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// 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
//
//
// 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.
#ifndef ABSL_STRINGS_INTERNAL_CORDZ_INFO_H_
#define ABSL_STRINGS_INTERNAL_CORDZ_INFO_H_
#include <atomic>
#include <cstdint>
#include <functional>
#include "absl/base/config.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/base/internal/spinlock.h"
#include "absl/base/thread_annotations.h"
#include "absl/strings/internal/cord_internal.h"
#include "absl/strings/internal/cordz_functions.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 {
// CordzInfo tracks a profiled Cord. Each of these objects can be in two places.
// If a Cord is alive, the CordzInfo will be in the global_cordz_infos map, and
// can also be retrieved via the linked list starting with
// global_cordz_infos_head and continued via the cordz_info_next() method. When
// a Cord has reached the end of its lifespan, the CordzInfo object will be
// migrated out of the global_cordz_infos list and the global_cordz_infos_map,
// and will either be deleted or appended to the global_delete_queue. If it is
// placed on the global_delete_queue, the CordzInfo object will be cleaned in
// the destructor of a CordzSampleToken object.
class ABSL_LOCKABLE CordzInfo : public CordzHandle {
public:
using MethodIdentifier = CordzUpdateTracker::MethodIdentifier;
// TrackCord creates a CordzInfo instance which tracks important metrics of
// a sampled cord, and stores the created CordzInfo instance into `cord'. All
// CordzInfo instances are placed in a global list which is used to discover
// and snapshot all actively tracked cords. Callers are responsible for
// calling UntrackCord() before the tracked Cord instance is deleted, or to
// stop tracking the sampled Cord. Callers are also responsible for guarding
// changes to the 'tree' value of a Cord (InlineData.tree) through the Lock()
// and Unlock() calls. Any change resulting in a new tree value for the cord
// requires a call to SetCordRep() before the old tree has been unreffed
// and/or deleted. `method` identifies the Cord public API method initiating
// the cord to be sampled.
// Requires `cord` to hold a tree, and `cord.cordz_info()` to be null.
static void TrackCord(InlineData& cord, MethodIdentifier method);
// Identical to TrackCord(), except that this function fills the
// `parent_stack` and `parent_method` properties of the returned CordzInfo
// instance from the provided `src` instance if `src` is sampled.
// This function should be used for sampling 'copy constructed' and 'copy
// assigned' cords. This function allows 'cord` to be already sampled, in
// which case the CordzInfo will be newly created from `src`.
static void TrackCord(InlineData& cord, const InlineData& src,
MethodIdentifier method);
// Maybe sample the cord identified by 'cord' for method 'method'.
// Uses `cordz_should_profile` to randomly pick cords to be sampled, and if
// so, invokes `TrackCord` to start sampling `cord`.
static void MaybeTrackCord(InlineData& cord, MethodIdentifier method);
// Maybe sample the cord identified by 'cord' for method 'method'.
// `src` identifies a 'parent' cord which is assigned to `cord`, typically the
// input cord for a copy constructor, or an assign method such as `operator=`
// `cord` will be sampled if (and only if) `src` is sampled.
// If `cord` is currently being sampled and `src` is not being sampled, then
// this function will stop sampling the cord and reset the cord's cordz_info.
//
// Previously this function defined that `cord` will be sampled if either
// `src` is sampled, or if `cord` is randomly picked for sampling. However,
// this can cause issues, as there may be paths where some cord is assigned an
// indirect copy of it's own value. As such a 'string of copies' would then
// remain sampled (`src.is_profiled`), then assigning such a cord back to
// 'itself' creates a cycle where the cord will converge to 'always sampled`.
//
// For example:
//
// Cord x;
// for (...) {
// // Copy ctor --> y.is_profiled := x.is_profiled | random(...)
// Cord y = x;
// ...
// // Assign x = y --> x.is_profiled = y.is_profiled | random(...)
// // ==> x.is_profiled |= random(...)
