1 /* 2 * Copyright 2013 Google Inc. 3 * 4 * Use of this source code is governed by a BSD-style license that can be 5 * found in the LICENSE file. 6 */ 7 8 #ifndef SkOnce_DEFINED 9 #define SkOnce_DEFINED 10 11 // Before trying SkOnce, see if SkLazyPtr or SkLazyFnPtr will work for you. 12 // They're smaller and faster, if slightly less versatile. 13 14 15 // SkOnce.h defines SK_DECLARE_STATIC_ONCE and SkOnce(), which you can use 16 // together to create a threadsafe way to call a function just once. E.g. 17 // 18 // static void register_my_stuff(GlobalRegistry* registry) { 19 // registry->register(...); 20 // } 21 // ... 22 // void EnsureRegistered() { 23 // SK_DECLARE_STATIC_ONCE(once); 24 // SkOnce(&once, register_my_stuff, GetGlobalRegistry()); 25 // } 26 // 27 // No matter how many times you call EnsureRegistered(), register_my_stuff will be called just once. 28 // OnceTest.cpp also should serve as a few other simple examples. 29 30 #include "SkDynamicAnnotations.h" 31 #include "SkThread.h" 32 #include "SkTypes.h" 33 34 // This must be used in a global or function scope, not as a class member. 35 #define SK_DECLARE_STATIC_ONCE(name) static SkOnceFlag name 36 37 class SkOnceFlag; 38 39 inline void SkOnce(SkOnceFlag* once, void (*f)()); 40 41 template <typename Arg> 42 inline void SkOnce(SkOnceFlag* once, void (*f)(Arg), Arg arg); 43 44 // If you've already got a lock and a flag to use, this variant lets you avoid an extra SkOnceFlag. 45 template <typename Lock> 46 inline void SkOnce(bool* done, Lock* lock, void (*f)()); 47 48 template <typename Lock, typename Arg> 49 inline void SkOnce(bool* done, Lock* lock, void (*f)(Arg), Arg arg); 50 51 // ---------------------- Implementation details below here. ----------------------------- 52 53 // This class has no constructor and must be zero-initialized (the macro above does this). 54 class SkOnceFlag { 55 public: 56 bool* mutableDone() { return &fDone; } 57 58 void acquire() { 59 // To act as a mutex, this needs an acquire barrier on success. 60 // sk_atomic_cas doesn't guarantee this ... 61 while (!sk_atomic_cas(&fSpinlock, 0, 1)) { 62 // spin 63 } 64 // ... so make sure to issue one of our own. 65 SkAssertResult(sk_acquire_load(&fSpinlock)); 66 } 67 68 void release() { 69 // To act as a mutex, this needs a release barrier. sk_atomic_cas guarantees this. 70 SkAssertResult(sk_atomic_cas(&fSpinlock, 1, 0)); 71 } 72 73 private: 74 bool fDone; 75 int32_t fSpinlock; 76 }; 77 78 // We've pulled a pretty standard double-checked locking implementation apart 79 // into its main fast path and a slow path that's called when we suspect the 80 // one-time code hasn't run yet. 81 82 // This is the guts of the code, called when we suspect the one-time code hasn't been run yet. 83 // This should be rarely called, so we separate it from SkOnce and don't mark it as inline. 84 // (We don't mind if this is an actual function call, but odds are it'll be inlined anyway.) 85 template <typename Lock, typename Arg> 86 static void sk_once_slow(bool* done, Lock* lock, void (*f)(Arg), Arg arg) { 87 lock->acquire(); 88 if (!*done) { 89 f(arg); 90 // Also known as a store-store/load-store barrier, this makes sure that the writes 91 // done before here---in particular, those done by calling f(arg)---are observable 92 // before the writes after the line, *done = true. 93 // 94 // In version control terms this is like saying, "check in the work up 95 // to and including f(arg), then check in *done=true as a subsequent change". 96 // 97 // We'll use this in the fast path to make sure f(arg)'s effects are 98 // observable whenever we observe *done == true. 99 sk_release_store(done, true); 100 } 101 lock->release(); 102 } 103 104 // This is our fast path, called all the time. We do really want it to be inlined. 105 template <typename Lock, typename Arg> 106 inline void SkOnce(bool* done, Lock* lock, void (*f)(Arg), Arg arg) { 107 if (!SK_ANNOTATE_UNPROTECTED_READ(*done)) { 108 sk_once_slow(done, lock, f, arg); 109 } 110 // Also known as a load-load/load-store barrier, this acquire barrier makes 111 // sure that anything we read from memory---in particular, memory written by 112 // calling f(arg)---is at least as current as the value we read from done. 113 // 114 // In version control terms, this is a lot like saying "sync up to the 115 // commit where we wrote done = true". 116 // 117 // The release barrier in sk_once_slow guaranteed that done = true 118 // happens after f(arg), so by syncing to done = true here we're 119 // forcing ourselves to also wait until the effects of f(arg) are readble. 120 SkAssertResult(sk_acquire_load(done)); 121 } 122 123 template <typename Arg> 124 inline void SkOnce(SkOnceFlag* once, void (*f)(Arg), Arg arg) { 125 return SkOnce(once->mutableDone(), once, f, arg); 126 } 127 128 // Calls its argument. 129 // This lets us use functions that take no arguments with SkOnce methods above. 130 // (We pass _this_ as the function and the no-arg function as its argument. Cute eh?) 131 static void sk_once_no_arg_adaptor(void (*f)()) { 132 f(); 133 } 134 135 inline void SkOnce(SkOnceFlag* once, void (*func)()) { 136 return SkOnce(once, sk_once_no_arg_adaptor, func); 137 } 138 139 template <typename Lock> 140 inline void SkOnce(bool* done, Lock* lock, void (*func)()) { 141 return SkOnce(done, lock, sk_once_no_arg_adaptor, func); 142 } 143 144 #endif // SkOnce_DEFINED 145
