| /* |
| * 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 two macros, DEF_SK_ONCE and SK_ONCE. |
| // You can use these macros together to create a threadsafe block of code that |
| // runs at most once, no matter how many times you call it. This is |
| // particularly useful for lazy singleton initialization. E.g. |
| // |
| // DEF_SK_ONCE(set_up_my_singleton, SingletonType* singleton) { |
| // // Code in this block will run at most once. |
| // *singleton = new Singleton(...); |
| // } |
| // ... |
| // const Singleton& getSingleton() { |
| // static Singleton* singleton = NULL; |
| // // Always call SK_ONCE. It's very cheap to call after the first time. |
| // SK_ONCE(set_up_my_singleton, singleton); |
| // SkASSERT(NULL != singleton); |
| // return *singleton; |
| // } |
| // |
| // OnceTest.cpp also should serve as another simple example. |
| |
| #include "SkThread.h" |
| #include "SkTypes.h" |
| |
| |
| // Pass a unique name (at least in this scope) for name, and a type and name |
| // for arg (as if writing a function declaration). |
| // E.g. |
| // DEF_SK_ONCE(my_onetime_setup, int* foo) { |
| // *foo += 5; |
| // } |
| #define DEF_SK_ONCE(name, arg) \ |
| static bool sk_once_##name##_done = false; \ |
| SK_DECLARE_STATIC_MUTEX(sk_once_##name##_mutex); \ |
| static void sk_once_##name##_function(arg) |
| |
| // Call this anywhere you need to guarantee that the corresponding DEF_SK_ONCE |
| // block of code has run. name should match the DEF_SK_ONCE, and here you pass |
| // the actual value of the argument. |
| // E.g |
| // int foo = 0; |
| // SK_ONCE(my_onetime_setup, &foo); |
| // SkASSERT(5 == foo); |
| #define SK_ONCE(name, arg) \ |
| sk_once(&sk_once_##name##_done, &sk_once_##name##_mutex, sk_once_##name##_function, arg) |
| |
| |
| // ---------------------- Implementation details below here. ----------------------------- |
| |
| |
| // TODO(bungeman, mtklein): move all these *barrier* functions to SkThread when refactoring lands. |
| |
| #ifdef SK_BUILD_FOR_WIN |
| #include <intrin.h> |
| inline static void compiler_barrier() { |
| _ReadWriteBarrier(); |
| } |
| #else |
| inline static void compiler_barrier() { |
| asm volatile("" : : : "memory"); |
| } |
| #endif |
| |
| inline static void full_barrier_on_arm() { |
| #ifdef SK_CPU_ARM |
| asm volatile("dmb" : : : "memory"); |
| #endif |
| } |
| |
| // 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() { |
| compiler_barrier(); |
| full_barrier_on_arm(); |
| } |
| |
| inline static void acquire_barrier() { |
| compiler_barrier(); |
| full_barrier_on_arm(); |
| } |
| |
| // 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 sk_once 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 Arg> |
| static void sk_once_slow(bool* done, SkBaseMutex* mutex, void (*once)(Arg), Arg arg) { |
| const SkAutoMutexAcquire lock(*mutex); |
| if (!*done) { |
| once(arg); |
| // Also known as a store-store/load-store barrier, this makes sure that the writes |
| // done before here---in particular, those done by calling once(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 once(arg), then check in *done=true as a subsequent change". |
| // |
| // We'll use this in the fast path to make sure once(arg)'s effects are |
| // observable whenever we observe *done == true. |
| release_barrier(); |
| *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. |
| |
| #if DYNAMIC_ANNOTATIONS_ENABLED |
| // 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) |
| #else |
| #define ANNOTATE_BENIGN_RACE(mem, desc) |
| #endif |
| |
| // This is our fast path, called all the time. We do really want it to be inlined. |
| template <typename Arg> |
| inline static void sk_once(bool* done, SkBaseMutex* mutex, void (*once)(Arg), Arg arg) { |
| ANNOTATE_BENIGN_RACE(done, "Don't worry TSAN, we're sure this is safe."); |
| if (!*done) { |
| sk_once_slow(done, mutex, once, 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 once(arg)---is at least as current as the value we read from done. |
| // |
| // In version control terms, this is a lot like saying "sync up to the |
| // commit where we wrote *done = true". |
| // |
| // The release barrier in sk_once_slow guaranteed that *done = true |
| // happens after once(arg), so by syncing to *done = true here we're |
| // forcing ourselves to also wait until the effects of once(arg) are readble. |
| acquire_barrier(); |
| } |
| |
| #undef ANNOTATE_BENIGN_RACE |
| |
| |
| #endif // SkOnce_DEFINED |