abseil-cpp/absl/strings/internal/cordz_info.cc

// Copyright 2019 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "absl/strings/internal/cordz_info.h"

#include "absl/base/config.h"
#include "absl/base/internal/spinlock.h"
#include "absl/debugging/stacktrace.h"
#include "absl/strings/internal/cord_internal.h"
#include "absl/strings/internal/cordz_handle.h"
#include "absl/strings/internal/cordz_statistics.h"
#include "absl/strings/internal/cordz_update_tracker.h"
#include "absl/synchronization/mutex.h"
#include "absl/types/span.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace cord_internal {

using ::absl::base_internal::SpinLockHolder;

constexpr int CordzInfo::kMaxStackDepth;

ABSL_CONST_INIT CordzInfo::List CordzInfo::global_list_{absl::kConstInit};

CordzInfo* CordzInfo::Head(const CordzSnapshot& snapshot) {
  ABSL_ASSERT(snapshot.is_snapshot());

  // We can do an 'unsafe' load of 'head', as we are guaranteed that the
  // instance it points to is kept alive by the provided CordzSnapshot, so we
  // can simply return the current value using an acquire load.
  // We do enforce in DEBUG builds that the 'head' value is present in the
  // delete queue: ODR violations may lead to 'snapshot' and 'global_list_'
  // being in different libraries / modules.
  CordzInfo* head = global_list_.head.load(std::memory_order_acquire);
  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(head));
  return head;
}

CordzInfo* CordzInfo::Next(const CordzSnapshot& snapshot) const {
  ABSL_ASSERT(snapshot.is_snapshot());

  // Similar to the 'Head()' function, we do not need a mutex here.
  CordzInfo* next = ci_next_.load(std::memory_order_acquire);
  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(this));
  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(next));
  return next;
}

void CordzInfo::TrackCord(InlineData& cord, MethodIdentifier method) {
  assert(cord.is_tree());
  assert(!cord.is_profiled());
  CordzInfo* cordz_info = new CordzInfo(cord.as_tree(), nullptr, method);
  cord.set_cordz_info(cordz_info);
  cordz_info->Track();
}

void CordzInfo::TrackCord(InlineData& cord, const InlineData& src,
                          MethodIdentifier method) {
  assert(cord.is_tree());
  assert(!cord.is_profiled());
  auto* info = src.is_tree() && src.is_profiled() ? src.cordz_info() : nullptr;
  CordzInfo* cordz_info = new CordzInfo(cord.as_tree(), info, method);
  cord.set_cordz_info(cordz_info);
  cordz_info->Track();
}

CordzInfo::MethodIdentifier CordzInfo::GetParentMethod(const CordzInfo* src) {
  if (src == nullptr) return MethodIdentifier::kUnknown;
  return src->parent_method_ != MethodIdentifier::kUnknown ? src->parent_method_
                                                           : src->method_;
}

int CordzInfo::FillParentStack(const CordzInfo* src, void** stack) {
  assert(stack);
  if (src == nullptr) return 0;
  if (src->parent_stack_depth_) {
    memcpy(stack, src->parent_stack_, src->parent_stack_depth_ * sizeof(void*));
    return src->parent_stack_depth_;
  }
  memcpy(stack, src->stack_, src->stack_depth_ * sizeof(void*));
  return src->stack_depth_;
}

CordzInfo::CordzInfo(CordRep* rep, const CordzInfo* src,
                     MethodIdentifier method)
    : rep_(rep),
      stack_depth_(absl::GetStackTrace(stack_, /*max_depth=*/kMaxStackDepth,
                                       /*skip_count=*/1)),
      parent_stack_depth_(FillParentStack(src, parent_stack_)),
      method_(method),
      parent_method_(GetParentMethod(src)),
      create_time_(absl::Now()),
      size_(rep->length) {
  update_tracker_.LossyAdd(method);
}

CordzInfo::~CordzInfo() {
  // `rep_` is potentially kept alive if CordzInfo is included
  // in a collection snapshot (which should be rare).
  if (ABSL_PREDICT_FALSE(rep_)) {
    CordRep::Unref(rep_);
  }
}

void CordzInfo::Track() {
  SpinLockHolder l(&list_->mutex);

  CordzInfo* const head = list_->head.load(std::memory_order_acquire);
  if (head != nullptr) {
    head->ci_prev_.store(this, std::memory_order_release);
  }
  ci_next_.store(head, std::memory_order_release);
  list_->head.store(this, std::memory_order_release);
}

void CordzInfo::Untrack() {
  ODRCheck();
  {
    SpinLockHolder l(&list_->mutex);

    CordzInfo* const head = list_->head.load(std::memory_order_acquire);
    CordzInfo* const next = ci_next_.load(std::memory_order_acquire);
    CordzInfo* const prev = ci_prev_.load(std::memory_order_acquire);

    if (next) {
      ABSL_ASSERT(next->ci_prev_.load(std::memory_order_acquire) == this);
      next->ci_prev_.store(prev, std::memory_order_release);
    }
    if (prev) {
      ABSL_ASSERT(head != this);
      ABSL_ASSERT(prev->ci_next_.load(std::memory_order_acquire) == this);
      prev->ci_next_.store(next, std::memory_order_release);
    } else {
      ABSL_ASSERT(head == this);
      list_->head.store(next, std::memory_order_release);
    }
  }

  // We can no longer be discovered: perform a fast path check if we are not
  // listed on any delete queue, so we can directly delete this instance.
  if (SafeToDelete()) {
    UnsafeSetCordRep(nullptr);
    delete this;
    return;
  }

  // We are likely part of a snapshot, extend the life of the CordRep
  {
    absl::MutexLock lock(&mutex_);
    if (rep_) CordRep::Ref(rep_);
  }
  CordzHandle::Delete(this);
}

void CordzInfo::Lock(MethodIdentifier method)
    ABSL_EXCLUSIVE_LOCK_FUNCTION(mutex_) {
  mutex_.Lock();
  update_tracker_.LossyAdd(method);
  assert(rep_);
}

void CordzInfo::Unlock() ABSL_UNLOCK_FUNCTION(mutex_) {
  bool tracked = rep_ != nullptr;
  if (rep_) {
    size_.store(rep_->length);
  }
  mutex_.Unlock();
  if (!tracked) {
    Untrack();
  }
}

absl::Span<void* const> CordzInfo::GetStack() const {
  return absl::MakeConstSpan(stack_, stack_depth_);
}

absl::Span<void* const> CordzInfo::GetParentStack() const {
  return absl::MakeConstSpan(parent_stack_, parent_stack_depth_);
}

CordzStatistics CordzInfo::GetCordzStatistics() const {
  CordzStatistics stats;
  stats.method = method_;
  stats.parent_method = parent_method_;
  stats.update_tracker = update_tracker_;
  stats.size = size_.load(std::memory_order_relaxed);
  return stats;
}

}  // namespace cord_internal
ABSL_NAMESPACE_END
}  // namespace absl