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/*
* Copyright (c) 2020, Sergey Bugaev <bugaevc@serenityos.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#include <AK/Assertions.h>
#include <AK/Atomic.h>
#include <AK/Types.h>
#include <pthread.h>
#include <serenity.h>
enum State : i32 {
INITIAL = PTHREAD_ONCE_INIT,
DONE,
PERFORMING_NO_WAITERS,
PERFORMING_WITH_WAITERS,
};
// https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_once.html
int pthread_once(pthread_once_t* self, void (*callback)(void))
{
auto& state = reinterpret_cast<Atomic<State>&>(*self);
// See what the current state is, and at the same time grab the lock if we
// got here first. We need acquire ordering here because if we see
// State::DONE, everything we do after that should "happen after" everything
// the other thread has done before writing the State::DONE.
State state2 = State::INITIAL;
bool have_exchanged = state.compare_exchange_strong(
state2, State::PERFORMING_NO_WAITERS, AK::memory_order_acquire);
if (have_exchanged) {
// We observed State::INITIAL and we've changed it to
// State::PERFORMING_NO_WAITERS, so it's us who should perform the
// operation.
callback();
// Now, record that we're done.
state2 = state.exchange(State::DONE, AK::memory_order_release);
switch (state2) {
case State::INITIAL:
case State::DONE:
VERIFY_NOT_REACHED();
case State::PERFORMING_NO_WAITERS:
// The fast path: there's no contention, so we don't have to wake
// anyone.
break;
case State::PERFORMING_WITH_WAITERS:
futex_wake(self, INT_MAX);
break;
}
return 0;
}
// We did not get there first. Let's see if we have to wait.
// state2 contains the observed state.
while (true) {
switch (state2) {
case State::INITIAL:
VERIFY_NOT_REACHED();
case State::DONE:
// Awesome, nothing to do then.
return 0;
case State::PERFORMING_NO_WAITERS:
// We're going to wait for it, but we have to record that we're
// waiting and the other thread should wake us up. We need acquire
// ordering here for the same reason as above.
have_exchanged = state.compare_exchange_strong(
state2, State::PERFORMING_WITH_WAITERS, AK::memory_order_acquire);
if (!have_exchanged) {
// Something has changed already, reevaluate without waiting.
continue;
}
state2 = State::PERFORMING_WITH_WAITERS;
[[fallthrough]];
case State::PERFORMING_WITH_WAITERS:
// Let's wait for it.
futex_wait(self, state2, nullptr, 0);
// We have been woken up, but that might have been due to a signal
// or something, so we have to reevaluate. We need acquire ordering
// here for the same reason as above. Hopefully we'll just see
// State::DONE this time, but who knows.
state2 = state.load(AK::memory_order_acquire);
continue;
}
}
}
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