[libc] rework mutex (#92168)

libc/config/config.json

@@ -40,5 +40,15 @@
       "value": true,
       "doc": "Enable -fstack-protector-strong to defend against stack smashing attack."
     }
+  },
+  "pthread": {
+    "LIBC_CONF_TIMEOUT_ENSURE_MONOTONICITY": {
+      "value": true,
+      "doc": "Automatically adjust timeout to CLOCK_MONOTONIC (default to true). POSIX API may require CLOCK_REALTIME, which can be unstable and leading to unexpected behavior. This option will convert the real-time timestamp to monotonic timestamp relative to the time of call."
+    },
+    "LIBC_CONF_RAW_MUTEX_DEFAULT_SPIN_COUNT": {
+      "value": 100,
+      "doc": "Default number of spins before blocking if a mutex is in contention (default to 100)."
+    }
   }
 }

libc/docs/configure.rst

@@ -34,6 +34,9 @@ to learn about the defaults for your platform and target.
     - ``LIBC_CONF_PRINTF_DISABLE_INDEX_MODE``: Disable index mode in the printf format string.
     - ``LIBC_CONF_PRINTF_DISABLE_WRITE_INT``: Disable handling of %n in printf format string.
     - ``LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_MEGA_LONG_DOUBLE_TABLE``: Use large table for better printf long double performance.
+* **"pthread" options**
+    - ``LIBC_CONF_RAW_MUTEX_DEFAULT_SPIN_COUNT``: Default number of spins before blocking if a mutex is in contention (default to 100).
+    - ``LIBC_CONF_TIMEOUT_ENSURE_MONOTONICITY``: Automatically adjust timeout to CLOCK_MONOTONIC (default to true). POSIX API may require CLOCK_REALTIME, which can be unstable and leading to unexpected behavior. This option will convert the real-time timestamp to monotonic timestamp relative to the time of call.
 * **"string" options**
     - ``LIBC_CONF_MEMSET_X86_USE_SOFTWARE_PREFETCHING``: Inserts prefetch for write instructions (PREFETCHW) for memset on x86 to recover performance when hardware prefetcher is disabled.
     - ``LIBC_CONF_STRING_UNSAFE_WIDE_READ``: Read more than a byte at a time to perform byte-string operations like strlen.

libc/hdr/types/CMakeLists.txt

@@ -108,3 +108,12 @@ add_proxy_header_library(
     libc.include.llvm-libc-types.struct_timeval
     libc.include.sys_time
 )
+
+add_proxy_header_library(
+  pid_t
+  HDRS
+    pid_t.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-types.pid_t
+    libc.include.sys_types
+)

libc/hdr/types/pid_t.h

@@ -0,0 +1,22 @@
+//===-- Proxy for pid_t ---------------------------------------------------===//
+//
+// 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_HDR_TYPES_PID_T_H
+#define LLVM_LIBC_HDR_TYPES_PID_T_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-types/pid_t.h"
+
+#else // Overlay mode
+
+#include <sys/types.h>
+
+#endif // LLVM_LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_TYPES_PID_T_H

libc/include/llvm-libc-types/CMakeLists.txt

@@ -7,7 +7,8 @@ add_header(__call_once_func_t HDR __call_once_func_t.h)
 add_header(__exec_argv_t HDR __exec_argv_t.h)
 add_header(__exec_envp_t HDR __exec_envp_t.h)
 add_header(__futex_word HDR __futex_word.h)
-add_header(__mutex_type HDR __mutex_type.h DEPENDS .__futex_word)
+add_header(pid_t HDR pid_t.h)
+add_header(__mutex_type HDR __mutex_type.h DEPENDS .__futex_word .pid_t)
 add_header(__pthread_once_func_t HDR __pthread_once_func_t.h)
 add_header(__pthread_start_t HDR __pthread_start_t.h)
 add_header(__pthread_tss_dtor_t HDR __pthread_tss_dtor_t.h)
@@ -20,7 +21,7 @@ add_header(blksize_t HDR blksize_t.h)
 add_header(cc_t HDR cc_t.h)
 add_header(clock_t HDR clock_t.h)
 add_header(clockid_t HDR clockid_t.h)
-add_header(cnd_t HDR cnd_t.h)
+add_header(cnd_t HDR cnd_t.h DEPENDS .__futex_word)
 add_header(cookie_io_functions_t HDR cookie_io_functions_t.h DEPENDS .off64_t .ssize_t)
 add_header(cpu_set_t HDR cpu_set_t.h)
 add_header(double_t HDR double_t.h)
@@ -45,7 +46,6 @@ add_header(mtx_t HDR mtx_t.h DEPENDS .__futex_word .__mutex_type)
 add_header(nlink_t HDR nlink_t.h)
 add_header(off_t HDR off_t.h)
 add_header(once_flag HDR once_flag.h DEPENDS .__futex_word)
-add_header(pid_t HDR pid_t.h)
 add_header(posix_spawn_file_actions_t HDR posix_spawn_file_actions_t.h)
 add_header(posix_spawnattr_t HDR posix_spawnattr_t.h)
 add_header(pthread_attr_t HDR pthread_attr_t.h DEPENDS .size_t)

