespace internal
 } // namespace LIBC_NAMESPACE_DECL
 
-#endif // LLVM_LIBC_SRC___SUPPORT_TIME_LINUX_MONOTONICITY_H
+#endif // LLVM_LIBC_SRC___SUPPORT_TIME_MONOTONICITY_H
diff --git a/libc/src/pthread/pthread_rwlock_clockwrlock.cpp b/libc/src/pthread/pthread_rwlock_clockwrlock.cpp
index 8f58c7f24bb10..787a1b1484df7 100644
--- a/libc/src/pthread/pthread_rwlock_clockwrlock.cpp
+++ b/libc/src/pthread/pthread_rwlock_clockwrlock.cpp
@@ -12,7 +12,7 @@
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
 #include "src/__support/threads/linux/rwlock.h"
-#include "src/__support/time/linux/abs_timeout.h"
+#include "src/__support/time/abs_timeout.h"
 
 #include <pthread.h>
 
diff --git a/libc/src/pthread/pthread_rwlock_timedrdlock.cpp b/libc/src/pthread/pthread_rwlock_timedrdlock.cpp
index fcddfed224906..745da508cf140 100644
--- a/libc/src/pthread/pthread_rwlock_timedrdlock.cpp
+++ b/libc/src/pthread/pthread_rwlock_timedrdlock.cpp
@@ -13,7 +13,7 @@
 #include "src/__support/macros/config.h"
 #include "src/__support/macros/optimization.h"
 #include "src/__support/threads/linux/rwlock.h"
-#include "src/__support/time/linux/abs_timeout.h"
+#include "src/__support/time/abs_timeout.h"
 
 #include <pthread.h>
 
diff --git a/libc/src/pthread/pthread_rwlock_timedwrlock.cpp b/libc/src/pthread/pthread_rwlock_timedwrlock.cpp
index d2dc70e992e82..9666fc5b47284 100644
--- a/libc/src/pthread/pthread_rwlock_timedwrlock.cpp
+++ b/libc/src/pthread/pthread_rwlock_timedwrlock.cpp
@@ -13,7 +13,7 @@
 #include "src/__support/macros/config.h"
 #include "src/__support/macros/optimization.h"
 #include "src/__support/threads/linux/rwlock.h"
-#include "src/__support/time/linux/abs_timeout.h"
+#include "src/__support/time/abs_timeout.h"
 
 #include <pthread.h>
 
diff --git a/libc/test/src/__support/threads/darwin/mutex_test.cpp b/libc/test/src/__support/threads/darwin/mutex_test.cpp
new file mode 100644
index 0000000000000..b87da8670e422
--- /dev/null
+++ b/libc/test/src/__support/threads/darwin/mutex_test.cpp
@@ -0,0 +1,45 @@
+//===-- Unittests for Darwin's Mutex ------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/threads/mutex.h"
+#include "src/__support/threads/mutex_common.h"
+#include "src/__support/threads/raw_mutex.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcSupportThreadsMutexTest, SmokeTest) {
+  LIBC_NAMESPACE::Mutex mutex(0, 0, 0, 0);
+  ASSERT_EQ(mutex.lock(), LIBC_NAMESPACE::MutexError::NONE);
+  ASSERT_EQ(mutex.unlock(), LIBC_NAMESPACE::MutexError::NONE);
+  ASSERT_EQ(mutex.try_lock(), LIBC_NAMESPACE::MutexError::NONE);
+  ASSERT_EQ(mutex.try_lock(), LIBC_NAMESPACE::MutexError::BUSY);
+  ASSERT_EQ(mutex.unlock(), LIBC_NAMESPACE::MutexError::NONE);
+  ASSERT_EQ(mutex.unlock(), LIBC_NAMESPACE::MutexError::UNLOCK_WITHOUT_LOCK);
+}
+
+TEST(LlvmLibcSupportThreadsRawMutexTest, Timeout) {
+  LIBC_NAMESPACE::RawMutex mutex;
+  ASSERT_TRUE(mutex.lock());
+  timespec ts;
+  LIBC_NAMESPACE::internal::clock_gettime(CLOCK_MONOTONIC, &ts);
+  ts.tv_sec += 1;
+  // Timeout will be respected when deadlock happens.
+  auto timeout = LIBC_NAMESPACE::internal::AbsTimeout::from_timespec(ts, false);
+  ASSERT_TRUE(timeout.has_value());
+  // The following will timeout
+  ASSERT_FALSE(mutex.lock(*timeout));
+  ASSERT_TRUE(mutex.unlock());
+  // Test that the mutex works after the timeout.
+  ASSERT_TRUE(mutex.lock());
+  ASSERT_TRUE(mutex.unlock());
+  // If a lock can be acquired directly, expired timeout will not count.
+  // Notice that the timeout is already reached during preivous deadlock.
+  ASSERT_TRUE(mutex.lock(*timeout));
+  ASSERT_TRUE(mutex.unlock());
+}
+
+// TODO(bojle): add other tests a la linux