Index: include/private/SkOnce.h |
diff --git a/include/private/SkOnce.h b/include/private/SkOnce.h |
index d83b63f2837382f0179a0c987a67b8a2f979f3b7..1c68fb7da1396771aa3bed37365b321111270dde 100644 |
--- a/include/private/SkOnce.h |
+++ b/include/private/SkOnce.h |
@@ -8,9 +8,9 @@ |
#ifndef SkOnce_DEFINED |
#define SkOnce_DEFINED |
-#include "../private/SkSpinlock.h" |
#include <atomic> |
#include <utility> |
+#include "SkTypes.h" |
// SkOnce provides call-once guarantees for Skia, much like std::once_flag/std::call_once(). |
// |
@@ -21,20 +21,50 @@ class SkOnce { |
public: |
template <typename Fn, typename... Args> |
void operator()(Fn&& fn, Args&&... args) { |
- // Vanilla double-checked locking. |
- if (!fDone.load(std::memory_order_acquire)) { |
- fLock.acquire(); |
- if (!fDone.load(std::memory_order_relaxed)) { |
+ auto state = fState.load(std::memory_order_acquire); |
+ |
+ if (state == Done) { |
+ return; |
+ } |
+ |
+ if (state == NotStarted) { |
+ // Try to claim the job of calling fn() by swapping from NotStarted to Calling. |
+ // See [1] below for why we use std::memory_order_acquire instead of relaxed. |
+ if (fState.compare_exchange_strong(state, Calling, std::memory_order_acquire)) { |
+ // Claimed! Call fn(), then mark this SkOnce as Done. |
fn(std::forward<Args>(args)...); |
- fDone.store(true, std::memory_order_release); |
+ return fState.store(Done, std::memory_order_release); |
} |
- fLock.release(); |
} |
+ |
+ while (state == Calling) { |
+ // Some other thread is calling fn(). Wait for them to finish. |
+ state = fState.load(std::memory_order_acquire); |
+ } |
+ SkASSERT(state == Done); |
} |
private: |
- std::atomic<bool> fDone{false}; |
- SkSpinlock fLock; |
+ enum State : uint8_t { NotStarted, Calling, Done}; |
+ std::atomic<uint8_t> fState{NotStarted}; |
}; |
+/* [1] Why do we compare_exchange_strong() with std::memory_order_acquire instead of relaxed? |
+ * |
+ * If we succeed, we really only need a relaxed compare_exchange_strong()... we're the ones |
+ * who are about to do a release store, so there's certainly nothing yet for an acquire to |
+ * synchronize with. |
+ * |
+ * If that compare_exchange_strong() fails, we're either in Calling or Done state. |
+ * Again, if we're in Calling state, relaxed would have been fine: the spin loop will |
+ * acquire up to the Calling thread's release store. |
+ * |
+ * But if that compare_exchange_strong() fails and we find ourselves in the Done state, |
+ * we've never done an acquire load to sync up to the store of that Done state. |
+ * |
+ * So on failure we need an acquire load. Generally the failure memory order cannot be |
+ * stronger than the success memory order, so we need acquire on success too. The single |
+ * memory order version of compare_exchange_strong() uses the same acquire order for both. |
+ */ |
+ |
#endif // SkOnce_DEFINED |