libc/include/llvm-libc-types/cnd_t.h

@@ -9,12 +9,12 @@
 #ifndef LLVM_LIBC_TYPES_CND_T_H
 #define LLVM_LIBC_TYPES_CND_T_H
 
-#include "mtx_t.h"
+#include "llvm-libc-types/__futex_word.h"
 
 typedef struct {
   void *__qfront;
   void *__qback;
-  mtx_t __qmtx;
+  __futex_word __qmtx;
 } cnd_t;
 
 #endif // LLVM_LIBC_TYPES_CND_T_H

libc/src/__support/File/dir.h

@@ -50,14 +50,16 @@ class Dir {
   // A directory is to be opened by the static method open and closed
   // by the close method. So, all constructors and destructor are declared
   // as private. Inappropriate constructors are declared as deleted.
-  Dir() = delete;
-  Dir(const Dir &) = delete;
+  LIBC_INLINE Dir() = delete;
+  LIBC_INLINE Dir(const Dir &) = delete;
 
-  explicit Dir(int fdesc)
-      : fd(fdesc), readptr(0), fillsize(0), mutex(false, false, false) {}
-  ~Dir() = default;
+  LIBC_INLINE explicit Dir(int fdesc)
+      : fd(fdesc), readptr(0), fillsize(0),
+        mutex(/*timed=*/false, /*recursive=*/false, /*robust=*/false,
+              /*pshared=*/false) {}
+  LIBC_INLINE ~Dir() = default;
 
-  Dir &operator=(const Dir &) = delete;
+  LIBC_INLINE Dir &operator=(const Dir &) = delete;
 
 public:
   static ErrorOr<Dir *> open(const char *path);
@@ -69,7 +71,7 @@ public:
   // cleaned up.
   int close();
 
-  int getfd() { return fd; }
+  LIBC_INLINE int getfd() { return fd; }
 };
 
 } // namespace LIBC_NAMESPACE

libc/src/__support/File/file.h

@@ -154,10 +154,11 @@ public:
                  uint8_t *buffer, size_t buffer_size, int buffer_mode,
                  bool owned, ModeFlags modeflags)
       : platform_write(wf), platform_read(rf), platform_seek(sf),
-        platform_close(cf), mutex(false, false, false), ungetc_buf(0),
-        buf(buffer), bufsize(buffer_size), bufmode(buffer_mode), own_buf(owned),
-        mode(modeflags), pos(0), prev_op(FileOp::NONE), read_limit(0),
-        eof(false), err(false) {
+        platform_close(cf), mutex(/*timed=*/false, /*recursive=*/false,
+                                  /*robust=*/false, /*pshared=*/false),
+        ungetc_buf(0), buf(buffer), bufsize(buffer_size), bufmode(buffer_mode),
+        own_buf(owned), mode(modeflags), pos(0), prev_op(FileOp::NONE),
+        read_limit(0), eof(false), err(false) {
     adjust_buf();
   }
 

libc/src/__support/threads/CndVar.h

@@ -10,13 +10,14 @@
 #define LLVM_LIBC___SUPPORT_SRC_THREADS_LINUX_CNDVAR_H
 
 #include "src/__support/threads/linux/futex_utils.h" // Futex
+#include "src/__support/threads/linux/raw_mutex.h"   // RawMutex
 #include "src/__support/threads/mutex.h"             // Mutex
 
