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//===-- xray_buffer_queue.h ------------------------------------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file is a part of XRay, a dynamic runtime instrumentation system.
// Defines the interface for a buffer queue implementation.
#include "sanitizer_common/sanitizer_atomic.h"
#include "sanitizer_common/sanitizer_common.h"
#include "sanitizer_common/sanitizer_mutex.h"
#include "xray_defs.h"
#include <cstddef>
#include <cstdint>
namespace __xray {
/// BufferQueue implements a circular queue of fixed sized buffers (much like a
/// freelist) but is concerned with making it quick to initialise, finalise, and
/// get from or return buffers to the queue. This is one key component of the
/// "flight data recorder" (FDR) mode to support ongoing XRay function call
/// trace collection.
class BufferQueue {
/// ControlBlock represents the memory layout of how we interpret the backing
/// store for all buffers and extents managed by a BufferQueue instance. The
/// ControlBlock has the reference count as the first member, sized according
/// to platform-specific cache-line size. We never use the Buffer member of
/// the union, which is only there for compiler-supported alignment and
/// sizing.
/// This ensures that the `Data` member will be placed at least kCacheLineSize
/// bytes from the beginning of the structure.
struct ControlBlock {
union {
atomic_uint64_t RefCount;
char Buffer[kCacheLineSize];
/// We need to make this size 1, to conform to the C++ rules for array data
/// members. Typically, we want to subtract this 1 byte for sizing
/// information.
char Data[1];
struct Buffer {
atomic_uint64_t *Extents = nullptr;
uint64_t Generation{0};
void *Data = nullptr;
size_t Size = 0;
friend class BufferQueue;
ControlBlock *BackingStore = nullptr;
ControlBlock *ExtentsBackingStore = nullptr;
size_t Count = 0;
struct BufferRep {
// The managed buffer.
Buffer Buff;
// This is true if the buffer has been returned to the available queue, and
// is considered "used" by another thread.
bool Used = false;
// This models a ForwardIterator. |T| Must be either a `Buffer` or `const
// Buffer`. Note that we only advance to the "used" buffers, when
// incrementing, so that at dereference we're always at a valid point.
template <class T> class Iterator {
BufferRep *Buffers = nullptr;
size_t Offset = 0;
size_t Max = 0;
Iterator &operator++() {
DCHECK_NE(Offset, Max);
do {
} while (!Buffers[Offset].Used && Offset != Max);
return *this;
Iterator operator++(int) {
Iterator C = *this;
return C;
T &operator*() const { return Buffers[Offset].Buff; }
T *operator->() const { return &(Buffers[Offset].Buff); }
Iterator(BufferRep *Root, size_t O, size_t M) XRAY_NEVER_INSTRUMENT
: Buffers(Root),
Max(M) {
// We want to advance to the first Offset where the 'Used' property is
// true, or to the end of the list/queue.
while (!Buffers[Offset].Used && Offset != Max) {
Iterator() = default;
Iterator(const Iterator &) = default;
Iterator(Iterator &&) = default;
Iterator &operator=(const Iterator &) = default;
Iterator &operator=(Iterator &&) = default;
~Iterator() = default;
template <class V>
friend bool operator==(const Iterator &L, const Iterator<V> &R) {
DCHECK_EQ(L.Max, R.Max);
return L.Buffers == R.Buffers && L.Offset == R.Offset;
template <class V>
friend bool operator!=(const Iterator &L, const Iterator<V> &R) {
return !(L == R);
// Size of each individual Buffer.
size_t BufferSize;
// Amount of pre-allocated buffers.
size_t BufferCount;
SpinMutex Mutex;
atomic_uint8_t Finalizing;
// The collocated ControlBlock and buffer storage.
ControlBlock *BackingStore;
// The collocated ControlBlock and extents storage.
ControlBlock *ExtentsBackingStore;
// A dynamically allocated array of BufferRep instances.
BufferRep *Buffers;
// Pointer to the next buffer to be handed out.
