Track thread activities in order to diagnose hangs.

base/debug/activity_tracker.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 "base/debug/activity_tracker.h"
+
+#include <atomic>
+
+#include "base/feature_list.h"
+#include "base/files/file.h"
+#include "base/files/file_path.h"
+#include "base/files/memory_mapped_file.h"
+#include "base/logging.h"
+#include "base/memory/ptr_util.h"
+#include "base/metrics/field_trial.h"
+#include "base/metrics/histogram_macros.h"
+#include "base/pending_task.h"
+#include "base/process/process.h"
+#include "base/process/process_handle.h"
+#include "base/stl_util.h"
+#include "base/strings/string_util.h"
+#include "base/threading/platform_thread.h"
+
+namespace base {
+namespace debug {
+
+namespace {
+
+// A number that identifies the memory as having been initialized. It's
+// arbitrary but happens to be the first 8 bytes of SHA1(ThreadActivityTracker).
+// A version number is added on so that major structure changes won't try to
+// read an older version (since the cookie won't match).
+const uint64_t kHeaderCookie = 0xC0029B240D4A3092ULL + 1;  // v1
+
+// The minimum depth a stack should support.
+const int kMinStackDepth = 2;
+
+}  // namespace
+
+
+#if !defined(OS_NACL)  // NACL doesn't support any kind of file access in build.
+void SetupGlobalActivityTrackerFieldTrial(const FilePath& file) {
+  const Feature kActivityTrackerFeature{
+      "ActivityTracking", FEATURE_DISABLED_BY_DEFAULT
+  };
+
+  if (!base::FeatureList::IsEnabled(kActivityTrackerFeature))
+    return;
+
+  // TODO(bcwhite): Adjust these numbers once there is real data to show
+  // just how much of an arena is necessary.
+  const size_t kMemorySize = 1 << 20;  // 1 MiB
+  const int kStackDepth = 4;
+  const uint64_t kAllocatorId = 0;
+  const char kAllocatorName[] = "ActivityTracker";
+
+  GlobalActivityTracker::CreateWithFile(
+      file.AddExtension(PersistentMemoryAllocator::kFileExtension),
+      kMemorySize, kAllocatorId, kAllocatorName, kStackDepth);
+}
+#endif  // !defined(OS_NACL)
+
+
+// This information is kept for every thread that is tracked. It is filled
+// the very first time the thread is seen. All fields must be of exact sizes
+// so there is no issue moving between 32 and 64-bit builds.
+struct ThreadActivityTracker::Header {
+  // This unique number indicates a valid initialization of the memory.
+  uint64_t cookie;
+
+  // The process-id and thread-id to which this data belongs. These identifiers
+  // are not guaranteed to mean anything but are unique, in combination, among
+  // all active trackers.
+  int64_t process_id;
+  union {
+    int64_t as_id;
+    PlatformThreadHandle::Handle as_handle;
+  } thread_ref;
+
+  // The start-time and start-ticks when the data was created. Each activity
+  // record has a |time_internal| value that can be converted to a "wall time"
+  // with these two values.
+  int64_t start_time;
+  int64_t start_ticks;
+
+  // The number of Activity slots in the data.
+  uint32_t stack_slots;
+
+  // The current depth of the stack. This may be greater than the number of
+  // slots. If the depth exceeds the number of slots, the newest entries
+  // won't be recorded.
+  std::atomic<uint32_t> current_depth;
+
+  // A memory location used to indicate if changes have been made to the stack
+  // that would invalidate an in-progress read of its contents. The active
+  // tracker will zero the value whenever something gets popped from the
+  // stack. A monitoring tracker can write a non-zero value here, copy the
+  // stack contents, and read the value to know, if it is still non-zero, that
+  // the contents didn't change while being copied.
+  std::atomic<uint32_t> stack_unchanged;
+
+  // The name of the thread (up to a maximum length). Dynamic-length names
+  // are not practical since the memory has to come from the same persistent
+  // allocator that holds this structure and to which this object has no
+  // reference.
