1//===-- xray_buffer_queue.h ------------------------------------*- C++ -*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file is a part of XRay, a dynamic runtime instrumentation system.
10//
11// Defines the interface for a buffer queue implementation.
12//
13//===----------------------------------------------------------------------===//
14#ifndef XRAY_BUFFER_QUEUE_H
15#define XRAY_BUFFER_QUEUE_H
16
17#include "sanitizer_common/sanitizer_atomic.h"
18#include "sanitizer_common/sanitizer_common.h"
19#include "sanitizer_common/sanitizer_mutex.h"
20#include "xray_defs.h"
21#include <cstddef>
22#include <cstdint>
23
24namespace __xray {
25
26/// BufferQueue implements a circular queue of fixed sized buffers (much like a
27/// freelist) but is concerned with making it quick to initialise, finalise, and
28/// get from or return buffers to the queue. This is one key component of the
29/// "flight data recorder" (FDR) mode to support ongoing XRay function call
30/// trace collection.
31class BufferQueue {
32public:
33 /// ControlBlock represents the memory layout of how we interpret the backing
34 /// store for all buffers and extents managed by a BufferQueue instance. The
35 /// ControlBlock has the reference count as the first member, sized according
36 /// to platform-specific cache-line size. We never use the Buffer member of
37 /// the union, which is only there for compiler-supported alignment and
38 /// sizing.
39 ///
40 /// This ensures that the `Data` member will be placed at least kCacheLineSize
41 /// bytes from the beginning of the structure.
42 struct ControlBlock {
43 union {
44 atomic_uint64_t RefCount;
45 char Buffer[kCacheLineSize];
46 };
47
48 /// We need to make this size 1, to conform to the C++ rules for array data
49 /// members. Typically, we want to subtract this 1 byte for sizing
50 /// information.
51 char Data[1];
52 };
53
54 struct Buffer {
55 atomic_uint64_t *Extents = nullptr;
56 uint64_t Generation{0};
57 void *Data = nullptr;
58 size_t Size = 0;
59
60 private:
61 friend class BufferQueue;
62 ControlBlock *BackingStore = nullptr;
63 ControlBlock *ExtentsBackingStore = nullptr;
64 size_t Count = 0;
65 };
66
67 struct BufferRep {
68 // The managed buffer.
69 Buffer Buff;
70
71 // This is true if the buffer has been returned to the available queue, and
72 // is considered "used" by another thread.
73 bool Used = false;
74 };
75
76private:
77 // This models a ForwardIterator. |T| Must be either a `Buffer` or `const
78 // Buffer`. Note that we only advance to the "used" buffers, when
79 // incrementing, so that at dereference we're always at a valid point.
80 template <class T> class Iterator {
81 public:
82 BufferRep *Buffers = nullptr;
83 size_t Offset = 0;
84 size_t Max = 0;
85
86 Iterator &operator++() {
87 DCHECK_NE(Offset, Max);
88 do {
89 ++Offset;
90 } while (Offset != Max && !Buffers[Offset].Used);
91 return *this;
92 }
93
94 Iterator operator++(int) {
95 Iterator C = *this;
96 ++(*this);
97 return C;
98 }
99
100 T &operator*() const { return Buffers[Offset].Buff; }
101
102 T *operator->() const { return &(Buffers[Offset].Buff); }
103
104 Iterator(BufferRep *Root, size_t O, size_t M) XRAY_NEVER_INSTRUMENT
105 : Buffers(Root),
106 Offset(O),
107 Max(M) {
108 // We want to advance to the first Offset where the 'Used' property is
109 // true, or to the end of the list/queue.
110 while (Offset != Max && !Buffers[Offset].Used) {
111 ++Offset;
112 }
113 }
114
115 Iterator() = default;
116 Iterator(const Iterator &) = default;
117 Iterator(Iterator &&) = default;
118 Iterator &operator=(const Iterator &) = default;
119 Iterator &operator=(Iterator &&) = default;
120 ~Iterator() = default;
121
122 template <class V>
123 friend bool operator==(const Iterator &L, const Iterator<V> &R) {
124 DCHECK_EQ(L.Max, R.Max);
125 return L.Buffers == R.Buffers && L.Offset == R.Offset;
126 }
127
128 template <class V>
129 friend bool operator!=(const Iterator &L, const Iterator<V> &R) {
130 return !(L == R);
131 }
132 };
133
134 // Size of each individual Buffer.
135 size_t BufferSize;
136
137 // Amount of pre-allocated buffers.
138 size_t BufferCount;
139
140 SpinMutex Mutex;
141 atomic_uint8_t Finalizing;
142
143 // The collocated ControlBlock and buffer storage.
144 ControlBlock *BackingStore;
145
146 // The collocated ControlBlock and extents storage.
147 ControlBlock *ExtentsBackingStore;
148
149 // A dynamically allocated array of BufferRep instances.
150 BufferRep *Buffers;
151
152 // Pointer to the next buffer to be handed out.
