blob: 89de1124421a69ccd9000e2eb05c6a5a192aa0fd [file] [log] [blame]
* Copyright 2013 Google Inc.
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
#ifndef SkOnce_DEFINED
#define SkOnce_DEFINED
// SkOnce.h defines SK_DECLARE_STATIC_ONCE and SkOnce(), which you can use
// together to create a threadsafe way to call a function just once. This
// is particularly useful for lazy singleton initialization. E.g.
// static void set_up_my_singleton(Singleton** singleton) {
// *singleton = new Singleton(...);
// }
// ...
// const Singleton& GetSingleton() {
// static Singleton* singleton = NULL;
// SkOnce(&once, set_up_my_singleton, &singleton);
// SkASSERT(NULL != singleton);
// return *singleton;
// }
// OnceTest.cpp also should serve as a few other simple examples.
#include "SkThread.h"
#include "SkTypes.h"
# define SK_ONCE_INIT { false, SkBaseMutex() }
#define SK_DECLARE_STATIC_ONCE(name) static SkOnceFlag name = SK_ONCE_INIT
struct SkOnceFlag; // If manually created, initialize with SkOnceFlag once = SK_ONCE_INIT
template <typename Func, typename Arg>
inline void SkOnce(SkOnceFlag* once, Func f, Arg arg);
// ---------------------- Implementation details below here. -----------------------------
struct SkOnceFlag {
bool done;
SkBaseMutex mutex;
// TODO(bungeman, mtklein): move all these *barrier* functions to SkThread when refactoring lands.
#include <intrin.h>
inline static void compiler_barrier() {
inline static void compiler_barrier() {
asm volatile("" : : : "memory");
inline static void full_barrier_on_arm() {
#ifdef SK_CPU_ARM
#if SK_ARM_ARCH >= 7
asm volatile("dmb" : : : "memory");
asm volatile("mcr p15, 0, %0, c7, c10, 5" : : "r" (0) : "memory");
// On every platform, we issue a compiler barrier to prevent it from reordering
// code. That's enough for platforms like x86 where release and acquire
// barriers are no-ops. On other platforms we may need to be more careful;
// ARM, in particular, needs real code for both acquire and release. We use a
// full barrier, which acts as both, because that the finest precision ARM
// provides.
inline static void release_barrier() {
inline static void acquire_barrier() {
// We've pulled a pretty standard double-checked locking implementation apart
// into its main fast path and a slow path that's called when we suspect the
// one-time code hasn't run yet.
// This is the guts of the code, called when we suspect the one-time code hasn't been run yet.
// This should be rarely called, so we separate it from SkOnce and don't mark it as inline.
// (We don't mind if this is an actual function call, but odds are it'll be inlined anyway.)
template <typename Func, typename Arg>
static void sk_once_slow(SkOnceFlag* once, Func f, Arg arg) {
const SkAutoMutexAcquire lock(once->mutex);
if (!once->done) {
// Also known as a store-store/load-store barrier, this makes sure that the writes
// done before here---in particular, those done by calling f(arg)---are observable
// before the writes after the line, *done = true.
// In version control terms this is like saying, "check in the work up
// to and including f(arg), then check in *done=true as a subsequent change".
// We'll use this in the fast path to make sure f(arg)'s effects are
// observable whenever we observe *done == true.
once->done = true;
// We nabbed this code from the dynamic_annotations library, and in their honor
// we check the same define. If you find yourself wanting more than just
// ANNOTATE_BENIGN_RACE, it might make sense to pull that in as a dependency
// rather than continue to reproduce it here.
// TSAN provides this hook to supress a known-safe apparent race.
extern "C" {
void AnnotateBenignRace(const char* file, int line, const volatile void* mem, const char* desc);
#define ANNOTATE_BENIGN_RACE(mem, desc) AnnotateBenignRace(__FILE__, __LINE__, mem, desc)
#define ANNOTATE_BENIGN_RACE(mem, desc)
// This is our fast path, called all the time. We do really want it to be inlined.
template <typename Func, typename Arg>
inline void SkOnce(SkOnceFlag* once, Func f, Arg arg) {
ANNOTATE_BENIGN_RACE(&(once->done), "Don't worry TSAN, we're sure this is safe.");
if (!once->done) {
sk_once_slow(once, f, arg);
// Also known as a load-load/load-store barrier, this acquire barrier makes
// sure that anything we read from memory---in particular, memory written by
// calling f(arg)---is at least as current as the value we read from once->done.
// In version control terms, this is a lot like saying "sync up to the
// commit where we wrote once->done = true".
// The release barrier in sk_once_slow guaranteed that once->done = true
// happens after f(arg), so by syncing to once->done = true here we're
// forcing ourselves to also wait until the effects of f(arg) are readble.
#endif // SkOnce_DEFINED