+  char thread_name[32];
+};
+
+// It doesn't matter what is contained in this (though it will be all zeros)
+// as only the address of it is important.
+const ThreadActivityTracker::ActivityData
+    ThreadActivityTracker::kNullActivityData = {};
+
+ThreadActivityTracker::ActivityData
+ThreadActivityTracker::ActivityData::ForThread(
+    const PlatformThreadHandle& handle) {
+  // Header already has a conversion union; reuse that.
+  ThreadActivityTracker::Header header;
+  header.thread_ref.as_id = 0;  // Zero the union in case as_handle is smaller.
+  header.thread_ref.as_handle = handle.platform_handle();
+  return ForThread(header.thread_ref.as_id);
+}
+
+ThreadActivityTracker::ActivitySnapshot::ActivitySnapshot() {}
+ThreadActivityTracker::ActivitySnapshot::~ActivitySnapshot() {}
+
+
+ThreadActivityTracker::ThreadActivityTracker(void* base, size_t size)
+    : header_(static_cast<Header*>(base)),
+      stack_(reinterpret_cast<Activity*>(reinterpret_cast<char*>(base) +
+                                         sizeof(Header))),
+      stack_slots_(
+          static_cast<uint32_t>((size - sizeof(Header)) / sizeof(Activity))) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+
+  // Verify the parameters but fail gracefully if they're not valid so that
+  // production code based on external inputs will not crash.  IsValid() will
+  // return false in this case.
+  if (!base ||
+      // Ensure there is enough space for the header and at least a few records.
+      size < sizeof(Header) + kMinStackDepth * sizeof(Activity) ||
+      // Ensure that the |stack_slots_| calculation didn't overflow.
+      (size - sizeof(Header)) / sizeof(Activity) >
+          std::numeric_limits<uint32_t>::max()) {
+    NOTREACHED();
+    return;
+  }
+
+  // Ensure that the thread reference doesn't exceed the size of the ID number.
+  // This won't compile at the global scope because Header is a private struct.
+  static_assert(
+      sizeof(header_->thread_ref) == sizeof(header_->thread_ref.as_id),
+      "PlatformThreadHandle::Handle is too big to hold in 64-bit ID");
+
+  // Provided memory should either be completely initialized or all zeros.
+  if (header_->cookie == 0) {
+    // This is a new file. Double-check other fields and then initialize.
+    DCHECK_EQ(0, header_->process_id);
+    DCHECK_EQ(0, header_->thread_ref.as_id);
+    DCHECK_EQ(0, header_->start_time);
+    DCHECK_EQ(0, header_->start_ticks);
+    DCHECK_EQ(0U, header_->stack_slots);
+    DCHECK_EQ(0U, header_->current_depth.load(std::memory_order_relaxed));
+    DCHECK_EQ(0U, header_->stack_unchanged.load(std::memory_order_relaxed));
+    DCHECK_EQ(0, stack_[0].time_internal);
+    DCHECK_EQ(0U, stack_[0].source_address);
+    DCHECK_EQ(0U, stack_[0].data.task.sequence_id);
+
+    header_->process_id = GetCurrentProcId();
+    header_->thread_ref.as_handle =
+        PlatformThread::CurrentHandle().platform_handle();
+    header_->start_time = base::Time::Now().ToInternalValue();
+    header_->start_ticks = base::TimeTicks::Now().ToInternalValue();
+    header_->stack_slots = stack_slots_;
+    strlcpy(header_->thread_name, PlatformThread::GetName(),
+            sizeof(header_->thread_name));
+    header_->cookie = kHeaderCookie;
+    valid_ = true;
+    DCHECK(IsValid());
+  } else {
+    // This is a file with existing data. Perform basic consistency checks.
