tracing v2: Add TraceRingBuffer class.

components/tracing/core/trace_ring_buffer.cc

+// Copyright 2016 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 "components/tracing/core/trace_ring_buffer.h"
+
+#include "base/threading/platform_thread.h"
+
+namespace tracing {
+namespace v2 {
+
+TraceRingBuffer::TraceRingBuffer(uint8_t* begin, size_t size)
+    : num_chunks_(size / Chunk::kSize), current_chunk_idx_(0) {

[kraynov, 2016/06/28 13:15:16]

What if (size % Chunk::kSize) != 0 ?

[Primiano Tucci (use gerrit), 2016/06/28 14:52:11]

we'll waste the reminder and use less than the passed buffer. shouldn't cause
problems right?

+  DCHECK_GT(num_chunks_, 0u);
+  DCHECK_EQ(0ul, reinterpret_cast<uintptr_t>(begin) % sizeof(uintptr_t));
+  chunks_.reset(new Chunk[num_chunks_]);
+  uint8_t* chunk_begin = begin;
+  for (size_t i = 0; i < num_chunks_; ++i) {
+    chunks_[i].Initialize(chunk_begin);
+    chunk_begin += Chunk::kSize;
+  }
+}
+
+TraceRingBuffer::~TraceRingBuffer() {}
+
+TraceRingBuffer::Chunk* TraceRingBuffer::TakeChunk() {
+  base::AutoLock lock(lock_);
+  DCHECK_GT(num_chunks_, 0ul);
+  DCHECK_LT(current_chunk_idx_, num_chunks_);
+  for (size_t i = 0; i < num_chunks_; ++i) {
+    Chunk* chunk = &chunks_[current_chunk_idx_];
+    current_chunk_idx_ = (current_chunk_idx_ + 1) % num_chunks_;
+    if (!chunk->is_owned()) {
+      chunk->Clear();
+      chunk->set_owner(base::PlatformThread::CurrentId());
+      return chunk;
+    }
+  }
+
+  // Bankrupcy: there are more threads than chunks. All chunks were in flight.
+  if (!bankrupcy_chunk_storage_) {
+    bankrupcy_chunk_storage_.reset(new uint8_t[Chunk::kSize]);
+    bankrupcy_chunk_.Initialize(&bankrupcy_chunk_storage_.get()[0]);
+  }
+  bankrupcy_chunk_.Clear();
+  return &bankrupcy_chunk_;
+}
+
+void TraceRingBuffer::ReturnChunk(TraceRingBuffer::Chunk* chunk,
+                                  uint32_t used_size) {
+  base::AutoLock lock(lock_);
+  chunk->set_used_size(used_size);
+  chunk->clear_owner();
+}
+
+TraceRingBuffer::Chunk::Chunk()
+    : begin_(nullptr), owner_(base::kInvalidThreadId) {}
+
+TraceRingBuffer::Chunk::~Chunk() {}
+
+void TraceRingBuffer::Chunk::Clear() {
+  set_used_size(0);
+}
+
+void TraceRingBuffer::Chunk::Initialize(uint8_t* begin) {
+  DCHECK_EQ(0ul, reinterpret_cast<uintptr_t>(begin) % sizeof(uintptr_t));
+  begin_ = begin;
+}
+
+}  // namespace v2
+}  // namespace tracing