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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
//
// 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.
#ifndef ABSL_STRINGS_CORDZ_HANDLE_H_
#define ABSL_STRINGS_CORDZ_HANDLE_H_
#include <atomic>
#include <vector>
#include "absl/base/config.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/base/internal/spinlock.h"
#include "absl/synchronization/mutex.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace cord_internal {
// This base class allows multiple types of object (CordzInfo and
// CordzSampleToken) to exist simultaneously on the delete queue (pointed to by
// global_dq_tail and traversed using dq_prev_ and dq_next_). The
// delete queue guarantees that once a profiler creates a CordzSampleToken and
// has gained visibility into a CordzInfo object, that CordzInfo object will not
// be deleted prematurely. This allows the profiler to inspect all CordzInfo
// objects that are alive without needing to hold a global lock.
class CordzHandle {
public:
CordzHandle() : CordzHandle(false) {}
bool is_snapshot() const { return is_snapshot_; }
// Returns true if this instance is safe to be deleted because it is either a
// snapshot, which is always safe to delete, or not included in the global
// delete queue and thus not included in any snapshot.
// Callers are responsible for making sure this instance can not be newly
// discovered by other threads. For example, CordzInfo instances first de-list
// themselves from the global CordzInfo list before determining if they are
// safe to be deleted directly.
// If SafeToDelete returns false, callers MUST use the Delete() method to
// safely queue CordzHandle instances for deletion.
bool SafeToDelete() const;
// Deletes the provided instance, or puts it on the delete queue to be deleted
// once there are no more sample tokens (snapshot) instances potentially
// referencing the instance. `handle` should not be null.
static void Delete(CordzHandle* handle);
// Returns the current entries in the delete queue in LIFO order.
static std::vector<const CordzHandle*> DiagnosticsGetDeleteQueue();
// Returns true if the provided handle is nullptr or guarded by this handle.
// Since the CordzSnapshot token is itself a CordzHandle, this method will
// allow tests to check if that token is keeping an arbitrary CordzHandle
// alive.
bool DiagnosticsHandleIsSafeToInspect(const CordzHandle* handle) const;
// Returns the current entries in the delete queue, in LIFO order, that are
// protected by this. CordzHandle objects are only placed on the delete queue
// after CordzHandle::Delete is called with them as an argument. Only
// CordzHandle objects that are not also CordzSnapshot objects will be
// included in the return vector. For each of the handles in the return
// vector, the earliest that their memory can be freed is when this
// CordzSnapshot object is deleted.
std::vector<const CordzHandle*> DiagnosticsGetSafeToInspectDeletedHandles();
protected:
explicit CordzHandle(bool is_snapshot);
virtual ~CordzHandle();
private:
// Global queue data. CordzHandle stores a pointer to the global queue
// instance to harden against ODR violations.
struct Queue {
constexpr explicit Queue(absl::ConstInitType)
: mutex(absl::kConstInit,
absl::base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL) {}
absl::base_internal::SpinLock mutex;
std::atomic<CordzHandle*> dq_tail ABSL_GUARDED_BY(mutex){nullptr};
// Returns true if this delete queue is empty. This method does not acquire
// the lock, but does a 'load acquire' observation on the delete queue tail.
// It is used inside Delete() to check for the presence of a delete queue
// without holding the lock. The assumption is that the caller is in the
// state of 'being deleted', and can not be newly discovered by a concurrent
// 'being constructed' snapshot instance. Practically, this means that any
// such discovery (`find`, 'first' or 'next', etc) must have proper 'happens
// before / after' semantics and atomic fences.
bool IsEmpty() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
return dq_tail.load(std::memory_order_acquire) == nullptr;
}
};
void ODRCheck() const {
#ifndef NDEBUG
ABSL_RAW_CHECK(queue_ == &global_queue_, "ODR violation in Cord");
#endif
}
ABSL_CONST_INIT static Queue global_queue_;
Queue* const queue_ = &global_queue_;
const bool is_snapshot_;
// dq_prev_ and dq_next_ require the global queue mutex to be held.
// Unfortunately we can't use thread annotations such that the thread safety
// analysis understands that queue_ and global_queue_ are one and the same.
CordzHandle* dq_prev_ = nullptr;
CordzHandle* dq_next_ = nullptr;
};
class CordzSnapshot : public CordzHandle {
public:
CordzSnapshot() : CordzHandle(true) {}
};
} // namespace cord_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_STRINGS_CORDZ_HANDLE_H_
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