BufferRep *Next;
// Pointer to the entry in the array where the next released buffer will be
// placed.
BufferRep *First;
// Count of buffers that have been handed out through 'getBuffer'.
size_t LiveBuffers;
// We use a generation number to identify buffers and which generation they're
// associated with.
atomic_uint64_t Generation;
/// Releases references to the buffers backed by the current buffer queue.
void cleanupBuffers();
enum class ErrorCode : unsigned {
static const char *getErrorString(ErrorCode E) {
switch (E) {
case ErrorCode::Ok:
return "(none)";
case ErrorCode::NotEnoughMemory:
return "no available buffers in the queue";
case ErrorCode::QueueFinalizing:
return "queue already finalizing";
case ErrorCode::UnrecognizedBuffer:
return "buffer being returned not owned by buffer queue";
case ErrorCode::AlreadyFinalized:
return "queue already finalized";
case ErrorCode::AlreadyInitialized:
return "queue already initialized";
return "unknown error";
/// Initialise a queue of size |N| with buffers of size |B|. We report success
/// through |Success|.
BufferQueue(size_t B, size_t N, bool &Success);
/// Updates |Buf| to contain the pointer to an appropriate buffer. Returns an
/// error in case there are no available buffers to return when we will run
/// over the upper bound for the total buffers.
/// Requirements:
/// - BufferQueue is not finalising.
/// Returns:
/// - ErrorCode::NotEnoughMemory on exceeding MaxSize.
/// - ErrorCode::Ok when we find a Buffer.
/// - ErrorCode::QueueFinalizing or ErrorCode::AlreadyFinalized on
/// a finalizing/finalized BufferQueue.
ErrorCode getBuffer(Buffer &Buf);
/// Updates |Buf| to point to nullptr, with size 0.
/// Returns:
/// - ErrorCode::Ok when we successfully release the buffer.
/// - ErrorCode::UnrecognizedBuffer for when this BufferQueue does not own
/// the buffer being released.
ErrorCode releaseBuffer(Buffer &Buf);
/// Initializes the buffer queue, starting a new generation. We can re-set the
/// size of buffers with |BS| along with the buffer count with |BC|.
/// Returns:
/// - ErrorCode::Ok when we successfully initialize the buffer. This
/// requires that the buffer queue is previously finalized.
/// - ErrorCode::AlreadyInitialized when the buffer queue is not finalized.
ErrorCode init(size_t BS, size_t BC);
bool finalizing() const {
return atomic_load(&Finalizing, memory_order_acquire);
uint64_t generation() const {
return atomic_load(&Generation, memory_order_acquire);
/// Returns the configured size of the buffers in the buffer queue.
size_t ConfiguredBufferSize() const { return BufferSize; }
/// Sets the state of the BufferQueue to finalizing, which ensures that:
/// - All subsequent attempts to retrieve a Buffer will fail.
/// - All releaseBuffer operations will not fail.
/// After a call to finalize succeeds, all subsequent calls to finalize will
/// fail with ErrorCode::QueueFinalizing.
ErrorCode finalize();
/// Applies the provided function F to each Buffer in the queue, only if the
/// Buffer is marked 'used' (i.e. has been the result of getBuffer(...) and a
/// releaseBuffer(...) operation).
template <class F> void apply(F Fn) XRAY_NEVER_INSTRUMENT {
SpinMutexLock G(&Mutex);
for (auto I = begin(), E = end(); I != E; ++I)
using const_iterator = Iterator<const Buffer>;
using iterator = Iterator<Buffer>;
/// Provides iterator access to the raw Buffer instances.
iterator begin() const { return iterator(Buffers, 0, BufferCount); }
const_iterator cbegin() const {
return const_iterator(Buffers, 0, BufferCount);
iterator end() const { return iterator(Buffers, BufferCount, BufferCount); }
const_iterator cend() const {
return const_iterator(Buffers, BufferCount, BufferCount);
// Cleans up allocated buffers.
} // namespace __xray