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/*
* Copyright (c) 2020, Sergey Bugaev <bugaevc@serenityos.org>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice, this
* list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#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,
};
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:
ASSERT_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(self, FUTEX_WAKE, INT_MAX, nullptr, nullptr, 0);
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:
ASSERT_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(self, FUTEX_WAIT, state2, nullptr, 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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