diff options
author | mtklein <mtklein@chromium.org> | 2016-05-04 13:57:30 -0700 |
---|---|---|
committer | Commit bot <commit-bot@chromium.org> | 2016-05-04 13:57:30 -0700 |
commit | b37c68ad42ae7880e702649a01655165c7e12acc (patch) | |
tree | 0a322e08933a489cb0000d6d7d55798f88a2a65b /include/private/SkOnce.h | |
parent | 06077565b18714ff3fc0db9118e2c21f6f25243f (diff) |
Simplify implementation of SkOnce to not need so many comments.
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
Diffstat (limited to 'include/private/SkOnce.h')
-rw-r--r-- | include/private/SkOnce.h | 42 |
1 files changed, 10 insertions, 32 deletions
diff --git a/include/private/SkOnce.h b/include/private/SkOnce.h index 507dcb5b8c..65334e3a26 100644 --- a/include/private/SkOnce.h +++ b/include/private/SkOnce.h @@ -29,44 +29,22 @@ public: 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)...); - return fState.store(Done, std::memory_order_release); - } + // 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); } - while (state == Calling) { - // Some other thread is calling fn(). Wait for them to finish. - state = fState.load(std::memory_order_acquire); - } - SkASSERT(state == Done); + // 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, Calling, Done}; + enum State : uint8_t { NotStarted, Claimed, 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 |