// // ==> x converges to 'always profiled'
// x = y;
// }
static void MaybeTrackCord(InlineData& cord, const InlineData& src,
MethodIdentifier method);
// Stops tracking changes for a sampled cord, and deletes the provided info.
// This function must be called before the sampled cord instance is deleted,
// and before the root cordrep of the sampled cord is unreffed.
// This function may extend the lifetime of the cordrep in cases where the
// CordInfo instance is being held by a concurrent collection thread.
void Untrack();
// Invokes UntrackCord() on `info` if `info` is not null.
static void MaybeUntrackCord(CordzInfo* info);
CordzInfo() = delete;
CordzInfo(const CordzInfo&) = delete;
CordzInfo& operator=(const CordzInfo&) = delete;
// Retrieves the oldest existing CordzInfo.
static CordzInfo* Head(const CordzSnapshot& snapshot)
ABSL_NO_THREAD_SAFETY_ANALYSIS;
// Retrieves the next oldest existing CordzInfo older than 'this' instance.
CordzInfo* Next(const CordzSnapshot& snapshot) const
ABSL_NO_THREAD_SAFETY_ANALYSIS;
// Locks this instance for the update identified by `method`.
// Increases the count for `method` in `update_tracker`.
void Lock(MethodIdentifier method) ABSL_EXCLUSIVE_LOCK_FUNCTION(mutex_);
// Unlocks this instance. If the contained `rep` has been set to null
// indicating the Cord has been cleared or is otherwise no longer sampled,
// then this method will delete this CordzInfo instance.
void Unlock() ABSL_UNLOCK_FUNCTION(mutex_);
// Asserts that this CordzInfo instance is locked.
void AssertHeld() ABSL_ASSERT_EXCLUSIVE_LOCK(mutex_);
// Updates the `rep` property of this instance. This methods is invoked by
// Cord logic each time the root node of a sampled Cord changes, and before
// the old root reference count is deleted. This guarantees that collection
// code can always safely take a reference on the tracked cord.
// Requires a lock to be held through the `Lock()` method.
// TODO(b/117940323): annotate with ABSL_EXCLUSIVE_LOCKS_REQUIRED once all
// Cord code is in a state where this can be proven true by the compiler.
void SetCordRep(CordRep* rep);
// Returns the current `rep` property of this instance with a reference
// added, or null if this instance represents a cord that has since been
// deleted or untracked.
CordRep* RefCordRep() const ABSL_LOCKS_EXCLUDED(mutex_);
// Returns the current value of `rep_` for testing purposes only.
CordRep* GetCordRepForTesting() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
return rep_;
}
// Sets the current value of `rep_` for testing purposes only.
void SetCordRepForTesting(CordRep* rep) ABSL_NO_THREAD_SAFETY_ANALYSIS {
rep_ = rep;
}
// Returns the stack trace for where the cord was first sampled. Cords are
// potentially sampled when they promote from an inlined cord to a tree or
// ring representation, which is not necessarily the location where the cord
// was first created. Some cords are created as inlined cords, and only as
// data is added do they become a non-inlined cord. However, typically the
// location represents reasonably well where the cord is 'created'.
absl::Span<void* const> GetStack() const;
// Returns the stack trace for a sampled cord's 'parent stack trace'. This
// value may be set if the cord is sampled (promoted) after being created
// from, or being assigned the value of an existing (sampled) cord.
absl::Span<void* const> GetParentStack() const;
// Retrieves the CordzStatistics associated with this Cord. The statistics
// are only updated when a Cord goes through a mutation, such as an Append
// or RemovePrefix.
