[libc-commits] [libc] [libc] add rwlock (PR #94156)

[Michael Jones via libc-commits]

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+//===--- Implementation of a Linux RwLock 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_RWLOCK_H
+#define LLVM_LIBC_SRC_SUPPORT_THREADS_LINUX_RWLOCK_H
+
+#include "hdr/errno_macros.h"
+#include "hdr/types/pid_t.h"
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/CPP/limits.h"
+#include "src/__support/CPP/optional.h"
+#include "src/__support/OSUtil/syscall.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_assert.h"
+#include "src/__support/macros/attributes.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/linux/raw_mutex.h"
+#include "src/__support/threads/sleep.h"
+
+#ifndef LIBC_COPT_RWLOCK_DEFAULT_SPIN_COUNT
+#define LIBC_COPT_RWLOCK_DEFAULT_SPIN_COUNT 100
+#endif
+
+#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
+
+namespace LIBC_NAMESPACE {
+// Forward declaration of the RwLock class.
+class RwLock;
+// A namespace to rwlock specific utilities.
+namespace rwlock {
+// The role of the thread in the RwLock.
+enum class Role { Reader = 0, Writer = 1 };
+
+// A waiting queue to keep track of the pending readers and writers.
+class WaitingQueue final : private RawMutex {
+  /* FutexWordType raw_mutex;  (from base class) */
+
+  // Pending reader count (protected by the mutex)
+  FutexWordType pending_readers;
+  // Pending writer count (protected by the mutex)
+  FutexWordType pending_writers;
+  // Reader serialization (increases on each reader-waking operation)
+  Futex reader_serialization;
+  // Writer serialization (increases on each writer-waking operation)
+  Futex writer_serialization;
+
+public:
+  // RAII guard to lock and unlock the waiting queue.
+  class Guard {
+    WaitingQueue &queue;
+    bool is_pshared;
+
+    LIBC_INLINE constexpr Guard(WaitingQueue &queue, bool is_pshared)
+        : queue(queue), is_pshared(is_pshared) {
+      queue.lock(cpp::nullopt, is_pshared);
+    }
+
+  public:
+    LIBC_INLINE ~Guard() { queue.unlock(is_pshared); }
+    template <Role role> LIBC_INLINE FutexWordType &pending_count() {
+      if constexpr (role == Role::Reader)
+        return queue.pending_readers;
+      else
+        return queue.pending_writers;
+    }
+    template <Role role> LIBC_INLINE FutexWordType &serialization() {
+      if constexpr (role == Role::Reader)
+        return queue.reader_serialization.val;
+      else
+        return queue.writer_serialization.val;
+    }
+    friend WaitingQueue;
+  };
+
+public:
+  LIBC_INLINE constexpr WaitingQueue()
+      : RawMutex(), pending_readers(0), pending_writers(0),
+        reader_serialization(0), writer_serialization(0) {}
+
+  LIBC_INLINE Guard acquire(bool is_pshared) {
+    return Guard(*this, is_pshared);
+  }
+
+  template <Role role>
+  LIBC_INLINE long wait(FutexWordType expected,
+                        cpp::optional<Futex::Timeout> timeout,
+                        bool is_pshared) {
+    if constexpr (role == Role::Reader)
+      return reader_serialization.wait(expected, timeout, is_pshared);
+    else
+      return writer_serialization.wait(expected, timeout, is_pshared);
+  }
+
+  template <Role role> LIBC_INLINE long notify(bool is_pshared) {
+    if constexpr (role == Role::Reader)
+      return reader_serialization.notify_all(is_pshared);
+    else
+      return writer_serialization.notify_one(is_pshared);
+  }
+};
+
+// The State of the RwLock is stored in an integer word, consisting of the
+// following components:
+// -----------------------------------------------
+// | Range    |           Description            |
+// ===============================================
+// | 0        | Pending Reader Bit               |
+// -----------------------------------------------
+// | 1        | Pending Writer Bit               |
+// -----------------------------------------------
+// | [2, MSB) | Active Reader Count              |
+// -----------------------------------------------
+// | MSB      | Active Writer Bit                |
+// -----------------------------------------------
+class State {
----------------
michaelrj-google wrote:

the name `State` isn't very descriptive. I'd prefer something like `RwState` to make it clear what it's storing state for.

https://github.com/llvm/llvm-project/pull/94156