+    valid_ = true;
+    valid_ = IsValid();
+  }
+}
+
+ThreadActivityTracker::~ThreadActivityTracker() {}
+
+void ThreadActivityTracker::PushActivity(const void* source,
+                                         ActivityType type,
+                                         const ActivityData& data) {
+  // A thread-checker creates a lock to check the thread-id which means
+  // re-entry into this code if lock acquisitions are being tracked.
+  DCHECK(type == ACT_LOCK_ACQUIRE || thread_checker_.CalledOnValidThread());
+
+  // Get the current depth of the stack. No access to other memory guarded
+  // by this variable is done here so a "relaxed" load is acceptable.
+  uint32_t depth = header_->current_depth.load(std::memory_order_relaxed);
+
+  // Handle the case where the stack depth has exceeded the storage capacity.
+  // Extra entries will be lost leaving only the base of the stack.
+  if (depth >= stack_slots_) {
+    // Since no other threads modify the data, no compare/exchange is needed.
+    // Since no other memory is being modified, a "relaxed" store is acceptable.
+    header_->current_depth.store(depth + 1, std::memory_order_relaxed);
+    return;
+  }
+
+  // Get a pointer to the next activity and load it. No atomicity is required
+  // here because the memory is known only to this thread. It will be made
+  // known to other threads once the depth is incremented.
+  Activity* activity = &stack_[depth];
+  activity->time_internal = base::TimeTicks::Now().ToInternalValue();
+  activity->source_address = reinterpret_cast<uintptr_t>(source);
+  activity->activity_type = type;
+  activity->data = data;
+
+  // Save the incremented depth. Because this guards |activity| memory filled
+  // above that may be read by another thread once the recorded depth changes,
+  // a "release" store is required.
+  header_->current_depth.store(depth + 1, std::memory_order_release);
+}
+
+void ThreadActivityTracker::ChangeActivity(const void* source,
+                                           ActivityType type,
+                                           const ActivityData& data) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(type != ACT_NULL || &data != &kNullActivityData);
+
+  // Get the current depth of the stack.
+  uint32_t depth = header_->current_depth.load(std::memory_order_relaxed);
+  DCHECK_LT(0U, depth);
+
+  // Update the information if it is being recorded (i.e. within slot limit).
+  if (depth <= stack_slots_) {
+    Activity* activity = &stack_[depth - 1];
+    DCHECK_EQ(reinterpret_cast<uintptr_t>(source), activity->source_address);
+
+    if (type != ACT_NULL) {
+      DCHECK_EQ(activity->activity_type & ACT_CATEGORY_MASK,
+                type & ACT_CATEGORY_MASK);
+      activity->activity_type = type;
+    }
+
+    if (&data != &kNullActivityData)
+      activity->data = data;
+  }
+}
+
+void ThreadActivityTracker::PopActivity(const void* source) {
+  // Do an atomic decrement of the depth. No changes to stack entries guarded
+  // by this variable are done here so a "relaxed" operation is acceptable.
+  // |depth| will receive the value BEFORE it was modified.
+  uint32_t depth =
+      header_->current_depth.fetch_sub(1, std::memory_order_relaxed);
+
+  // Validate that everything is running correctly.
+  DCHECK_LT(0U, depth);
+  if (depth <= stack_slots_) {
+    DCHECK_EQ(reinterpret_cast<uintptr_t>(source),
+              stack_[depth - 1].source_address);
+    DCHECK(stack_[depth - 1].activity_type == ACT_LOCK_ACQUIRE ||
+           thread_checker_.CalledOnValidThread());
+  }
+
+  // The stack has shrunk meaning that some other thread trying to copy the
+  // contents for reporting purposes could get bad data. That thread would
+  // have written a non-zero value into |stack_unchanged|; clearing it here
+  // will let that thread detect that something did change. This needs to
+  // happen after the atomic |depth| operation above so a "release" store
+  // is required.