 #include <stdint.h> // uint32_t
 
 namespace LIBC_NAMESPACE {
 
-struct CndVar {
+class CndVar {
   enum CndWaiterStatus : uint32_t {
     WS_Waiting = 0xE,
     WS_Signalled = 0x5,
@@ -29,15 +30,16 @@ struct CndVar {
 
   CndWaiter *waitq_front;
   CndWaiter *waitq_back;
-  Mutex qmtx;
+  RawMutex qmtx;
 
-  static int init(CndVar *cv) {
+public:
+  LIBC_INLINE static int init(CndVar *cv) {
     cv->waitq_front = cv->waitq_back = nullptr;
-    auto err = Mutex::init(&cv->qmtx, false, false, false);
-    return err == MutexError::NONE ? 0 : -1;
+    RawMutex::init(&cv->qmtx);
+    return 0;
   }
 
-  static void destroy(CndVar *cv) {
+  LIBC_INLINE static void destroy(CndVar *cv) {
     cv->waitq_front = cv->waitq_back = nullptr;
   }
 

libc/src/__support/threads/fork_callbacks.cpp

@@ -33,7 +33,10 @@ class AtForkCallbackManager {
   size_t next_index;
 
 public:
-  constexpr AtForkCallbackManager() : mtx(false, false, false), next_index(0) {}
+  constexpr AtForkCallbackManager()
+      : mtx(/*timed=*/false, /*recursive=*/false, /*robust=*/false,
+            /*pshared=*/false),
+        next_index(0) {}
 
   bool register_triple(const ForkCallbackTriple &triple) {
     cpp::lock_guard lock(mtx);

libc/src/__support/threads/gpu/mutex.h

@@ -19,7 +19,7 @@ namespace LIBC_NAMESPACE {
 /// define the Mutex interface and require that only a single thread executes
 /// code requiring a mutex lock.
 struct Mutex {
-  LIBC_INLINE constexpr Mutex(bool, bool, bool) {}
+  LIBC_INLINE constexpr Mutex(bool, bool, bool, bool) {}
 
   LIBC_INLINE MutexError lock() { return MutexError::NONE; }
   LIBC_INLINE MutexError unlock() { return MutexError::NONE; }

libc/src/__support/threads/linux/CMakeLists.txt

@@ -22,12 +22,37 @@ add_header_library(
     libc.src.__support.time.linux.abs_timeout
 )
 
+set(raw_mutex_additional_flags)
+if (LIBC_CONF_TIMEOUT_ENSURE_MONOTONICITY)
+  set(raw_mutex_additional_flags -DLIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY=1)
+else()
+  set(raw_mutex_additional_flags -DLIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY=0)
+endif()
+
+add_header_library(
+  raw_mutex
+  HDRS
+    mutex.h
+  DEPENDS
+    .futex_utils
+    libc.src.__support.threads.sleep
+    libc.src.__support.time.linux.abs_timeout
+    libc.src.__support.time.linux.monotonicity
+    libc.src.__support.CPP.optional
+    libc.hdr.types.pid_t
+  COMPILE_OPTIONS
+    -DLIBC_COPT_RAW_MUTEX_DEFAULT_SPIN_COUNT=${LIBC_CONF_RAW_MUTEX_DEFAULT_SPIN_COUNT}
+    ${raw_mutex_additional_flags}
+  
+)
+
 add_header_library(
   mutex
   HDRS
     mutex.h
   DEPENDS
     .futex_utils
+    .raw_mutex
     libc.src.__support.threads.mutex_common
 )
 
@@ -75,5 +100,6 @@ add_object_library(
     libc.src.__support.OSUtil.osutil
     libc.src.__support.threads.linux.futex_word_type
     libc.src.__support.threads.mutex
+    libc.src.__support.threads.linux.raw_mutex
     libc.src.__support.CPP.mutex
 )

libc/src/__support/threads/linux/CndVar.cpp

@@ -10,6 +10,7 @@
 #include "src/__support/CPP/mutex.h"
 #include "src/__support/OSUtil/syscall.h"           // syscall_impl
 #include "src/__support/threads/linux/futex_word.h" // FutexWordType
+#include "src/__support/threads/linux/raw_mutex.h"  // RawMutex
 #include "src/__support/threads/mutex.h"            // Mutex
 
