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/**
* Copyright (C) 2023-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#define MONGO_LOGV2_DEFAULT_COMPONENT ::mongo::logv2::LogComponent::kDefault
#include "ticket_pool.h"
// TODO SERVER-72616: Remove futex usage from this class in favour of atomic waits.
#include <linux/futex.h> /* Definition of FUTEX_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
#include "mongo/logv2/log.h"
#include "mongo/platform/atomic_word.h"
#include "mongo/platform/mutex.h"
#include "mongo/stdx/condition_variable.h"
#include "mongo/util/errno_util.h"
namespace mongo {
namespace {
static int futex(uint32_t* uaddr,
int futex_op,
uint32_t val,
const struct timespec* timeout,
uint32_t* uaddr2,
uint32_t val3) noexcept {
return syscall(SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
}
// TODO SERVER-72616: This can go away once we're on C++20 and std::atomic<T>::wait exists
static stdx::cv_status atomic_wait(AtomicWord<uint32_t>& atomic,
uint32_t expectedValue,
Date_t until) noexcept {
while (atomic.load() == expectedValue) {
// Prepare the timeout value for the futex call.
timespec ts;
auto now = Date_t::now();
if (now >= until) {
return stdx::cv_status::timeout;
}
auto millis = until - now;
ts.tv_sec = millis.count() / 1'000;
ts.tv_nsec = (millis.count() % 1'000) * 1'000'000;
auto futexResult = futex(reinterpret_cast<uint32_t*>(&atomic),
FUTEX_WAIT_PRIVATE,
expectedValue,
&ts,
nullptr,
0);
if (futexResult != 0) {
switch (errno) {
// The value has changed before we called futex wait, we treat this as a
// notification and exit.
case EAGAIN:
return stdx::cv_status::no_timeout;
case ETIMEDOUT:
return stdx::cv_status::timeout;
// We ignore signal interruptions as other signals are handled by either crashing
// the server or gracefully exiting the server and waiting for operations to finish.
case EINTR:
break;
// All other errors are unrecoverable, fassert and crash the server.
default: {
LOGV2_FATAL(7206704,
"Error in atomic wait for ticket",
"error"_attr = errorMessage(posixError(errno)));
}
}
}
}
return stdx::cv_status::no_timeout;
}
// TODO SERVER-72616: This can go away once we're on C++20 and std::atomic<T>::notify_one exists.
static void atomic_notify_one(AtomicWord<uint32_t>& atomic) noexcept {
auto result =
futex(reinterpret_cast<uint32_t*>(&atomic), FUTEX_WAKE_PRIVATE, 1, nullptr, nullptr, 0);
if (result < 0) {
// We treat possible errors here as a server crash since we cannot recover from them.
LOGV2_FATAL(7206703,
"Error in atomic notify for ticket",
"error"_attr = errorMessage(posixError(errno)));
}
}
} // namespace
template <class Queue>
bool TicketPool<Queue>::tryAcquire() {
auto available = _available.load();
bool gotTicket = false;
while (available > 0 && !gotTicket) {
gotTicket = _available.compareAndSwap(&available, available - 1);
}
return gotTicket;
}
template <class Queue>
bool TicketPool<Queue>::acquire(AdmissionContext* admCtx, Date_t deadline) {
auto waiter = std::make_shared<TicketWaiter>();
waiter->context = admCtx;
{
stdx::unique_lock<Mutex> lk(_mutex);
// It is important to check for a ticket one more time before queueing, as a ticket may have
// just become available.
if (tryAcquire()) {
return true;
}
_waiters.push(waiter);
}
_queued.addAndFetch(1);
auto res = atomic_wait(waiter->futexWord, TicketWaiter::State::Waiting, deadline);
if (res == stdx::cv_status::timeout) {
// If we timed out, we need to invalidate ourselves, but ensure that we take a ticket if
// it was given.
auto state = static_cast<uint32_t>(TicketWaiter::State::Waiting);
if (waiter->futexWord.compareAndSwap(&state, TicketWaiter::State::TimedOut)) {
// Successfully set outselves to timed out so nobody tries to give us a ticket.
return false;
} else {
// We were given a ticket anyways. We must take it.
invariant(state == TicketWaiter::State::Acquired);
return true;
}
}
invariant(waiter->futexWord.load() == TicketWaiter::State::Acquired);
return true;
}
template <class Queue>
std::shared_ptr<TicketWaiter> TicketPool<Queue>::_popWaiterOrAddTicketToPool() {
stdx::unique_lock<Mutex> lock(_mutex);
auto waiter = _waiters.pop();
if (!waiter) {
// We need to ensure we add the ticket back to the pool while holding the mutex. This
// prevents a soon-to-be waiter from missing an available ticket. Otherwise, we could
// leave a waiter in the queue without ever waking it.
_available.addAndFetch(1);
}
return waiter;
}
template <class Queue>
void TicketPool<Queue>::_release() {
while (auto waiter = _popWaiterOrAddTicketToPool()) {
_queued.subtractAndFetch(1);
auto state = static_cast<uint32_t>(TicketWaiter::State::Waiting);
if (waiter->futexWord.compareAndSwap(&state, TicketWaiter::State::Acquired)) {
atomic_notify_one(waiter->futexWord);
return;
} else {
// We raced with the waiter timing out, so we didn't transfer the ticket. Try again.
invariant(state == TicketWaiter::State::TimedOut);
}
}
}
template <class Queue>
void TicketPool<Queue>::release() {
_release();
}
template class TicketPool<FifoTicketQueue>;
template class TicketPool<SimplePriorityTicketQueue>;
} // namespace mongo
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