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 "SkAtomics.h"
31#include "SkSpinlock.h"
32
33// This must be used in a global scope, not in function scope or as a class member.
34#define SK_DECLARE_STATIC_ONCE(name) namespace {} static SkOnceFlag name
35
36class SkOnceFlag;
37
38inline void SkOnce(SkOnceFlag* once, void (*f)());
39
40template <typename Arg>
41inline void SkOnce(SkOnceFlag* once, void (*f)(Arg), Arg arg);
42
43// If you've already got a lock and a flag to use, this variant lets you avoid an extra SkOnceFlag.
44template <typename Lock>
45inline void SkOnce(bool* done, Lock* lock, void (*f)());
46
47template <typename Lock, typename Arg>
48inline void SkOnce(bool* done, Lock* lock, void (*f)(Arg), Arg arg);
49
50//  ----------------------  Implementation details below here. -----------------------------
51
52// This class has no constructor and must be zero-initialized (the macro above does this).
53class SkOnceFlag {
54public:
55    bool* mutableDone() { return &fDone; }
56
57    void acquire() { fSpinlock.acquire(); }
58    void release() { fSpinlock.release(); }
59
60private:
61    bool fDone;
62    SkPODSpinlock fSpinlock;
63};
64
65// We've pulled a pretty standard double-checked locking implementation apart
66// into its main fast path and a slow path that's called when we suspect the
67// one-time code hasn't run yet.
68
69// This is the guts of the code, called when we suspect the one-time code hasn't been run yet.
70// This should be rarely called, so we separate it from SkOnce and don't mark it as inline.
71// (We don't mind if this is an actual function call, but odds are it'll be inlined anyway.)
72template <typename Lock, typename Arg>
73static void sk_once_slow(bool* done, Lock* lock, void (*f)(Arg), Arg arg) {
74    lock->acquire();
75    if (!sk_atomic_load(done, sk_memory_order_relaxed)) {
76        f(arg);
77        // Also known as a store-store/load-store barrier, this makes sure that the writes
78        // done before here---in particular, those done by calling f(arg)---are observable
79        // before the writes after the line, *done = true.
80        //
81        // In version control terms this is like saying, "check in the work up
82        // to and including f(arg), then check in *done=true as a subsequent change".
83        //
84        // We'll use this in the fast path to make sure f(arg)'s effects are
85        // observable whenever we observe *done == true.
86        sk_release_store(done, true);
87    }
88    lock->release();
89}
90
91// This is our fast path, called all the time.  We do really want it to be inlined.
92template <typename Lock, typename Arg>
93inline void SkOnce(bool* done, Lock* lock, void (*f)(Arg), Arg arg) {
94    // When *done == true:
95    //   Also known as a load-load/load-store barrier, this acquire barrier makes
96    //   sure that anything we read from memory---in particular, memory written by
97    //   calling f(arg)---is at least as current as the value we read from done.
98    //
99    //   In version control terms, this is a lot like saying "sync up to the
100    //   commit where we wrote done = true".
101    //
102    //   The release barrier in sk_once_slow guaranteed that done = true
103    //   happens after f(arg), so by syncing to done = true here we're
104    //   forcing ourselves to also wait until the effects of f(arg) are readble.
105    //
106    // When *done == false:
107    //   We'll try to call f(arg) in sk_once_slow.
108    //   If we get the lock, great, we call f(arg), release true into done, and drop the lock.
109    //   If we race and don't get the lock first, we'll wait for the first guy to finish.
110    //   Then lock acquire() will give us at least an acquire memory barrier to get the same
111    //   effect as the acquire load in the *done == true fast case.  We'll see *done is true,
112    //   then just drop the lock and return.
113    if (!sk_atomic_load(done, sk_memory_order_acquire)) {
114        sk_once_slow(done, lock, f, arg);
115    }
116}
117
118template <typename Arg>
119inline void SkOnce(SkOnceFlag* once, void (*f)(Arg), Arg arg) {
120    return SkOnce(once->mutableDone(), once, f, arg);
121}
122
123// Calls its argument.
124// This lets us use functions that take no arguments with SkOnce methods above.
125// (We pass _this_ as the function and the no-arg function as its argument.  Cute eh?)
126static void sk_once_no_arg_adaptor(void (*f)()) {
127    f();
128}
129
130inline void SkOnce(SkOnceFlag* once, void (*func)()) {
131    return SkOnce(once, sk_once_no_arg_adaptor, func);
132}
133
134template <typename Lock>
135inline void SkOnce(bool* done, Lock* lock, void (*func)()) {
136    return SkOnce(done, lock, sk_once_no_arg_adaptor, func);
137}
138
139#endif  // SkOnce_DEFINED
140