 #include <sys/syscall.h> // For syscall numbers.
@@ -74,11 +75,11 @@ void CndVar::notify_one() {
   if (waitq_front == nullptr)
     waitq_back = nullptr;
 
-  qmtx.futex_word = FutexWordType(Mutex::LockState::Free);
+  qmtx.reset();
 
   // this is a special WAKE_OP, so we use syscall directly
   LIBC_NAMESPACE::syscall_impl<long>(
-      FUTEX_SYSCALL_ID, &qmtx.futex_word.val, FUTEX_WAKE_OP, 1, 1,
+      FUTEX_SYSCALL_ID, &qmtx.get_raw_futex(), FUTEX_WAKE_OP, 1, 1,
       &first->futex_word.val,
       FUTEX_OP(FUTEX_OP_SET, WS_Signalled, FUTEX_OP_CMP_EQ, WS_Waiting));
 }

libc/src/__support/threads/linux/mutex.h

@@ -9,114 +9,79 @@
 #ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_MUTEX_H
 #define LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_MUTEX_H
 
+#include "hdr/types/pid_t.h"
+#include "src/__support/CPP/optional.h"
+#include "src/__support/libc_assert.h"
 #include "src/__support/threads/linux/futex_utils.h"
+#include "src/__support/threads/linux/raw_mutex.h"
 #include "src/__support/threads/mutex_common.h"
 
 namespace LIBC_NAMESPACE {
-struct Mutex {
+
+// TODO: support shared/recursive/robust mutexes.
+class Mutex final : private RawMutex {
+  // reserved timed, may be useful when combined with other flags.
   unsigned char timed;
   unsigned char recursive;
   unsigned char robust;
+  unsigned char pshared;
 
-  void *owner;
+  // TLS address may not work across forked processes. Use thread id instead.
+  pid_t owner;
   unsigned long long lock_count;
 
-  Futex futex_word;
-
-  enum class LockState : FutexWordType {
-    Free,
-    Locked,
-    Waiting,
-  };
-
 public:
-  constexpr Mutex(bool istimed, bool isrecursive, bool isrobust)
-      : timed(istimed), recursive(isrecursive), robust(isrobust),
-        owner(nullptr), lock_count(0),
-        futex_word(FutexWordType(LockState::Free)) {}
+  LIBC_INLINE constexpr Mutex(bool is_timed, bool is_recursive, bool is_robust,
+                              bool is_pshared)
+      : RawMutex(), timed(is_timed), recursive(is_recursive), robust(is_robust),
+        pshared(is_pshared), owner(0), lock_count(0) {}
 
-  static MutexError init(Mutex *mutex, bool istimed, bool isrecur,
-                         bool isrobust) {
-    mutex->timed = istimed;
+  LIBC_INLINE static MutexError init(Mutex *mutex, bool is_timed, bool isrecur,
+                                     bool isrobust, bool is_pshared) {
+    RawMutex::init(mutex);
+    mutex->timed = is_timed;
     mutex->recursive = isrecur;
     mutex->robust = isrobust;
-    mutex->owner = nullptr;
+    mutex->pshared = is_pshared;
+    mutex->owner = 0;
     mutex->lock_count = 0;
-    mutex->futex_word.set(FutexWordType(LockState::Free));
     return MutexError::NONE;
   }
 