+  header_->stack_unchanged.store(0, std::memory_order_release);
+}
+
+bool ThreadActivityTracker::IsValid() const {
+  if (header_->cookie != kHeaderCookie ||
+      header_->process_id == 0 ||
+      header_->thread_ref.as_id == 0 ||
+      header_->start_time == 0 ||
+      header_->start_ticks == 0 ||
+      header_->stack_slots != stack_slots_ ||
+      header_->thread_name[sizeof(header_->thread_name) - 1] != '\0') {
+    return false;
+  }
+
+  return valid_;
+}
+
+bool ThreadActivityTracker::Snapshot(ActivitySnapshot* output_snapshot) const {
+  DCHECK(output_snapshot);
+
+  // There is no "called on valid thread" check for this method as it can be
+  // called from other threads or even other processes. It is also the reason
+  // why atomic operations must be used in certain places above.
+
+  // It's possible for the data to change while reading it in such a way that it
+  // invalidates the read. Make several attempts but don't try forever.
+  const int kMaxAttempts = 10;
+  uint32_t depth;
+
+  // Stop here if the data isn't valid.
+  if (!IsValid())
+    return false;
+
+  // Start with an empty return stack.
+  output_snapshot->activity_stack.clear();
+
+  for (int attempt = 0; attempt < kMaxAttempts; ++attempt) {
+    // Remember the process and thread IDs to ensure they aren't replaced
+    // during the snapshot operation.
+    const int64_t starting_process_id = header_->process_id;
+    const int64_t starting_thread_id = header_->thread_ref.as_id;
+
+    // Write a non-zero value to |stack_unchanged| so it's possible to detect
+    // at the end that nothing has changed since copying the data began. A
+    // "cst" operation is required to ensure it occurs before everything else.
+    header_->stack_unchanged.store(1, std::memory_order_seq_cst);
+
+    // Fetching the current depth also "acquires" the contents of the stack.
+    depth = header_->current_depth.load(std::memory_order_acquire);
+    if (depth > 0) {
+      // Copy the existing contents. Memcpy is used for speed.
+      uint32_t count = std::min(depth, stack_slots_);
+      output_snapshot->activity_stack.resize(count);
+      memcpy(&output_snapshot->activity_stack[0], stack_,
+             count * sizeof(Activity));
+    }
+
+    // Retry if something changed during the copy. A "cst" operation ensures
+    // it must happen after all the above operations.
+    if (!header_->stack_unchanged.load(std::memory_order_seq_cst))
+      continue;
+
+    // Stack copied. Record it's full depth.
+    output_snapshot->activity_stack_depth = depth;
+
+    // TODO(bcwhite): Snapshot other things here.
+
+    // Get the general thread information.
+    output_snapshot->process_id = header_->process_id;
+    output_snapshot->thread_id = header_->thread_ref.as_id;
+    output_snapshot->thread_name =
+        std::string(header_->thread_name, sizeof(header_->thread_name) - 1);
+
+    // All characters of the thread-name buffer were copied so as to not break
+    // if the trailing NUL were missing. Now limit the length if the actual
+    // name is shorter.
+    output_snapshot->thread_name.resize(
+        strlen(output_snapshot->thread_name.c_str()));
+
+    // If the process or thread ID has changed then the tracker has exited and
+    // the memory reused by a new one. Try again.
+    if (output_snapshot->process_id != starting_process_id ||
+        output_snapshot->thread_id != starting_thread_id) {
+      continue;
+    }
+
+    // Only successful if the data is still valid once everything is done since
+    // it's possible for the thread to end somewhere in the middle and all its
+    // values become garbage.
+    if (!IsValid())
+      return false;
+
+    // Change all the timestamps in the activities from "ticks" to "wall" time.
+    const Time start_time = Time::FromInternalValue(header_->start_time);
+    const int64_t start_ticks = header_->start_ticks;
+    for (Activity& activity : output_snapshot->activity_stack) {
+      activity.time_internal =
+          (start_time +
+           TimeDelta::FromInternalValue(activity.time_internal - start_ticks))
+              .ToInternalValue();
+    }
+
+    // Success!
