THUAI6/CAPI/cpp/grpc/include/absl/synchronization/internal/waiter.h

// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//

#ifndef ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
#define ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_

#include "absl/base/config.h"

#ifdef _WIN32
#include <sdkddkver.h>
#else
#include <pthread.h>
#endif

#ifdef __linux__
#include <linux/futex.h>
#endif

#ifdef ABSL_HAVE_SEMAPHORE_H
#include <semaphore.h>
#endif

#include <atomic>
#include <cstdint>

#include "absl/base/internal/thread_identity.h"
#include "absl/synchronization/internal/futex.h"
#include "absl/synchronization/internal/kernel_timeout.h"

// May be chosen at compile time via -DABSL_FORCE_WAITER_MODE=<index>
#define ABSL_WAITER_MODE_FUTEX 0
#define ABSL_WAITER_MODE_SEM 1
#define ABSL_WAITER_MODE_CONDVAR 2
#define ABSL_WAITER_MODE_WIN32 3

#if defined(ABSL_FORCE_WAITER_MODE)
#define ABSL_WAITER_MODE ABSL_FORCE_WAITER_MODE
#elif defined(_WIN32) && _WIN32_WINNT >= _WIN32_WINNT_VISTA
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_WIN32
#elif defined(ABSL_INTERNAL_HAVE_FUTEX)
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
#elif defined(ABSL_HAVE_SEMAPHORE_H)
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_SEM
#else
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_CONDVAR
#endif

namespace absl
{
    ABSL_NAMESPACE_BEGIN
    namespace synchronization_internal
    {

        // Waiter is an OS-specific semaphore.
        class Waiter
        {
        public:
            // Prepare any data to track waits.
            Waiter();

            // Not copyable or movable
            Waiter(const Waiter&) = delete;
            Waiter& operator=(const Waiter&) = delete;

            // Blocks the calling thread until a matching call to `Post()` or
            // `t` has passed. Returns `true` if woken (`Post()` called),
            // `false` on timeout.
            bool Wait(KernelTimeout t);

            // Restart the caller of `Wait()` as with a normal semaphore.
            void Post();

            // If anyone is waiting, wake them up temporarily and cause them to
            // call `MaybeBecomeIdle()`. They will then return to waiting for a
            // `Post()` or timeout.
            void Poke();

            // Returns the Waiter associated with the identity.
            static Waiter* GetWaiter(base_internal::ThreadIdentity* identity)
            {
                static_assert(
                    sizeof(Waiter) <= sizeof(base_internal::ThreadIdentity::WaiterState),
                    "Insufficient space for Waiter"
                );
                return reinterpret_cast<Waiter*>(identity->waiter_state.data);
            }

            // How many periods to remain idle before releasing resources
#ifndef ABSL_HAVE_THREAD_SANITIZER
            static constexpr int kIdlePeriods = 60;
#else
            // Memory consumption under ThreadSanitizer is a serious concern,
            // so we release resources sooner. The value of 1 leads to 1 to 2 second
            // delay before marking a thread as idle.
            static const int kIdlePeriods = 1;
#endif

        private:
            // The destructor must not be called since Mutex/CondVar
            // can use PerThreadSem/Waiter after the thread exits.
            // Waiter objects are embedded in ThreadIdentity objects,
            // which are reused via a freelist and are never destroyed.
            ~Waiter() = delete;

#if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
            // Futexes are defined by specification to be 32-bits.
            // Thus std::atomic<int32_t> must be just an int32_t with lockfree methods.
            std::atomic<int32_t> futex_;
            static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");

#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
            // REQUIRES: mu_ must be held.
            void InternalCondVarPoke();

            pthread_mutex_t mu_;
            pthread_cond_t cv_;
            int waiter_count_;
            int wakeup_count_;  // Unclaimed wakeups.

#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
            sem_t sem_;
            // This seems superfluous, but for Poke() we need to cause spurious
            // wakeups on the semaphore. Hence we can't actually use the
            // semaphore's count.
            std::atomic<int> wakeups_;

#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
            // WinHelper - Used to define utilities for accessing the lock and
            // condition variable storage once the types are complete.
            class WinHelper;

            // REQUIRES: WinHelper::GetLock(this) must be held.
            void InternalCondVarPoke();

            // We can't include Windows.h in our headers, so we use aligned character
            // buffers to define the storage of SRWLOCK and CONDITION_VARIABLE.
            // SRW locks and condition variables do not need to be explicitly destroyed.
            // https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
            // https://stackoverflow.com/questions/28975958/why-does-windows-have-no-deleteconditionvariable-function-to-go-together-with
            alignas(void*) unsigned char mu_storage_[sizeof(void*)];
            alignas(void*) unsigned char cv_storage_[sizeof(void*)];
            int waiter_count_;
            int wakeup_count_;

#else
#error Unknown ABSL_WAITER_MODE
#endif
        };

    }  // namespace synchronization_internal
    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_