-  static MutexError destroy(Mutex *) { return MutexError::NONE; }
-
-  MutexError reset();
-
-  MutexError lock() {
-    bool was_waiting = false;
-    while (true) {
-      FutexWordType mutex_status = FutexWordType(LockState::Free);
-      FutexWordType locked_status = FutexWordType(LockState::Locked);
-
-      if (futex_word.compare_exchange_strong(
-              mutex_status, FutexWordType(LockState::Locked))) {
-        if (was_waiting)
-          futex_word = FutexWordType(LockState::Waiting);
-        return MutexError::NONE;
-      }
-
-      switch (LockState(mutex_status)) {
-      case LockState::Waiting:
-        // If other threads are waiting already, then join them. Note that the
-        // futex syscall will block if the futex data is still
-        // `LockState::Waiting` (the 4th argument to the syscall function
-        // below.)
-        futex_word.wait(FutexWordType(LockState::Waiting));
-        was_waiting = true;
-        // Once woken up/unblocked, try everything all over.
-        continue;
-      case LockState::Locked:
-        // Mutex has been locked by another thread so set the status to
-        // LockState::Waiting.
-        if (futex_word.compare_exchange_strong(
-                locked_status, FutexWordType(LockState::Waiting))) {
-          // If we are able to set the futex data to `LockState::Waiting`, then
-          // we will wait for the futex to be woken up. Note again that the
-          // following syscall will block only if the futex data is still
-          // `LockState::Waiting`.
-          futex_word.wait(FutexWordType(LockState::Waiting));
-          was_waiting = true;
-        }
-        continue;
-      case LockState::Free:
-        // If it was LockState::Free, we shouldn't be here at all.
-        return MutexError::BAD_LOCK_STATE;
-      }
-    }
+  LIBC_INLINE static MutexError destroy(Mutex *lock) {
+    LIBC_ASSERT(lock->owner == 0 && lock->lock_count == 0 &&
+                "Mutex destroyed while being locked.");
+    RawMutex::destroy(lock);
+    return MutexError::NONE;
   }
 
-  MutexError unlock() {
-    while (true) {
-      FutexWordType mutex_status = FutexWordType(LockState::Waiting);
-      if (futex_word.compare_exchange_strong(mutex_status,
-                                             FutexWordType(LockState::Free))) {
-        // If any thread is waiting to be woken up, then do it.
-        futex_word.notify_one();
-        return MutexError::NONE;
-      }
-
-      if (mutex_status == FutexWordType(LockState::Locked)) {
-        // If nobody was waiting at this point, just free it.
-        if (futex_word.compare_exchange_strong(mutex_status,
-                                               FutexWordType(LockState::Free)))
-          return MutexError::NONE;
-      } else {
-        // This can happen, for example if some thread tries to unlock an
-        // already free mutex.
-        return MutexError::UNLOCK_WITHOUT_LOCK;
-      }
-    }
+  // TODO: record owner and lock count.
+  LIBC_INLINE MutexError lock() {
+    // Since timeout is not specified, we do not need to check the return value.
+    this->RawMutex::lock(
+        /* timeout=*/cpp::nullopt, this->pshared);
+    return MutexError::NONE;
   }
 
-  MutexError trylock();
+  // TODO: record owner and lock count.
+  LIBC_INLINE MutexError timed_lock(internal::AbsTimeout abs_time) {
+    if (this->RawMutex::lock(abs_time, this->pshared))
+      return MutexError::NONE;
+    return MutexError::TIMEOUT;
+  }
+
+  LIBC_INLINE MutexError unlock() {
+    if (this->RawMutex::unlock(this->pshared))
+      return MutexError::NONE;
+    return MutexError::UNLOCK_WITHOUT_LOCK;
+  }
+
+  // TODO: record owner and lock count.
+  LIBC_INLINE MutexError try_lock() {
+    if (this->RawMutex::try_lock())
+      return MutexError::NONE;
+    return MutexError::BUSY;
+  }
 };
 
