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d0b1ad675d
Summary: I'd misunderstood the point of SharedMutex's upgrade locks - unless they're used in rare paths, they don't allow for increased concurrency. This diff and D7885245 remove all of Eden's ulocks, replacing them with a helper which checks once with an rlock held, and if the check fails, switches to a wlock (and checks again). Reviewed By: yfeldblum Differential Revision: D7886046 fbshipit-source-id: 545bb0dbb4898cbb71412efc6222ef12e4ee374e
53 lines
1.5 KiB
C++
53 lines
1.5 KiB
C++
/*
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* Copyright (c) 2018-present, Facebook, Inc.
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* All rights reserved.
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*
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* This source code is licensed under the BSD-style license found in the
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* LICENSE file in the root directory of this source tree. An additional grant
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* of patent rights can be found in the PATENTS file in the same directory.
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*
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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&) -> folly::Optional<T>
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* update should have type (State&) -> 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 *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 *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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