base/synchronization/lock_impl.h

// Copyright 2011 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_SYNCHRONIZATION_LOCK_IMPL_H_
#define BASE_SYNCHRONIZATION_LOCK_IMPL_H_

#include <utility>

#include "base/base_export.h"
#include "base/check.h"
#include "base/dcheck_is_on.h"
#include "base/memory/raw_ptr_exclusion.h"
#include "base/memory/stack_allocated.h"
#include "base/synchronization/lock_subtle.h"
#include "base/thread_annotations.h"
#include "build/build_config.h"

#if BUILDFLAG(IS_WIN)
#include "base/win/windows_types.h"
#elif BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
#include <errno.h>
#include <pthread.h>
#include <string.h>
#endif

namespace base {
class Lock;
class ConditionVariable;

namespace win {
namespace internal {
class AutoNativeLock;
class ScopedHandleVerifier;
}  // namespace internal
}  // namespace win

namespace internal {

// This class implements the underlying platform-specific spin-lock mechanism
// used for the Lock class. Do not use, use Lock instead.
class BASE_EXPORT LockImpl {
 public:
  LockImpl(const LockImpl&) = delete;
  LockImpl& operator=(const LockImpl&) = delete;

 private:
  friend class base::Lock;
  friend class base::ConditionVariable;
  friend class base::win::internal::AutoNativeLock;
  friend class base::win::internal::ScopedHandleVerifier;

#if BUILDFLAG(IS_WIN)
  using NativeHandle = CHROME_SRWLOCK;
#elif BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
  using NativeHandle = pthread_mutex_t;
#endif

  LockImpl();
  ~LockImpl();

  // If the lock is not held, take it and return true.  If the lock is already
  // held by something else, immediately return false.
  inline bool Try();

  // Take the lock, blocking until it is available if necessary.
  inline void Lock();

  // Release the lock.  This must only be called by the lock's holder: after
  // a successful call to Try, or a call to Lock.
  inline void Unlock();

  // Return the native underlying lock.
  // TODO(awalker): refactor lock and condition variables so that this is
  // unnecessary.
  NativeHandle* native_handle() { return &native_handle_; }

#if BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
  // Whether this lock will attempt to use priority inheritance.
  static bool PriorityInheritanceAvailable();
#endif

  void LockInternal();
  NativeHandle native_handle_;
};

void LockImpl::Lock() {
  // Try the lock first to acquire it cheaply if it's not contended. Try() is
  // cheap on platforms with futex-type locks, as it doesn't call into the
  // kernel. Not marked `[[likely]]`, as:
  // 1. We don't know how much contention the lock would experience
  // 2. This may lead to weird-looking code layout when inlined into a caller
  // with `[[(un)likely]]` attributes.
  if (Try()) {
    return;
  }

  LockInternal();
}

#if BUILDFLAG(IS_WIN)
bool LockImpl::Try() {
  return !!::TryAcquireSRWLockExclusive(
      reinterpret_cast<PSRWLOCK>(&native_handle_));
}

void LockImpl::Unlock() {
  ::ReleaseSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&native_handle_));
}

#elif BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)

#if DCHECK_IS_ON()
BASE_EXPORT void dcheck_trylock_result(int rv);
BASE_EXPORT void dcheck_unlock_result(int rv);
#endif

bool LockImpl::Try() {
  int rv = pthread_mutex_trylock(&native_handle_);
#if DCHECK_IS_ON()
  dcheck_trylock_result(rv);
#endif
  return rv == 0;
}

void LockImpl::Unlock() {
  [[maybe_unused]] int rv = pthread_mutex_unlock(&native_handle_);
#if DCHECK_IS_ON()
  dcheck_unlock_result(rv);
#endif
}
#endif

// This is an implementation used for AutoLock templated on the lock type.
template <class LockType>
class SCOPED_LOCKABLE BasicAutoLock {
  STACK_ALLOCATED();

 public:
  struct AlreadyAcquired {};

  explicit BasicAutoLock(
      LockType& lock,
      subtle::LockTracking tracking = subtle::LockTracking::kDisabled)
      EXCLUSIVE_LOCK_FUNCTION(lock)
      : lock_(lock) {
    lock_.Acquire(tracking);
  }

  BasicAutoLock(LockType& lock, const AlreadyAcquired&)
      EXCLUSIVE_LOCKS_REQUIRED(lock)
      : lock_(lock) {
    lock_.AssertAcquired();
  }

  BasicAutoLock(const BasicAutoLock&) = delete;
  BasicAutoLock& operator=(const BasicAutoLock&) = delete;

  ~BasicAutoLock() UNLOCK_FUNCTION() {
    lock_.AssertAcquired();
    lock_.Release();
  }

 private:
  LockType& lock_;
};

// This is an implementation used for MovableAutoLock templated on the lock
// type.
template <class LockType>
class SCOPED_LOCKABLE BasicMovableAutoLock {
 public:
  explicit BasicMovableAutoLock(
      LockType& lock,
      subtle::LockTracking tracking = subtle::LockTracking::kDisabled)
      EXCLUSIVE_LOCK_FUNCTION(lock)
      : lock_(&lock) {
    lock_->Acquire(tracking);
  }

