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// Copyright The OpenTelemetry Authors
// SPDX-License-Identifier: Apache-2.0
#pragma once
#include <atomic>
#include <chrono>
#include <thread>
#include "opentelemetry/version.h"
#if defined(_MSC_VER)
# ifndef NOMINMAX
# define NOMINMAX
# endif
# define _WINSOCKAPI_ // stops including winsock.h
# include <windows.h>
#elif defined(__i386__) || defined(__x86_64__)
# if defined(__clang__)
# include <emmintrin.h>
# endif
#endif
OPENTELEMETRY_BEGIN_NAMESPACE
namespace common
{
constexpr int SPINLOCK_FAST_ITERATIONS = 100;
constexpr int SPINLOCK_SLEEP_MS = 1;
/**
* A Mutex which uses atomic flags and spin-locks instead of halting threads.
*
* This mutex uses an incremental back-off strategy with the following phases:
* 1. A tight spin-lock loop (pending: using hardware PAUSE/YIELD instructions)
* 2. A loop where the current thread yields control after checking the lock.
* 3. Issuing a thread-sleep call before starting back in phase 1.
*
* This is meant to give a good balance of perofrmance and CPU consumption in
* practice.
*
* This mutex uses an incremental back-off strategy with the following phases:
* 1. A tight spin-lock loop (pending: using hardware PAUSE/YIELD instructions)
* 2. A loop where the current thread yields control after checking the lock.
* 3. Issuing a thread-sleep call before starting back in phase 1.
*
* This is meant to give a good balance of perofrmance and CPU consumption in
* practice.
*
* This class implements the `BasicLockable` specification:
* https://en.cppreference.com/w/cpp/named_req/BasicLockable
*/
class SpinLockMutex
{
public:
SpinLockMutex() noexcept {}
~SpinLockMutex() noexcept = default;
SpinLockMutex(const SpinLockMutex &) = delete;
SpinLockMutex &operator=(const SpinLockMutex &) = delete;
SpinLockMutex &operator=(const SpinLockMutex &) volatile = delete;
/**
* Attempts to lock the mutex. Return immediately with `true` (success) or `false` (failure).
*/
bool try_lock() noexcept
{
return !flag_.load(std::memory_order_relaxed) &&
!flag_.exchange(true, std::memory_order_acquire);
}
/**
* Blocks until a lock can be obtained for the current thread.
*
* This mutex will spin the current CPU waiting for the lock to be available. This can have
* decent performance in scenarios where there is low lock contention and lock-holders achieve
* their work quickly. It degrades in scenarios where locked tasks take a long time.
*/
void lock() noexcept
{
for (;;)
{
// Try once
if (!flag_.exchange(true, std::memory_order_acquire))
{
return;
}
// Spin-Fast (goal ~10ns)
for (std::size_t i = 0; i < SPINLOCK_FAST_ITERATIONS; ++i)
{
if (try_lock())
{
return;
}
// Issue a Pause/Yield instruction while spinning.
#if defined(_MSC_VER)
YieldProcessor();
#elif defined(__i386__) || defined(__x86_64__)
# if defined(__clang__)
_mm_pause();
# else
__builtin_ia32_pause();
# endif
#elif defined(__arm__)
__asm__ volatile("yield" ::: "memory");
#else
// TODO: Issue PAGE/YIELD on other architectures.
#endif
}
// Yield then try again (goal ~100ns)
std::this_thread::yield();
if (try_lock())
{
return;
}
// Sleep and then start the whole process again. (goal ~1000ns)
std::this_thread::sleep_for(std::chrono::milliseconds(SPINLOCK_SLEEP_MS));
}
return;
}
/** Releases the lock held by the execution agent. Throws no exceptions. */
void unlock() noexcept { flag_.store(false, std::memory_order_release); }
private:
std::atomic<bool> flag_{false};
};
} // namespace common
OPENTELEMETRY_END_NAMESPACE
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