b37c68ad42
I think this version reads more clearly, and the key invariants are expressed in code rather than comments: - race losers always go through an acquire - we never exit the function unless fState is Done BUG=skia: GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1951013004 Review-Url: https://codereview.chromium.org/1951013004
51 lines
1.6 KiB
C++
51 lines
1.6 KiB
C++
/*
|
|
* 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
|
|
|
|
#include <atomic>
|
|
#include <utility>
|
|
#include "SkTypes.h"
|
|
|
|
// SkOnce provides call-once guarantees for Skia, much like std::once_flag/std::call_once().
|
|
//
|
|
// There should be no particularly error-prone gotcha use cases when using SkOnce.
|
|
// It works correctly as a class member, a local, a global, a function-scoped static, whatever.
|
|
|
|
class SkOnce {
|
|
public:
|
|
constexpr SkOnce() = default;
|
|
|
|
template <typename Fn, typename... Args>
|
|
void operator()(Fn&& fn, Args&&... args) {
|
|
auto state = fState.load(std::memory_order_acquire);
|
|
|
|
if (state == Done) {
|
|
return;
|
|
}
|
|
|
|
// If it looks like no one has started calling fn(), try to claim that job.
|
|
if (state == NotStarted && fState.compare_exchange_strong(state, Claimed,
|
|
std::memory_order_relaxed)) {
|
|
// Great! We'll run fn() then notify the other threads by releasing Done into fState.
|
|
fn(std::forward<Args>(args)...);
|
|
return fState.store(Done, std::memory_order_release);
|
|
}
|
|
|
|
// Some other thread is calling fn().
|
|
// We'll just spin here acquiring until it releases Done into fState.
|
|
while (fState.load(std::memory_order_acquire) != Done) { /*spin*/ }
|
|
}
|
|
|
|
private:
|
|
enum State : uint8_t { NotStarted, Claimed, Done};
|
|
std::atomic<uint8_t> fState{NotStarted};
|
|
};
|
|
|
|
#endif // SkOnce_DEFINED
|