  BasicMovableAutoLock(const BasicMovableAutoLock&) = delete;
  BasicMovableAutoLock& operator=(const BasicMovableAutoLock&) = delete;
  BasicMovableAutoLock(BasicMovableAutoLock&& other) :
      lock_(std::exchange(other.lock_, nullptr)) {}
  BasicMovableAutoLock& operator=(BasicMovableAutoLock&& other) = delete;

  ~BasicMovableAutoLock() UNLOCK_FUNCTION() {
    // The lock may have been moved out.
    if (lock_) {
      lock_->AssertAcquired();
      lock_->Release();
    }
  }

 private:
  // RAW_PTR_EXCLUSION: Stack-scoped.
  RAW_PTR_EXCLUSION LockType* lock_;
};

// This is an implementation used for AutoTryLock templated on the lock type.
template <class LockType>
class SCOPED_LOCKABLE BasicAutoTryLock {
  STACK_ALLOCATED();

 public:
  // The `LOCKS_EXCLUDED(lock)` annotation requires that the caller has not
  // acquired `lock`. Without the annotation, Clang's Thread Safety Analysis
  // would generate a false positive despite correct usage. For instance, a
  // caller that checks `is_acquired()` before writing to guarded data would be
  // flagged with "writing variable 'foo' requires holding 'lock' exclusively."
  // See <https://crbug.com/340196356>.
  explicit BasicAutoTryLock(
      LockType& lock,
      subtle::LockTracking tracking = subtle::LockTracking::kDisabled)
      LOCKS_EXCLUDED(lock)
      : lock_(lock), is_acquired_(lock_.Try(tracking)) {}

  BasicAutoTryLock(const BasicAutoTryLock&) = delete;
  BasicAutoTryLock& operator=(const BasicAutoTryLock&) = delete;

  ~BasicAutoTryLock() UNLOCK_FUNCTION() {
    if (is_acquired_) {
      lock_.AssertAcquired();
      lock_.Release();
    }
  }

  bool is_acquired() const EXCLUSIVE_TRYLOCK_FUNCTION(true) {
    return is_acquired_;
  }

 private:
  LockType& lock_;
  const bool is_acquired_;
};

// This is an implementation used for AutoUnlock templated on the lock type.
template <class LockType>
class BasicAutoUnlock {
  STACK_ALLOCATED();

 public:
  explicit BasicAutoUnlock(LockType& lock) : lock_(lock) {
    // We require our caller to have the lock.
    lock_.AssertAcquired();
    lock_.Release();
  }

  BasicAutoUnlock(const BasicAutoUnlock&) = delete;
  BasicAutoUnlock& operator=(const BasicAutoUnlock&) = delete;

  ~BasicAutoUnlock() { lock_.Acquire(); }

 private:
  LockType& lock_;
};

// This is an implementation used for AutoLockMaybe templated on the lock type.
template <class LockType>
class SCOPED_LOCKABLE BasicAutoLockMaybe {
  STACK_ALLOCATED();

 public:
  explicit BasicAutoLockMaybe(
      LockType* lock,
      subtle::LockTracking tracking = subtle::LockTracking::kDisabled)
      EXCLUSIVE_LOCK_FUNCTION(lock)
      : lock_(lock) {
    if (lock_)
      lock_->Acquire(tracking);
  }

  BasicAutoLockMaybe(const BasicAutoLockMaybe&) = delete;
  BasicAutoLockMaybe& operator=(const BasicAutoLockMaybe&) = delete;

  ~BasicAutoLockMaybe() UNLOCK_FUNCTION() {
    if (lock_) {
      lock_->AssertAcquired();
      lock_->Release();
    }
  }

 private:
  LockType* const lock_;
};

// This is an implementation used for ReleasableAutoLock templated on the lock
// type.
template <class LockType>
class SCOPED_LOCKABLE BasicReleasableAutoLock {
  STACK_ALLOCATED();

 public:
  explicit BasicReleasableAutoLock(
      LockType* lock,
      subtle::LockTracking tracking = subtle::LockTracking::kDisabled)
      EXCLUSIVE_LOCK_FUNCTION(lock)
      : lock_(lock) {
    DCHECK(lock_);
    lock_->Acquire(tracking);
  }

  BasicReleasableAutoLock(const BasicReleasableAutoLock&) = delete;
  BasicReleasableAutoLock& operator=(const BasicReleasableAutoLock&) = delete;

  ~BasicReleasableAutoLock() UNLOCK_FUNCTION() {
    if (lock_) {
      lock_->AssertAcquired();
      lock_->Release();
    }
  }

  void Release() UNLOCK_FUNCTION() {
    DCHECK(lock_);
    lock_->AssertAcquired();
    lock_->Release();
    lock_ = nullptr;
  }

 private:
  LockType* lock_;
};

}  // namespace internal
}  // namespace base

#endif  // BASE_SYNCHRONIZATION_LOCK_IMPL_H_