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73 lines
2.4 KiB
73 lines
2.4 KiB
// Copyright 2019 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "platform/base/trace_logging_activation.h"
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#include <atomic>
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#include <cassert>
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#include <thread>
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namespace openscreen {
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namespace {
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// If tracing is active, this is a valid pointer to an object that implements
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// the TraceLoggingPlatform interface. If tracing is not active, this is
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// nullptr.
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std::atomic<TraceLoggingPlatform*> g_current_destination{};
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// The count of threads currently calling into the current TraceLoggingPlatform.
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std::atomic<int> g_use_count{};
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inline TraceLoggingPlatform* PinCurrentDestination() {
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// NOTE: It's important to increment the global use count *before* loading the
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// pointer, to ensure the referent is pinned-down (i.e., any thread executing
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// StopTracing() stays blocked) until CurrentTracingDestination's destructor
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// calls UnpinCurrentDestination().
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g_use_count.fetch_add(1);
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return g_current_destination.load(std::memory_order_relaxed);
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}
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inline void UnpinCurrentDestination() {
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g_use_count.fetch_sub(1);
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}
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} // namespace
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void StartTracing(TraceLoggingPlatform* destination) {
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assert(destination);
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auto* const old_destination = g_current_destination.exchange(destination);
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(void)old_destination; // Prevent "unused variable" compiler warnings.
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assert(old_destination == nullptr || old_destination == destination);
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}
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void StopTracing() {
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auto* const old_destination = g_current_destination.exchange(nullptr);
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if (!old_destination) {
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return; // Already stopped.
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}
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// Block the current thread until the global use count goes to zero. At that
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// point, there can no longer be any dangling references. Theoretically, this
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// loop may never terminate; but in practice, that should never happen. If it
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// did happen, that would mean one or more CPU cores are continuously spending
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// most of their time executing the TraceLoggingPlatform methods, yet those
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// methods are supposed to be super-cheap and take near-zero time to execute!
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int iters = 0;
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while (g_use_count.load(std::memory_order_relaxed) != 0) {
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assert(iters < 1024);
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std::this_thread::yield();
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++iters;
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}
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}
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CurrentTracingDestination::CurrentTracingDestination()
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: destination_(PinCurrentDestination()) {}
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CurrentTracingDestination::~CurrentTracingDestination() {
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UnpinCurrentDestination();
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}
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} // namespace openscreen
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