153 BufferRep *Next;
154
155 // Pointer to the entry in the array where the next released buffer will be
156 // placed.
157 BufferRep *First;
158
159 // Count of buffers that have been handed out through 'getBuffer'.
160 size_t LiveBuffers;
161
162 // We use a generation number to identify buffers and which generation they're
163 // associated with.
164 atomic_uint64_t Generation;
165
166 /// Releases references to the buffers backed by the current buffer queue.
167 void cleanupBuffers();
168
169public:
170 enum class ErrorCode : unsigned {
171 Ok,
172 NotEnoughMemory,
173 QueueFinalizing,
174 UnrecognizedBuffer,
175 AlreadyFinalized,
176 AlreadyInitialized,
177 };
178
179 static const char *getErrorString(ErrorCode E) {
180 switch (E) {
181 case ErrorCode::Ok:
182 return "(none)";
183 case ErrorCode::NotEnoughMemory:
184 return "no available buffers in the queue";
185 case ErrorCode::QueueFinalizing:
186 return "queue already finalizing";
187 case ErrorCode::UnrecognizedBuffer:
188 return "buffer being returned not owned by buffer queue";
189 case ErrorCode::AlreadyFinalized:
190 return "queue already finalized";
191 case ErrorCode::AlreadyInitialized:
192 return "queue already initialized";
193 }
194 return "unknown error";
195 }
196
197 /// Initialise a queue of size |N| with buffers of size |B|. We report success
198 /// through |Success|.
199 BufferQueue(size_t B, size_t N, bool &Success);
200
201 /// Updates |Buf| to contain the pointer to an appropriate buffer. Returns an
202 /// error in case there are no available buffers to return when we will run
203 /// over the upper bound for the total buffers.
204 ///
205 /// Requirements:
206 /// - BufferQueue is not finalising.
207 ///
208 /// Returns:
209 /// - ErrorCode::NotEnoughMemory on exceeding MaxSize.
210 /// - ErrorCode::Ok when we find a Buffer.
211 /// - ErrorCode::QueueFinalizing or ErrorCode::AlreadyFinalized on
212 /// a finalizing/finalized BufferQueue.
213 ErrorCode getBuffer(Buffer &Buf);
214
215 /// Updates |Buf| to point to nullptr, with size 0.
216 ///
217 /// Returns:
218 /// - ErrorCode::Ok when we successfully release the buffer.
219 /// - ErrorCode::UnrecognizedBuffer for when this BufferQueue does not own
220 /// the buffer being released.
221 ErrorCode releaseBuffer(Buffer &Buf);
222
223 /// Initializes the buffer queue, starting a new generation. We can re-set the
224 /// size of buffers with |BS| along with the buffer count with |BC|.
225 ///
226 /// Returns:
227 /// - ErrorCode::Ok when we successfully initialize the buffer. This
228 /// requires that the buffer queue is previously finalized.
229 /// - ErrorCode::AlreadyInitialized when the buffer queue is not finalized.
230 ErrorCode init(size_t BS, size_t BC);
231
232 bool finalizing() const {
233 return atomic_load(a: &Finalizing, mo: memory_order_acquire);
234 }
235
236 uint64_t generation() const {
237 return atomic_load(a: &Generation, mo: memory_order_acquire);
238 }
239
240 /// Returns the configured size of the buffers in the buffer queue.
241 size_t ConfiguredBufferSize() const { return BufferSize; }
242
243 /// Sets the state of the BufferQueue to finalizing, which ensures that:
244 ///
245 /// - All subsequent attempts to retrieve a Buffer will fail.
246 /// - All releaseBuffer operations will not fail.
247 ///
248 /// After a call to finalize succeeds, all subsequent calls to finalize will
249 /// fail with ErrorCode::QueueFinalizing.
250 ErrorCode finalize();
251
252 /// Applies the provided function F to each Buffer in the queue, only if the
253 /// Buffer is marked 'used' (i.e. has been the result of getBuffer(...) and a
254 /// releaseBuffer(...) operation).
255 template <class F> void apply(F Fn) XRAY_NEVER_INSTRUMENT {
256 SpinMutexLock G(&Mutex);
257 for (auto I = begin(), E = end(); I != E; ++I)
258 Fn(*I);
259 }
260
261 using const_iterator = Iterator<const Buffer>;
262 using iterator = Iterator<Buffer>;
263
264 /// Provides iterator access to the raw Buffer instances.
265 iterator begin() const { return iterator(Buffers, 0, BufferCount); }
266 const_iterator cbegin() const {
267 return const_iterator(Buffers, 0, BufferCount);
268 }
269 iterator end() const { return iterator(Buffers, BufferCount, BufferCount); }
270 const_iterator cend() const {
271 return const_iterator(Buffers, BufferCount, BufferCount);
272 }
273
274 // Cleans up allocated buffers.
275 ~BufferQueue();
276};
277
278} // namespace __xray
279
280#endif // XRAY_BUFFER_QUEUE_H
281