 } // namespace LIBC_NAMESPACE

libc/src/__support/threads/linux/raw_mutex.h

@@ -0,0 +1,130 @@
+//===--- Implementation of a Linux 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
+
+#include "src/__support/CPP/optional.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_assert.h"
+#include "src/__support/macros/attributes.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"
+
+#ifndef LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY
+#define LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY 1
+#warning "LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY is not defined, defaulting to 1"
+#endif
+
+#if LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY
+#include "src/__support/time/linux/monotonicity.h"
+#endif
+
+#ifndef LIBC_COPT_RAW_MUTEX_DEFAULT_SPIN_COUNT
+#define LIBC_COPT_RAW_MUTEX_DEFAULT_SPIN_COUNT 100
+#warning                                                                       \
+    "LIBC_COPT_RAW_MUTEX_DEFAULT_SPIN_COUNT is not defined, defaulting to 100"
+#endif
+
+namespace LIBC_NAMESPACE {
+// Lock is a simple timable lock for internal usage.
+// This is separated from Mutex because this one does not need to consider
+// robustness and reentrancy. Also, this one has spin optimization for shorter
+// critical sections.
+class RawMutex {
+protected:
+  Futex futex;
+  LIBC_INLINE_VAR static constexpr FutexWordType UNLOCKED = 0b00;
+  LIBC_INLINE_VAR static constexpr FutexWordType LOCKED = 0b01;
+  LIBC_INLINE_VAR static constexpr FutexWordType IN_CONTENTION = 0b10;
+
+private:
+  LIBC_INLINE FutexWordType spin(unsigned spin_count) {
+    FutexWordType result;
+    for (;;) {
+      result = futex.load(cpp::MemoryOrder::RELAXED);
+      // spin until one of the following conditions is met:
+      // - the mutex is unlocked
+      // - the mutex is in contention
+      // - the spin count reaches 0
+      if (result != LOCKED || spin_count == 0u)
+        return result;
+      // Pause the pipeline to avoid extraneous memory operations due to
+      // speculation.
+      sleep_briefly();
+      spin_count--;
+    };
+  }
+
+  // Return true if the lock is acquired. Return false if timeout happens before
+  // the lock is acquired.
+  LIBC_INLINE bool lock_slow(cpp::optional<Futex::Timeout> timeout,
+                             bool is_pshared, unsigned spin_count) {
+    FutexWordType state = spin(spin_count);
+    // Before go into contention state, try to grab the lock.
+    if (state == UNLOCKED &&
+        futex.compare_exchange_strong(state, LOCKED, cpp::MemoryOrder::ACQUIRE,
+                                      cpp::MemoryOrder::RELAXED))
+      return true;
+#if LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY
+    /* ADL should kick in */
+    if (timeout)
+      ensure_monotonicity(*timeout);
+#endif
+    for (;;) {
+      // Try to grab the lock if it is unlocked. Mark the contention flag if it
+      // is locked.
+      if (state != IN_CONTENTION &&
+          futex.exchange(IN_CONTENTION, cpp::MemoryOrder::ACQUIRE) == UNLOCKED)
+        return true;
+      // Contention persists. Park the thread and wait for further notification.
+      if (ETIMEDOUT == -futex.wait(IN_CONTENTION, timeout, is_pshared))
+        return false;
+      // Continue to spin after waking up.
+      state = spin(spin_count);
+    }
+  }
+
+  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 constexpr RawMutex() : futex(UNLOCKED) {}
+  [[nodiscard]] LIBC_INLINE bool try_lock() {
+    FutexWordType expected = UNLOCKED;
+    // Use strong version since this is a one-time operation.
+    return futex.compare_exchange_strong(
+        expected, LOCKED, cpp::MemoryOrder::ACQUIRE, cpp::MemoryOrder::RELAXED);
+  }
+  LIBC_INLINE bool
+  lock(cpp::optional<Futex::Timeout> timeout = cpp::nullopt,
+       bool is_shared = false,
+       unsigned spin_count = LIBC_COPT_RAW_MUTEX_DEFAULT_SPIN_COUNT) {
+    // Timeout will not be checked if immediate lock is possible.
+    if (LIBC_LIKELY(try_lock()))
+      return true;
+    return lock_slow(timeout, is_shared, spin_count);
+  }
+  LIBC_INLINE bool unlock(bool is_pshared = false) {
+    FutexWordType prev = futex.exchange(UNLOCKED, cpp::MemoryOrder::RELEASE);
+    // if there is someone waiting, wake them up
+    if (LIBC_UNLIKELY(prev == IN_CONTENTION))
+      wake(is_pshared);
+    // Detect invalid unlock operation.
+    return prev != UNLOCKED;
+  }
+  LIBC_INLINE void static destroy([[maybe_unused]] RawMutex *lock) {
+    LIBC_ASSERT(lock->futex == UNLOCKED && "Mutex destroyed while used.");
+  }
+  LIBC_INLINE Futex &get_raw_futex() { return futex; }
+  LIBC_INLINE void reset() { futex = UNLOCKED; }
+};
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_RAW_MUTEX_H

libc/src/__support/threads/thread.cpp

@@ -54,7 +54,9 @@ class TSSKeyMgr {
   cpp::array<TSSKeyUnit, TSS_KEY_COUNT> units;
 
