| /* |
| * 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 |
| |
| // Before trying SkOnce, see if SkLazyPtr or SkLazyFnPtr will work for you. |
| // They're smaller and faster, if slightly less versatile. |
| |
| |
| // 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. E.g. |
| // |
| // static void register_my_stuff(GlobalRegistry* registry) { |
| // registry->register(...); |
| // } |
| // ... |
| // void EnsureRegistered() { |
| // SK_DECLARE_STATIC_ONCE(once); |
| // SkOnce(&once, register_my_stuff, GetGlobalRegistry()); |
| // } |
| // |
| // No matter how many times you call EnsureRegistered(), register_my_stuff will be called just once. |
| // OnceTest.cpp also should serve as a few other simple examples. |
| |
| #include "../private/SkAtomics.h" |
| #include "../private/SkSpinlock.h" |
| |
| // This must be used in a global scope, not in function scope or as a class member. |
| #define SK_DECLARE_STATIC_ONCE(name) namespace {} static SkOnceFlag name |
| |
| class SkOnceFlag; |
| |
| inline void SkOnce(SkOnceFlag* once, void (*f)()); |
| |
| template <typename Arg> |
| inline void SkOnce(SkOnceFlag* once, void (*f)(Arg), Arg arg); |
| |
| // If you've already got a lock and a flag to use, this variant lets you avoid an extra SkOnceFlag. |
| template <typename Lock> |
| inline void SkOnce(bool* done, Lock* lock, void (*f)()); |
| |
| template <typename Lock, typename Arg> |
| inline void SkOnce(bool* done, Lock* lock, void (*f)(Arg), Arg arg); |
| |
| // ---------------------- Implementation details below here. ----------------------------- |
| |
| // This class has no constructor and must be zero-initialized (the macro above does this). |
| class SkOnceFlag { |
| public: |
| bool* mutableDone() { return &fDone; } |
| |
| void acquire() { fSpinlock.acquire(); } |
| void release() { fSpinlock.release(); } |
| |
| private: |
| bool fDone; |
| SkPODSpinlock fSpinlock; |
| }; |
| |
| // 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 Lock, typename Arg> |
| static void sk_once_slow(bool* done, Lock* lock, void (*f)(Arg), Arg arg) { |
| lock->acquire(); |
| if (!sk_atomic_load(done, sk_memory_order_relaxed)) { |
| 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. |
| sk_release_store(done, true); |
| } |
| lock->release(); |
| } |
| |
| // This is our fast path, called all the time. We do really want it to be inlined. |
| template <typename Lock, typename Arg> |
| inline void SkOnce(bool* done, Lock* lock, void (*f)(Arg), Arg arg) { |
| // When *done == true: |
| // 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 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 f(arg), so by syncing to done = true here we're |
| // forcing ourselves to also wait until the effects of f(arg) are readble. |
| // |
| // When *done == false: |
| // We'll try to call f(arg) in sk_once_slow. |
| // If we get the lock, great, we call f(arg), release true into done, and drop the lock. |
| // If we race and don't get the lock first, we'll wait for the first guy to finish. |
| // Then lock acquire() will give us at least an acquire memory barrier to get the same |
| // effect as the acquire load in the *done == true fast case. We'll see *done is true, |
| // then just drop the lock and return. |
| if (!sk_atomic_load(done, sk_memory_order_acquire)) { |
| sk_once_slow(done, lock, f, arg); |
| } |
| } |
| |
| template <typename Arg> |
| inline void SkOnce(SkOnceFlag* once, void (*f)(Arg), Arg arg) { |
| return SkOnce(once->mutableDone(), once, f, arg); |
| } |
| |
| // Calls its argument. |
| // This lets us use functions that take no arguments with SkOnce methods above. |
| // (We pass _this_ as the function and the no-arg function as its argument. Cute eh?) |
| static void sk_once_no_arg_adaptor(void (*f)()) { |
| f(); |
| } |
| |
| inline void SkOnce(SkOnceFlag* once, void (*func)()) { |
| return SkOnce(once, sk_once_no_arg_adaptor, func); |
| } |
| |
| template <typename Lock> |
| inline void SkOnce(bool* done, Lock* lock, void (*func)()) { |
| return SkOnce(done, lock, sk_once_no_arg_adaptor, func); |
| } |
| |
| #endif // SkOnce_DEFINED |