+    return true;
+  }
+
+  // Too many attempts.
+  return false;
+}
+
+// static
+size_t ThreadActivityTracker::SizeForStackDepth(int stack_depth) {
+  return static_cast<size_t>(stack_depth) * sizeof(Activity) + sizeof(Header);
+}
+
+
+GlobalActivityTracker* GlobalActivityTracker::g_tracker_ = nullptr;
+
+GlobalActivityTracker::ManagedActivityTracker::ManagedActivityTracker(
+    PersistentMemoryAllocator::Reference mem_reference,
+    void* base,
+    size_t size)
+    : ThreadActivityTracker(base, size),
+      mem_reference_(mem_reference),
+      mem_base_(base) {}
+
+GlobalActivityTracker::ManagedActivityTracker::~ManagedActivityTracker() {
+  // The global |g_tracker_| must point to the owner of this class since all
+  // objects of this type must be destructed before |g_tracker_| can be changed
+  // (something that only occurs in tests).
+  DCHECK(g_tracker_);
+  g_tracker_->ReturnTrackerMemory(this);
+}
+
+void GlobalActivityTracker::CreateWithAllocator(
+    std::unique_ptr<PersistentMemoryAllocator> allocator,
+    int stack_depth) {
+  // There's no need to do anything with the result. It is self-managing.
+  GlobalActivityTracker* global_tracker =
+      new GlobalActivityTracker(std::move(allocator), stack_depth);
+  // Create a tracker for this thread since it is known.
+  global_tracker->CreateTrackerForCurrentThread();
+}
+
+#if !defined(OS_NACL)
+// static
+void GlobalActivityTracker::CreateWithFile(const FilePath& file_path,
+                                           size_t size,
+                                           uint64_t id,
+                                           StringPiece name,
+                                           int stack_depth) {
+  DCHECK(!file_path.empty());
+  DCHECK_GE(std::numeric_limits<int64_t>::max(), size);
+
+  // Create and map the file into memory and make it globally available.
+  std::unique_ptr<MemoryMappedFile> mapped_file(new MemoryMappedFile());
+  bool success =
+      mapped_file->Initialize(File(file_path,
+                                   File::FLAG_CREATE_ALWAYS | File::FLAG_READ |
+                                   File::FLAG_WRITE | File::FLAG_SHARE_DELETE),
+                              {0, static_cast<int64_t>(size)},
+                              MemoryMappedFile::READ_WRITE_EXTEND);
+  DCHECK(success);
+  CreateWithAllocator(WrapUnique(new FilePersistentMemoryAllocator(
+                          std::move(mapped_file), size, id, name, false)),
+                      stack_depth);
+}
+#endif  // !defined(OS_NACL)
+
+// static
+void GlobalActivityTracker::CreateWithLocalMemory(size_t size,
+                                                  uint64_t id,
+                                                  StringPiece name,
+                                                  int stack_depth) {
+  CreateWithAllocator(
+      WrapUnique(new LocalPersistentMemoryAllocator(size, id, name)),
+      stack_depth);
+}
+
+ThreadActivityTracker* GlobalActivityTracker::CreateTrackerForCurrentThread() {
+  DCHECK(!this_thread_tracker_.Get());
+
+  PersistentMemoryAllocator::Reference mem_reference = 0;
+  void* mem_base = nullptr;
+
+  // Get the current count of available memories, acquiring the array values.
+  int count = available_memories_count_.load(std::memory_order_acquire);
+  while (count > 0) {
+    // There is a memory block that was previously released (and zero'd) so
+    // just re-use that rather than allocating a new one. Use "acquire" so
+    // operations below can be re-ordered above.
+    mem_reference =
+        available_memories_[count - 1].load(std::memory_order_acquire);
+    DCHECK(mem_reference);
+
+    // Decrement the count indicating that the value has been taken. If this
+    // fails then something else, another thread doing push or pop, has changed
+    // the stack; retry if so.
