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// Copyright 2019 The Fuchsia Authors
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file or at
#include <lib/ktrace.h>
#include <lib/ktrace/string_ref.h>
#include <ktl/atomic.h>
#include <ktl/enforce.h>
int StringRef::Register(StringRef* string_ref) {
// Return the id if the string ref is already registered.
int id = string_ref->id.load(ktl::memory_order_relaxed);
if (id != kInvalidId) {
return id;
// Try to set the id of the string ref. When there is a race with other
// threads or CPUs only one will succeed, in which case the id counter will
// harmlessly skip values equal to the number of agents racing between here
// and the check above.
const int new_id = id_counter_.fetch_add(1, ktl::memory_order_relaxed);
while (!string_ref->id.compare_exchange_weak(id, new_id, ktl::memory_order_relaxed,
ktl::memory_order_relaxed)) {
// If another agent set the id first simply return the id.
if (id != kInvalidId && id != new_id)
return id;
// Emit a name record the first time this string ref is encountered at
// runtime. This is ignored if tracing is not active and is replayed at the
// beginning of subsequent tracing sessions.
ktrace_name_etc(TAG_PROBE_NAME, string_ref->id, 0, string_ref->string, true);
// Register the string ref in the global linked list. When there is a race
// above only the winning agent that set the id will continue to this point.
string_ref->next = head_.load(ktl::memory_order_relaxed);
while (!head_.compare_exchange_weak(string_ref->next, string_ref, ktl::memory_order_release,
ktl::memory_order_relaxed)) {
return new_id;
ktl::atomic<int> StringRef::id_counter_{StringRef::kInvalidId + 1};
ktl::atomic<StringRef*> StringRef::head_{nullptr};