base/threading/scoped_thread_priority.h

// Copyright 2019 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_THREADING_SCOPED_THREAD_PRIORITY_H_
#define BASE_THREADING_SCOPED_THREAD_PRIORITY_H_

#include <atomic>

#include "base/base_export.h"
#include "base/compiler_specific.h"
#include "base/location.h"
#include "base/macros.h"
#include "base/optional.h"
#include "build/build_config.h"

namespace base {

class Location;
enum class ThreadPriority : int;

// INTERNAL_SCOPED_THREAD_PRIORITY_APPEND_LINE(name) produces an identifier by
// appending the current line number to |name|. This is used to avoid name
// collisions from variables defined inside a macro.
#define INTERNAL_SCOPED_THREAD_PRIORITY_CONCAT(a, b) a##b
// CONCAT1 provides extra level of indirection so that __LINE__ macro expands.
#define INTERNAL_SCOPED_THREAD_PRIORITY_CONCAT1(a, b) \
  INTERNAL_SCOPED_THREAD_PRIORITY_CONCAT(a, b)
#define INTERNAL_SCOPED_THREAD_PRIORITY_APPEND_LINE(name) \
  INTERNAL_SCOPED_THREAD_PRIORITY_CONCAT1(name, __LINE__)

// All code that may load a DLL on a background thread must be surrounded by a
// scope that starts with this macro.
//
// Example:
//   Foo();
//   {
//     SCOPED_MAY_LOAD_LIBRARY_AT_BACKGROUND_PRIORITY();
//     LoadMyDll();
//   }
//   Bar();
//
// The macro raises the thread priority to NORMAL for the scope if no other
// thread has completed the current scope already (multiple threads can racily
// begin the initialization and will all be boosted for it). On Windows, loading
// a DLL on a background thread can lead to a priority inversion on the loader
// lock and cause huge janks.
#define SCOPED_MAY_LOAD_LIBRARY_AT_BACKGROUND_PRIORITY()               \
  static std::atomic_bool INTERNAL_SCOPED_THREAD_PRIORITY_APPEND_LINE( \
      already_loaded){false};                                          \
  base::internal::ScopedMayLoadLibraryAtBackgroundPriority             \
      INTERNAL_SCOPED_THREAD_PRIORITY_APPEND_LINE(                     \
          scoped_may_load_library_at_background_priority)(             \
          FROM_HERE,                                                   \
          &INTERNAL_SCOPED_THREAD_PRIORITY_APPEND_LINE(already_loaded));

// Like SCOPED_MAY_LOAD_LIBRARY_AT_BACKGROUND_PRIORITY, but raises the thread
// priority every time the scope is entered. Use this around code that may
// conditionally load a DLL each time it is executed, or which repeatedly loads
// and unloads DLLs.
#define SCOPED_MAY_LOAD_LIBRARY_AT_BACKGROUND_PRIORITY_REPEATEDLY() \
  base::internal::ScopedMayLoadLibraryAtBackgroundPriority          \
      INTERNAL_SCOPED_THREAD_PRIORITY_APPEND_LINE(                  \
          scoped_may_load_library_at_background_priority)(FROM_HERE, nullptr);

namespace internal {

class BASE_EXPORT ScopedMayLoadLibraryAtBackgroundPriority {
 public:
  // Boosts thread priority to NORMAL within its scope if |already_loaded| is
  // nullptr or set to false.
  explicit ScopedMayLoadLibraryAtBackgroundPriority(
      const Location& from_here,
      std::atomic_bool* already_loaded);
  ~ScopedMayLoadLibraryAtBackgroundPriority();

 private:
#if defined(OS_WIN)
  // The original priority when invoking entering the scope().
  base::Optional<ThreadPriority> original_thread_priority_;
  std::atomic_bool* const already_loaded_;
#endif

  DISALLOW_COPY_AND_ASSIGN(ScopedMayLoadLibraryAtBackgroundPriority);
};

}  // namespace internal

}  // namespace base

#endif  // BASE_THREADING_SCOPED_THREAD_PRIORITY_H_