| /* |
| * 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; |
| // SK_DECLARE_STATIC_ONCE(once); |
| // 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, { 0, SkDEBUGCODE(0) } } |
| #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; |
| SkSpinlock lock; |
| }; |
| |
| // 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 |
| # if SK_ARM_ARCH >= 7 |
| asm volatile("dmb" : : : "memory"); |
| # else |
| asm volatile("mcr p15, 0, %0, c7, c10, 5" : : "r" (0) : "memory"); |
| # endif |
| #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 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 SkAutoSpinlock lock(&once->lock); |
| if (!once->done) { |
| f(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 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. |
| release_barrier(); |
| 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. |
| |
| #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 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. |
| acquire_barrier(); |
| } |
| |
| #undef ANNOTATE_BENIGN_RACE |
| |
| #endif // SkOnce_DEFINED |