132 lines
5.4 KiB
C++
132 lines
5.4 KiB
C++
// Copyright 2019 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef ABSL_STRINGS_CORDZ_HANDLE_H_
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#define ABSL_STRINGS_CORDZ_HANDLE_H_
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#include <atomic>
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#include <vector>
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#include "absl/base/config.h"
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#include "absl/base/internal/raw_logging.h"
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#include "absl/base/internal/spinlock.h"
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#include "absl/synchronization/mutex.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace cord_internal {
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// This base class allows multiple types of object (CordzInfo and
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// CordzSampleToken) to exist simultaneously on the delete queue (pointed to by
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// global_dq_tail and traversed using dq_prev_ and dq_next_). The
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// delete queue guarantees that once a profiler creates a CordzSampleToken and
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// has gained visibility into a CordzInfo object, that CordzInfo object will not
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// be deleted prematurely. This allows the profiler to inspect all CordzInfo
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// objects that are alive without needing to hold a global lock.
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class CordzHandle {
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public:
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CordzHandle() : CordzHandle(false) {}
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bool is_snapshot() const { return is_snapshot_; }
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// Returns true if this instance is safe to be deleted because it is either a
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// snapshot, which is always safe to delete, or not included in the global
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// delete queue and thus not included in any snapshot.
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// Callers are responsible for making sure this instance can not be newly
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// discovered by other threads. For example, CordzInfo instances first de-list
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// themselves from the global CordzInfo list before determining if they are
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// safe to be deleted directly.
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// If SafeToDelete returns false, callers MUST use the Delete() method to
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// safely queue CordzHandle instances for deletion.
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bool SafeToDelete() const;
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// Deletes the provided instance, or puts it on the delete queue to be deleted
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// once there are no more sample tokens (snapshot) instances potentially
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// referencing the instance. `handle` should not be null.
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static void Delete(CordzHandle* handle);
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// Returns the current entries in the delete queue in LIFO order.
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static std::vector<const CordzHandle*> DiagnosticsGetDeleteQueue();
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// Returns true if the provided handle is nullptr or guarded by this handle.
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// Since the CordzSnapshot token is itself a CordzHandle, this method will
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// allow tests to check if that token is keeping an arbitrary CordzHandle
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// alive.
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bool DiagnosticsHandleIsSafeToInspect(const CordzHandle* handle) const;
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// Returns the current entries in the delete queue, in LIFO order, that are
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// protected by this. CordzHandle objects are only placed on the delete queue
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// after CordzHandle::Delete is called with them as an argument. Only
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// CordzHandle objects that are not also CordzSnapshot objects will be
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// included in the return vector. For each of the handles in the return
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// vector, the earliest that their memory can be freed is when this
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// CordzSnapshot object is deleted.
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std::vector<const CordzHandle*> DiagnosticsGetSafeToInspectDeletedHandles();
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protected:
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explicit CordzHandle(bool is_snapshot);
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virtual ~CordzHandle();
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private:
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// Global queue data. CordzHandle stores a pointer to the global queue
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// instance to harden against ODR violations.
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struct Queue {
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constexpr explicit Queue(absl::ConstInitType)
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: mutex(absl::kConstInit,
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absl::base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL) {}
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absl::base_internal::SpinLock mutex;
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std::atomic<CordzHandle*> dq_tail ABSL_GUARDED_BY(mutex){nullptr};
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// Returns true if this delete queue is empty. This method does not acquire
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// the lock, but does a 'load acquire' observation on the delete queue tail.
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// It is used inside Delete() to check for the presence of a delete queue
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// without holding the lock. The assumption is that the caller is in the
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// state of 'being deleted', and can not be newly discovered by a concurrent
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// 'being constructed' snapshot instance. Practically, this means that any
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// such discovery (`find`, 'first' or 'next', etc) must have proper 'happens
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// before / after' semantics and atomic fences.
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bool IsEmpty() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
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return dq_tail.load(std::memory_order_acquire) == nullptr;
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}
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};
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void ODRCheck() const {
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#ifndef NDEBUG
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ABSL_RAW_CHECK(queue_ == &global_queue_, "ODR violation in Cord");
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#endif
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}
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ABSL_CONST_INIT static Queue global_queue_;
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Queue* const queue_ = &global_queue_;
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const bool is_snapshot_;
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// dq_prev_ and dq_next_ require the global queue mutex to be held.
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// Unfortunately we can't use thread annotations such that the thread safety
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// analysis understands that queue_ and global_queue_ are one and the same.
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CordzHandle* dq_prev_ = nullptr;
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CordzHandle* dq_next_ = nullptr;
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};
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class CordzSnapshot : public CordzHandle {
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public:
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CordzSnapshot() : CordzHandle(true) {}
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};
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} // namespace cord_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_STRINGS_CORDZ_HANDLE_H_
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