+    // NOTE: |count| will be loaded with the existing value and affect the
+    // "while" condition.
+    if (!available_memories_count_.compare_exchange_weak(
+            count, count - 1, std::memory_order_acquire,
+            std::memory_order_acquire)) {
+      continue;
+    }
+
+    // Clear the value just read from the array so that the "push" operation
+    // knows there is no value there and will work correctly.
+    available_memories_[count - 1].store(0, std::memory_order_relaxed);
+
+    // Turn the reference back into one of the activity-tracker type.
+    mem_base = allocator_->GetAsObject<char>(mem_reference,
+                                             kTypeIdActivityTrackerFree);
+    DCHECK(mem_base);
+    DCHECK_LE(stack_memory_size_, allocator_->GetAllocSize(mem_reference));
+    allocator_->ChangeType(mem_reference, kTypeIdActivityTracker,
+                           kTypeIdActivityTrackerFree);
+
+    // Success.
+    break;
+  }
+
+  // Handle the case where no previously-used memories are available.
+  if (count == 0) {
+    // Allocate a block of memory from the persistent segment.
+    mem_reference =
+        allocator_->Allocate(stack_memory_size_, kTypeIdActivityTracker);
+    if (mem_reference) {
+      // Success. Convert the reference to an actual memory address.
+      mem_base =
+          allocator_->GetAsObject<char>(mem_reference, kTypeIdActivityTracker);
+      // Make the allocation iterable so it can be found by other processes.
+      allocator_->MakeIterable(mem_reference);
+    } else {
+      // Failure. This shouldn't happen.
+      NOTREACHED();
+      // But if it does, probably because the allocator wasn't given enough
+      // memory to satisfy all possible requests, handle it gracefully by
+      // allocating the required memory from the heap.
+      mem_base = new char[stack_memory_size_];
+      memset(mem_base, 0, stack_memory_size_);
+      // Report the thread-count at which the allocator was full so that the
+      // failure can be seen and underlying memory resized appropriately.
+      UMA_HISTOGRAM_COUNTS_1000(
+          "UMA.ActivityTracker.ThreadTrackers.MemLimit",
+          thread_tracker_count_.load(std::memory_order_relaxed));
+    }
+  }
+
+  // Create a tracker with the acquired memory and set it as the tracker
+  // for this particular thread in thread-local-storage.
+  DCHECK(mem_base);
+  ManagedActivityTracker* tracker =
+      new ManagedActivityTracker(mem_reference, mem_base, stack_memory_size_);
+  DCHECK(tracker->IsValid());
+  this_thread_tracker_.Set(tracker);
+  int old_count = thread_tracker_count_.fetch_add(1, std::memory_order_relaxed);
+
+  UMA_HISTOGRAM_ENUMERATION("UMA.ActivityTracker.ThreadTrackers.Count",
+                            old_count + 1, kMaxThreadCount);
+  return tracker;
+}
+
+void GlobalActivityTracker::ReleaseTrackerForCurrentThreadForTesting() {
+  ThreadActivityTracker* tracker =
+      reinterpret_cast<ThreadActivityTracker*>(this_thread_tracker_.Get());
+  if (tracker) {
+    this_thread_tracker_.Free();
+    delete tracker;
+  }
+}
+
+GlobalActivityTracker::GlobalActivityTracker(
+    std::unique_ptr<PersistentMemoryAllocator> allocator,
+    int stack_depth)
+    : allocator_(std::move(allocator)),
+      stack_memory_size_(ThreadActivityTracker::SizeForStackDepth(stack_depth)),
+      this_thread_tracker_(&OnTLSDestroy),
+      thread_tracker_count_(0),
+      available_memories_count_(0) {
+  // Clear the available-memories array.
+  memset(available_memories_, 0, sizeof(available_memories_));
+
+  // Ensure the passed memory is valid and empty (iterator finds nothing).