 public:
-  constexpr TSSKeyMgr() : mtx(false, false, false) {}
+  constexpr TSSKeyMgr()
+      : mtx(/*timed=*/false, /*recursive=*/false, /*robust=*/false,
+            /*pshared=*/false) {}
 
   cpp::optional<unsigned int> new_key(TSSDtor *dtor) {
     cpp::lock_guard lock(mtx);
@@ -111,7 +113,9 @@ class ThreadAtExitCallbackMgr {
   FixedVector<AtExitUnit, 1024> callback_list;
 
 public:
-  constexpr ThreadAtExitCallbackMgr() : mtx(false, false, false) {}
+  constexpr ThreadAtExitCallbackMgr()
+      : mtx(/*timed=*/false, /*recursive=*/false, /*robust=*/false,
+            /*pshared=*/false) {}
 
   int add_callback(AtExitCallback *callback, void *obj) {
     cpp::lock_guard lock(mtx);

libc/src/pthread/pthread_mutex_init.cpp

@@ -26,9 +26,10 @@ LLVM_LIBC_FUNCTION(int, pthread_mutex_init,
                     const pthread_mutexattr_t *__restrict attr)) {
   auto mutexattr = attr == nullptr ? DEFAULT_MUTEXATTR : *attr;
   auto err =
-      Mutex::init(reinterpret_cast<Mutex *>(m), false,
+      Mutex::init(reinterpret_cast<Mutex *>(m), /*is_timed=*/true,
                   get_mutexattr_type(mutexattr) & PTHREAD_MUTEX_RECURSIVE,
-                  get_mutexattr_robust(mutexattr) & PTHREAD_MUTEX_ROBUST);
+                  get_mutexattr_robust(mutexattr) & PTHREAD_MUTEX_ROBUST,
+                  get_mutexattr_pshared(mutexattr) & PTHREAD_PROCESS_SHARED);
   return err == MutexError::NONE ? 0 : EAGAIN;
 }
 

libc/src/pthread/pthread_mutexattr.h

@@ -43,6 +43,11 @@ LIBC_INLINE int get_mutexattr_robust(pthread_mutexattr_t attr) {
          unsigned(PThreadMutexAttrPos::ROBUST_SHIFT);
 }
 
+LIBC_INLINE int get_mutexattr_pshared(pthread_mutexattr_t attr) {
+  return (attr & unsigned(PThreadMutexAttrPos::PSHARED_MASK)) >>
+         unsigned(PThreadMutexAttrPos::PSHARED_SHIFT);
+}
+
 } // namespace LIBC_NAMESPACE
 
 #endif // LLVM_LIBC_SRC_PTHREAD_PTHREAD_MUTEXATTR_H

libc/src/stdlib/atexit.cpp

@@ -17,7 +17,8 @@ namespace LIBC_NAMESPACE {
 
 namespace {
 
-Mutex handler_list_mtx(false, false, false);
+Mutex handler_list_mtx(/*timed=*/false, /*recursive=*/false, /*robust=*/false,
+                       /*pshared=*/false);
 
 using AtExitCallback = void(void *);
 using StdCAtExitCallback = void(void);

libc/src/threads/linux/CMakeLists.txt

@@ -9,6 +9,7 @@ add_header_library(
     libc.src.__support.CPP.mutex
     libc.src.__support.OSUtil.osutil
     libc.src.__support.threads.mutex
+    libc.src.__support.threads.linux.raw_mutex
     libc.src.__support.threads.linux.futex_utils
 )
 

libc/src/threads/mtx_init.cpp

@@ -20,7 +20,8 @@ static_assert(sizeof(Mutex) <= sizeof(mtx_t),
 
 LLVM_LIBC_FUNCTION(int, mtx_init, (mtx_t * m, int type)) {
   auto err = Mutex::init(reinterpret_cast<Mutex *>(m), type & mtx_timed,
-                         type & mtx_recursive, 0);
+                         type & mtx_recursive, /* is_robust */ false,
+                         /* is_pshared */ false);
   return err == MutexError::NONE ? thrd_success : thrd_error;
 }
 

libc/test/integration/src/__support/threads/thread_detach_test.cpp

@@ -10,7 +10,8 @@
 #include "src/__support/threads/thread.h"
 #include "test/IntegrationTest/test.h"
 
