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SPDX License Identifiers are a more compact / standardized way of representing file license information. See: https://spdx.dev/resources/use/#identifiers This was done with the `ambr` search and replace tool. ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
88 lines
3.1 KiB
C++
88 lines
3.1 KiB
C++
/*
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* Copyright (c) 2020, Sergey Bugaev <bugaevc@serenityos.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#include <AK/Assertions.h>
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#include <AK/Atomic.h>
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#include <AK/Types.h>
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#include <pthread.h>
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#include <serenity.h>
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enum State : i32 {
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INITIAL = PTHREAD_ONCE_INIT,
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DONE,
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PERFORMING_NO_WAITERS,
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PERFORMING_WITH_WAITERS,
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};
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int pthread_once(pthread_once_t* self, void (*callback)(void))
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{
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auto& state = reinterpret_cast<Atomic<State>&>(*self);
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// See what the current state is, and at the same time grab the lock if we
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// got here first. We need acquire ordering here because if we see
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// State::DONE, everything we do after that should "happen after" everything
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// the other thread has done before writing the State::DONE.
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State state2 = State::INITIAL;
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bool have_exchanged = state.compare_exchange_strong(
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state2, State::PERFORMING_NO_WAITERS, AK::memory_order_acquire);
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if (have_exchanged) {
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// We observed State::INITIAL and we've changed it to
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// State::PERFORMING_NO_WAITERS, so it's us who should perform the
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// operation.
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callback();
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// Now, record that we're done.
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state2 = state.exchange(State::DONE, AK::memory_order_release);
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switch (state2) {
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case State::INITIAL:
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case State::DONE:
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VERIFY_NOT_REACHED();
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case State::PERFORMING_NO_WAITERS:
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// The fast path: there's no contention, so we don't have to wake
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// anyone.
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break;
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case State::PERFORMING_WITH_WAITERS:
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futex(self, FUTEX_WAKE, INT_MAX, nullptr, nullptr, 0);
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break;
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}
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return 0;
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}
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// We did not get there first. Let's see if we have to wait.
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// state2 contains the observed state.
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while (true) {
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switch (state2) {
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case State::INITIAL:
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VERIFY_NOT_REACHED();
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case State::DONE:
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// Awesome, nothing to do then.
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return 0;
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case State::PERFORMING_NO_WAITERS:
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// We're going to wait for it, but we have to record that we're
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// waiting and the other thread should wake us up. We need acquire
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// ordering here for the same reason as above.
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have_exchanged = state.compare_exchange_strong(
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state2, State::PERFORMING_WITH_WAITERS, AK::memory_order_acquire);
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if (!have_exchanged) {
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// Something has changed already, reevaluate without waiting.
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continue;
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}
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state2 = State::PERFORMING_WITH_WAITERS;
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[[fallthrough]];
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case State::PERFORMING_WITH_WAITERS:
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// Let's wait for it.
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futex(self, FUTEX_WAIT, state2, nullptr, nullptr, 0);
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// We have been woken up, but that might have been due to a signal
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// or something, so we have to reevaluate. We need acquire ordering
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// here for the same reason as above. Hopefully we'll just see
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// State::DONE this time, but who knows.
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state2 = state.load(AK::memory_order_acquire);
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continue;
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}
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}
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}
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