src/dawn/native/ExecutionQueue.cpp

// Copyright 2023 The Dawn & Tint Authors
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#include "dawn/native/ExecutionQueue.h"

#include <atomic>
#include <utility>
#include <vector>

#include "dawn/common/Atomic.h"

namespace dawn::native {

ExecutionSerial ExecutionQueueBase::GetPendingCommandSerial() const {
    return ExecutionSerial(mLastSubmittedSerial.load(std::memory_order_acquire) + 1);
}

ExecutionSerial ExecutionQueueBase::GetLastSubmittedCommandSerial() const {
    return ExecutionSerial(mLastSubmittedSerial.load(std::memory_order_acquire));
}

ExecutionSerial ExecutionQueueBase::GetCompletedCommandSerial() const {
    return ExecutionSerial(mCompletedSerial.load(std::memory_order_acquire));
}

ResultOrError<bool> ExecutionQueueBase::WaitForQueueSerial(ExecutionSerial serial,
                                                           Nanoseconds timeout) {
    bool succeeded;
    DAWN_TRY_ASSIGN(succeeded, WaitForQueueSerialImpl(serial, timeout));

    if (!succeeded) {
        return false;
    }

    // It's critical to update the completed serial right away. If fences are processed by another
    // thread before CheckAndUpdateCompletedSerials() runs on the current thread,
    // the fence list will be empty, preventing the current thread from determining the true latest
    // serial. Pre-emptively updating mCompletedSerial ensures CheckAndUpdateCompletedSerials()
    // returns an accurate value, preventing stale data.
    FetchMax(mCompletedSerial, uint64_t(serial));

    return true;
}

MaybeError ExecutionQueueBase::CheckPassedSerials() {
    ExecutionSerial completedSerial;
    DAWN_TRY_ASSIGN(completedSerial, CheckAndUpdateCompletedSerials());

    DAWN_ASSERT(completedSerial <=
                ExecutionSerial(mLastSubmittedSerial.load(std::memory_order_acquire)));

    // Atomically set mCompletedSerial to completedSerial if completedSerial is larger.
    FetchMax(mCompletedSerial, uint64_t(completedSerial));
    return {};
}

MaybeError ExecutionQueueBase::UpdateCompletedSerial() {
    ExecutionSerial completedSerial;
    DAWN_TRY_ASSIGN(completedSerial, CheckAndUpdateCompletedSerials());

    DAWN_ASSERT(completedSerial <=
                ExecutionSerial(mLastSubmittedSerial.load(std::memory_order_acquire)));
    UpdateCompletedSerialTo(completedSerial);
    return {};
}

void ExecutionQueueBase::TrackSerialTask(ExecutionSerial serial, Task&& task) {
    if (serial <= GetCompletedCommandSerial()) {
        task();
        return;
    }
    mWaitingTasks->Enqueue(std::move(task), serial);
}

void ExecutionQueueBase::UpdateCompletedSerialTo(ExecutionSerial completedSerial) {
    // Atomically set mCompletedSerial to completedSerial if completedSerial is larger.
    FetchMax(mCompletedSerial, uint64_t(completedSerial));

    mWaitingTasks.Use([&](auto tasks) {
        // Note that the callbacks need to be called while holding this lock to ensure that callback
        // ordering is enforced. At the moment, that means that re-entrant callbacks that call async
        // APIs will deadlock. In the future we may consider making this a recursive lock or
        // providing some other mechanism in order to enable re-entrant async APIs.
        for (auto task : tasks->IterateUpTo(completedSerial)) {
            task();
        }
        tasks->ClearUpTo(completedSerial);
    });
}

MaybeError ExecutionQueueBase::EnsureCommandsFlushed(ExecutionSerial serial) {
    DAWN_ASSERT(serial <= GetPendingCommandSerial());
    if (serial > GetLastSubmittedCommandSerial()) {
        ForceEventualFlushOfCommands();
        DAWN_TRY(SubmitPendingCommands());
        DAWN_ASSERT(serial <= GetLastSubmittedCommandSerial());
    }
    return {};
}

MaybeError ExecutionQueueBase::SubmitPendingCommands() {
    if (mInSubmit) {
        return {};
    }

    mInSubmit = true;
    auto result = SubmitPendingCommandsImpl();
    mInSubmit = false;

    return result;
}

void ExecutionQueueBase::AssumeCommandsComplete() {
    // Bump serials so any pending callbacks can be fired.
    // TODO(crbug.com/dawn/831): This is called during device destroy, which is not
    // thread-safe yet. Two threads calling destroy would race setting these serials.
    ExecutionSerial completed =
        ExecutionSerial(mLastSubmittedSerial.fetch_add(1u, std::memory_order_release) + 1);
    UpdateCompletedSerialTo(completed);
}

void ExecutionQueueBase::IncrementLastSubmittedCommandSerial() {
    mLastSubmittedSerial.fetch_add(1u, std::memory_order_release);
}

bool ExecutionQueueBase::HasScheduledCommands() const {
    return mLastSubmittedSerial.load(std::memory_order_acquire) >
               mCompletedSerial.load(std::memory_order_acquire) ||
           HasPendingCommands();
}

// All prevously submitted works at the moment will supposedly complete at this serial.
// Internally the serial is computed according to whether frontend and backend have pending
// commands. There are 4 cases of combination:
//   1) Frontend(No), Backend(No)
//   2) Frontend(No), Backend(Yes)
//   3) Frontend(Yes), Backend(No)
//   4) Frontend(Yes), Backend(Yes)
// For case 1, we don't need the serial to track the task as we can ack it right now.
// For case 2 and 4, there will be at least an eventual submission, so we can use
// 'GetPendingCommandSerial' as the serial.
// For case 3, we can't use 'GetPendingCommandSerial' as it won't be submitted surely. Instead we
// use 'GetLastSubmittedCommandSerial', which must be fired eventually.
ExecutionSerial ExecutionQueueBase::GetScheduledWorkDoneSerial() const {
    return HasPendingCommands() ? GetPendingCommandSerial() : GetLastSubmittedCommandSerial();
}

}  // namespace dawn::native