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fbdb46f5cb
Reviewed By: chadaustin Differential Revision: D17872966 fbshipit-source-id: cd60a364a2146f0dadbeca693b1d4a5d7c97ff63
51 lines
1.4 KiB
C++
51 lines
1.4 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* This software may be used and distributed according to the terms of the
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* GNU General Public License version 2.
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*/
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#pragma once
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#include <folly/Synchronized.h>
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namespace facebook {
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namespace eden {
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/**
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* Helper function that optimizes for the case where a read-only check on a
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* contended data structure is likely to succeed. It first acquires the
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* synchronized object with an rlock. If check returns a true-ish value, then
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* the result of dereferencing it is returned. Otherwise, a wlock is acquired
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* and update is called.
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*
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* check should have type (const State&) -> std::optional<T>
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* update should have type (LockedPtr&) -> T
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*/
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template <typename Return, typename State, typename CheckFn, typename UpdateFn>
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Return tryRlockCheckBeforeUpdate(
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folly::Synchronized<State>& state,
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CheckFn&& check,
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UpdateFn&& update) {
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// First, acquire the rlock. If the check succeeds, acquiring a wlock is
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// unnecessary.
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{
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auto rlock = state.rlock();
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auto result = check(*rlock);
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if (LIKELY(bool(result))) {
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return *std::move(result);
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}
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}
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auto wlock = state.wlock();
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// Check again - something may have raced between the locks.
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auto result = check(*wlock);
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if (UNLIKELY(bool(result))) {
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return *std::move(result);
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}
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return update(wlock);
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}
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} // namespace eden
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} // namespace facebook
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