+  uint32_t type;
+  DCHECK(!PersistentMemoryAllocator::Iterator(allocator_.get()).GetNext(&type));
+
+  // Ensure that there is no other global object and then make this one such.
+  DCHECK(!g_tracker_);
+  g_tracker_ = this;
+}
+
+GlobalActivityTracker::~GlobalActivityTracker() {
+  DCHECK_EQ(g_tracker_, this);
+  DCHECK_EQ(0, thread_tracker_count_.load(std::memory_order_relaxed));
+  g_tracker_ = nullptr;
+}
+
+void GlobalActivityTracker::ReturnTrackerMemory(
+    ManagedActivityTracker* tracker) {
+  PersistentMemoryAllocator::Reference mem_reference = tracker->mem_reference_;
+  void* mem_base = tracker->mem_base_;
+
+  // Zero the memory so that it is ready for use if needed again later. It's
+  // better to clear the memory now, when a thread is exiting, than to do it
+  // when it is first needed by a thread doing actual work.
+  memset(mem_base, 0, stack_memory_size_);
+
+  // Remove the destructed tracker from the set of known ones.
+  DCHECK_LE(1, thread_tracker_count_.load(std::memory_order_relaxed));
+  thread_tracker_count_.fetch_sub(1, std::memory_order_relaxed);
+
+  // Deal with the memory that was used by the tracker.
+  if (mem_reference) {
+    // The memory was within the persistent memory allocator. Change its type
+    // so that iteration won't find it.
+    allocator_->ChangeType(mem_reference, kTypeIdActivityTrackerFree,
+                           kTypeIdActivityTracker);
+    // There is no way to free memory from a persistent allocator so instead
+    // push it on the internal list of available memory blocks.
+    while (true) {
+      // Get the existing count of available memories and ensure we won't
+      // burst the array. Acquire the values in the array.
+      int count = available_memories_count_.load(std::memory_order_acquire);
+      if (count >= kMaxThreadCount) {
+        NOTREACHED();
+        // Storage is full. Just forget about this memory. It won't be re-used
+        // but there's no real loss.
+        break;
+      }
+
+      // Write the reference of the memory being returned to this slot in the
+      // array. Empty slots have a value of zero so do an atomic compare-and-
+      // exchange to ensure that a race condition doesn't exist with another
+      // thread doing the same.
+      PersistentMemoryAllocator::Reference mem_expected = 0;
+      if (!available_memories_[count].compare_exchange_weak(
+              mem_expected, mem_reference, std::memory_order_release,
+              std::memory_order_relaxed)) {
+        continue;  // Try again.
+      }
+
+      // Increment the count, releasing the value written to the array. This
+      // could fail if a simultaneous "pop" operation decremented the counter.
+      // If that happens, clear the array slot and start over. Do a "strong"
+      // exchange to avoid spurious retries that can occur with a "weak" one.
+      int expected = count;  // Updated by compare/exchange.
+      if (!available_memories_count_.compare_exchange_strong(
+              expected, count + 1, std::memory_order_release,
+              std::memory_order_relaxed)) {
+        available_memories_[count].store(0, std::memory_order_relaxed);
+        continue;
+      }
+
+      // Count was successfully incremented to reflect the newly added value.
+      break;
+    }
+  } else {
+    // The memory was allocated from the process heap. This shouldn't happen
+    // because the persistent memory segment should be big enough for all
+    // thread stacks but it's better to support falling back to allocation
+    // from the heap rather than crash. Everything will work as normal but
+    // the data won't be persisted.
+    delete[] reinterpret_cast<char*>(mem_base);
+  }
+}
+
+// static
+void GlobalActivityTracker::OnTLSDestroy(void* value) {
+  delete reinterpret_cast<ManagedActivityTracker*>(value);
+}
+
+
+ScopedActivity::ScopedActivity(const tracked_objects::Location& location,
+                               uint8_t action,
+                               uint32_t id,
+                               uint32_t info)
+    : GlobalActivityTracker::ScopedThreadActivity(
+          location.program_counter(),
+          static_cast<ThreadActivityTracker::ActivityType>(
+              ThreadActivityTracker::ACT_GENERIC | action),
+          ThreadActivityTracker::ActivityData::ForGeneric(id, info),
+          /*lock_allowed=*/true),
+      id_(id) {
+  // The action must not affect the category bits of the activity type.
