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// Copyright 2021 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.
#ifndef CC_METRICS_SHARED_METRICS_BUFFER_H_
#define CC_METRICS_SHARED_METRICS_BUFFER_H_
#include "device/base/synchronization/one_writer_seqlock.h"
namespace cc {
// The struct written in shared memory to transport metrics across
// processes. |data| is protected by the sequence-lock |seq_lock|.
// Note: This template copies data between processes. Any class that uses this
// template would need security review.
template <class T>
struct SharedMetricsBuffer {
device::OneWriterSeqLock seq_lock;
T data;
static_assert(std::is_trivially_copyable<T>::value,
"Metrics shared across processes need to be trivially "
"copyable, otherwise it is dangerous to copy it.");
bool Read(T& out) const {
const uint32_t kMaxRetries = 5;
uint32_t retries = 0;
base::subtle::Atomic32 version;
do {
const uint32_t kMaxReadAttempts = 32;
version = seq_lock.ReadBegin(kMaxReadAttempts);
device::OneWriterSeqLock::AtomicReaderMemcpy(&out, &data, sizeof(T));
} while (seq_lock.ReadRetry(version) && ++retries < kMaxRetries);
// Consider the number of retries less than kMaxRetries as success.
return retries < kMaxRetries;
}
void Write(const T& in) {
seq_lock.WriteBegin();
device::OneWriterSeqLock::AtomicWriterMemcpy(&data, &in, sizeof(T));
seq_lock.WriteEnd();
}
};
} // namespace cc
#endif // CC_METRICS_SHARED_METRICS_BUFFER_H_
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