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