+  DCHECK_EQ(0, action & ThreadActivityTracker::ACT_CATEGORY_MASK);
+}
+
+void ScopedActivity::ChangeAction(uint8_t action) {
+  DCHECK_EQ(0, action & ThreadActivityTracker::ACT_CATEGORY_MASK);
+  ChangeTypeAndData(static_cast<ThreadActivityTracker::ActivityType>(
+                        ThreadActivityTracker::ACT_GENERIC | action),
+                    ThreadActivityTracker::kNullActivityData);
+}
+
+void ScopedActivity::ChangeInfo(uint32_t info) {
+  ChangeTypeAndData(ThreadActivityTracker::ACT_NULL,
+                    ThreadActivityTracker::ActivityData::ForGeneric(id_, info));
+}
+
+void ScopedActivity::ChangeActionAndInfo(uint8_t action, uint32_t info) {
+  DCHECK_EQ(0, action & ThreadActivityTracker::ACT_CATEGORY_MASK);
+  ChangeTypeAndData(static_cast<ThreadActivityTracker::ActivityType>(
+                        ThreadActivityTracker::ACT_GENERIC | action),
+                    ThreadActivityTracker::ActivityData::ForGeneric(id_, info));
+}
+
+ScopedTaskRunActivity::ScopedTaskRunActivity(const base::PendingTask& task)
+    : GlobalActivityTracker::ScopedThreadActivity(
+          task.posted_from.program_counter(),
+          ThreadActivityTracker::ACT_TASK_RUN,
+          ThreadActivityTracker::ActivityData::ForTask(task.sequence_num),
+          /*lock_allowed=*/true) {}
+
+ScopedLockAcquireActivity::ScopedLockAcquireActivity(
+    const base::internal::LockImpl* lock)
+    : GlobalActivityTracker::ScopedThreadActivity(
+          nullptr,  // TODO(bcwhite): Find a real address.
+          ThreadActivityTracker::ACT_LOCK_ACQUIRE,
+          ThreadActivityTracker::ActivityData::ForLock(lock),
+          /*lock_allowed=*/false) {}
+
+ScopedEventWaitActivity::ScopedEventWaitActivity(
+    const base::WaitableEvent* event)
+    : GlobalActivityTracker::ScopedThreadActivity(
+          nullptr,  // TODO(bcwhite): Find a real address.
+          ThreadActivityTracker::ACT_EVENT_WAIT,
+          ThreadActivityTracker::ActivityData::ForEvent(event),
+          /*lock_allowed=*/true) {}
+
+ScopedThreadJoinActivity::ScopedThreadJoinActivity(
+    const base::PlatformThreadHandle* thread)
+    : GlobalActivityTracker::ScopedThreadActivity(
+          nullptr,  // TODO(bcwhite): Find a real address.
+          ThreadActivityTracker::ACT_THREAD_JOIN,
+          ThreadActivityTracker::ActivityData::ForThread(*thread),
+          /*lock_allowed=*/true) {}
+
+#if !defined(OS_NACL) && !defined(OS_IOS)
+ScopedProcessWaitActivity::ScopedProcessWaitActivity(
+    const base::Process* process)
+    : GlobalActivityTracker::ScopedThreadActivity(
+          nullptr,  // TODO(bcwhite): Find a real address.
+          ThreadActivityTracker::ACT_PROCESS_WAIT,
+          ThreadActivityTracker::ActivityData::ForProcess(process->Pid()),
+          /*lock_allowed=*/true) {}
+#endif
+
+}  // namespace debug
+}  // namespace base