[libc-commits] [libc] [libc] implement PI mutex (PR #199393)

[Schrodinger ZHU Yifan via libc-commits]

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+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Linux priority inheritance mutex support.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_PI_MUTEX_H
+#define LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_PI_MUTEX_H
+
+#include "hdr/errno_macros.h"
+#include "hdr/types/size_t.h"
+#include "src/__support/CPP/limits.h"
+#include "src/__support/CPP/optional.h"
+#include "src/__support/OSUtil/syscall.h"
+#include "src/__support/macros/attributes.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/optimization.h"
+#include "src/__support/threads/identifier.h"
+#include "src/__support/threads/linux/futex_utils.h"
+#include "src/__support/threads/linux/futex_word.h"
+#include "src/__support/threads/mutex_common.h"
+
+#include <linux/futex.h>
+
+#ifdef LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY
+#include "src/__support/time/monotonicity.h"
+#endif
+
+namespace LIBC_NAMESPACE_DECL {
+
+class PIMutexRef {
+public:
+  enum class Type { Normal, ErrorChecking, Recursive };
+
+protected:
+  Futex &owner;
+  Type type;
+  // Number of recursive locks minus one. This pointer must be provided
+  // if type == Recursive
+  size_t *recursive_count;
+
+public:
+  LIBC_INLINE constexpr PIMutexRef(Futex &owner, Type type,
+                                   size_t *recursive_count = nullptr)
+      : owner(owner), type(type), recursive_count(recursive_count) {}
+  LIBC_INLINE MutexError try_lock() {
+    FutexWordType old_owner = 0;
+    auto current = static_cast<FutexWordType>(internal::gettid());
+    if (owner.compare_exchange_strong(old_owner, current,
+                                      cpp::MemoryOrder::ACQUIRE,
+                                      cpp::MemoryOrder::RELAXED))
+      return MutexError::NONE;
+
+    if (current == (old_owner & FUTEX_TID_MASK)) {
+      switch (type) {
+      case Type::Normal:
+        break;
+      case Type::ErrorChecking:
+        return MutexError::DEADLOCK;
+      case Type::Recursive:
+        if (LIBC_UNLIKELY(*recursive_count ==
+                          cpp::numeric_limits<size_t>::max()))
+          return MutexError::OVERFLOW;
+        *recursive_count += 1;
+        return MutexError::NONE;
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SchrodingerZhu wrote:

This is explicitly said

https://github.com/llvm/llvm-project/pull/199393