-LIBC_NAMESPACE::Mutex mutex(false, false, false);
+LIBC_NAMESPACE::Mutex mutex(/*timed=*/false, /*recursive=*/false,
+                            /*robust=*/false, /*pshared=*/false);
 
 int func(void *) {
   mutex.lock();

libc/test/src/__support/CMakeLists.txt

@@ -207,3 +207,4 @@ add_subdirectory(FPUtil)
 add_subdirectory(fixed_point)
 add_subdirectory(HashTable)
 add_subdirectory(time)
+add_subdirectory(threads)

libc/test/src/__support/threads/CMakeLists.txt

@@ -0,0 +1,4 @@
+add_custom_target(libc-support-threads-tests)
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
+  add_subdirectory(${LIBC_TARGET_OS})
+endif()

libc/test/src/__support/threads/linux/CMakeLists.txt

@@ -0,0 +1,12 @@
+add_libc_test(
+  raw_mutex_test
+  SUITE
+    libc-support-threads-tests
+  SRCS
+    raw_mutex_test.cpp
+  DEPENDS
+    libc.src.__support.threads.linux.raw_mutex
+    libc.src.sys.mman.mmap
+    libc.src.sys.mman.munmap
+    libc.src.stdlib.exit
+)

libc/test/src/__support/threads/linux/raw_mutex_test.cpp

@@ -0,0 +1,81 @@
+//===-- Unittests for Linux's RawMutex ------------------------------------===//
+//
+// 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 "include/llvm-libc-macros/linux/time-macros.h"
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/OSUtil/syscall.h"
+#include "src/__support/threads/linux/raw_mutex.h"
+#include "src/__support/threads/sleep.h"
+#include "src/__support/time/linux/clock_gettime.h"
+#include "src/stdlib/exit.h"
+#include "src/sys/mman/mmap.h"
+#include "src/sys/mman/munmap.h"
+#include "test/UnitTest/Test.h"
+#include <sys/syscall.h>
+
+TEST(LlvmLibcSupportThreadsRawMutexTest, SmokeTest) {
+  LIBC_NAMESPACE::RawMutex mutex;
+  ASSERT_TRUE(mutex.lock());
+  ASSERT_TRUE(mutex.unlock());
+  ASSERT_TRUE(mutex.try_lock());
+  ASSERT_FALSE(mutex.try_lock());
+  ASSERT_TRUE(mutex.unlock());
+  ASSERT_FALSE(mutex.unlock());
+}
+
+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());
+}
+
+TEST(LlvmLibcSupportThreadsRawMutexTest, PSharedLock) {
+  struct SharedData {
+    LIBC_NAMESPACE::RawMutex mutex;
+    LIBC_NAMESPACE::cpp::Atomic<size_t> finished;
+    int data;
+  };
+  void *addr =
+      LIBC_NAMESPACE::mmap(nullptr, sizeof(SharedData), PROT_READ | PROT_WRITE,
+                           MAP_ANONYMOUS | MAP_SHARED, -1, 0);
+  ASSERT_NE(addr, MAP_FAILED);
+  auto *shared = reinterpret_cast<SharedData *>(addr);
+  shared->data = 0;
+  LIBC_NAMESPACE::RawMutex::init(&shared->mutex);
+  // Avoid pull in our own implementation of pthread_t.
+  long pid = LIBC_NAMESPACE::syscall_impl<long>(SYS_fork);
+  for (int i = 0; i < 10000; ++i) {
+    shared->mutex.lock(LIBC_NAMESPACE::cpp::nullopt, true);
+    shared->data++;
+    shared->mutex.unlock(true);
+  }
+  // Mark the thread as finished.
+  shared->finished.fetch_add(1);
+  // let the child exit early to avoid output pollution
+  if (pid == 0)
+    LIBC_NAMESPACE::exit(0);
+  while (shared->finished.load() != 2)
+    LIBC_NAMESPACE::sleep_briefly();
+  ASSERT_EQ(shared->data, 20000);
+  LIBC_NAMESPACE::munmap(addr, sizeof(SharedData));
+}