CordzStatistics GetCordzStatistics() const;
private:
using SpinLock = absl::base_internal::SpinLock;
using SpinLockHolder = ::absl::base_internal::SpinLockHolder;
// Global cordz info list. CordzInfo stores a pointer to the global list
// instance to harden against ODR violations.
struct List {
constexpr explicit List(absl::ConstInitType)
: mutex(absl::kConstInit,
absl::base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL) {}
SpinLock mutex;
std::atomic<CordzInfo*> head ABSL_GUARDED_BY(mutex){nullptr};
};
static constexpr size_t kMaxStackDepth = 64;
explicit CordzInfo(CordRep* rep, const CordzInfo* src,
MethodIdentifier method);
~CordzInfo() override;
// Sets `rep_` without holding a lock.
void UnsafeSetCordRep(CordRep* rep) ABSL_NO_THREAD_SAFETY_ANALYSIS;
void Track();
// Returns the parent method from `src`, which is either `parent_method_` or
// `method_` depending on `parent_method_` being kUnknown.
// Returns kUnknown if `src` is null.
static MethodIdentifier GetParentMethod(const CordzInfo* src);
// Fills the provided stack from `src`, copying either `parent_stack_` or
// `stack_` depending on `parent_stack_` being empty, returning the size of
// the parent stack.
// Returns 0 if `src` is null.
static size_t FillParentStack(const CordzInfo* src, void** stack);
void ODRCheck() const {
#ifndef NDEBUG
ABSL_RAW_CHECK(list_ == &global_list_, "ODR violation in Cord");
#endif
}
// Non-inlined implementation of `MaybeTrackCord`, which is executed if
// either `src` is sampled or `cord` is sampled, and either untracks or
// tracks `cord` as documented per `MaybeTrackCord`.
static void MaybeTrackCordImpl(InlineData& cord, const InlineData& src,
MethodIdentifier method);
ABSL_CONST_INIT static List global_list_;
List* const list_ = &global_list_;
// ci_prev_ and ci_next_ require the global list mutex to be held.
// Unfortunately we can't use thread annotations such that the thread safety
// analysis understands that list_ and global_list_ are one and the same.
std::atomic<CordzInfo*> ci_prev_{nullptr};
std::atomic<CordzInfo*> ci_next_{nullptr};
mutable absl::Mutex mutex_;
CordRep* rep_ ABSL_GUARDED_BY(mutex_);
void* stack_[kMaxStackDepth];
void* parent_stack_[kMaxStackDepth];
const size_t stack_depth_;
const size_t parent_stack_depth_;
const MethodIdentifier method_;
const MethodIdentifier parent_method_;
CordzUpdateTracker update_tracker_;
const absl::Time create_time_;
};
inline ABSL_ATTRIBUTE_ALWAYS_INLINE void CordzInfo::MaybeTrackCord(
InlineData& cord, MethodIdentifier method) {
if (ABSL_PREDICT_FALSE(cordz_should_profile())) {
TrackCord(cord, method);
}
}
inline ABSL_ATTRIBUTE_ALWAYS_INLINE void CordzInfo::MaybeTrackCord(
InlineData& cord, const InlineData& src, MethodIdentifier method) {
if (ABSL_PREDICT_FALSE(InlineData::is_either_profiled(cord, src))) {
MaybeTrackCordImpl(cord, src, method);
}
}
inline ABSL_ATTRIBUTE_ALWAYS_INLINE void CordzInfo::MaybeUntrackCord(
CordzInfo* info) {
if (ABSL_PREDICT_FALSE(info)) {
info->Untrack();
}
}
inline void CordzInfo::AssertHeld() ABSL_ASSERT_EXCLUSIVE_LOCK(mutex_) {
#ifndef NDEBUG
mutex_.AssertHeld();
#endif
}
inline void CordzInfo::SetCordRep(CordRep* rep) {
AssertHeld();
rep_ = rep;
}
inline void CordzInfo::UnsafeSetCordRep(CordRep* rep) { rep_ = rep; }
inline CordRep* CordzInfo::RefCordRep() const ABSL_LOCKS_EXCLUDED(mutex_) {
MutexLock lock(&mutex_);
return rep_ ? CordRep::Ref(rep_) : nullptr;
}
} // namespace cord_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_STRINGS_INTERNAL_CORDZ_INFO_H_