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1 | |
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2 | // LTTng ltt-tracer.c atomic lockless buffering scheme Promela model v2 |
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3 | // Created for the Spin validator. |
4 | // Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> |
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5 | // October 2008 |
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6 | |
7 | // TODO : create test cases that will generate an overflow on the offset and |
8 | // counter type. Counter types smaller than a byte should be used. |
9 | |
10 | // Promela only has unsigned char, no signed char. |
11 | // Because detection of difference < 0 depends on a signed type, but we want |
12 | // compactness, check also for the values being higher than half of the unsigned |
13 | // char range (and consider them negative). The model, by design, does not use |
14 | // offsets or counts higher than 127 because we would then have to use a larger |
15 | // type (short or int). |
16 | #define HALF_UCHAR (255/2) |
17 | |
18 | // NUMPROCS 4 : causes event loss with some reader timings. |
19 | // e.g. 3 events, 1 switch, 1 event (lost, buffer full), read 1 subbuffer |
20 | #define NUMPROCS 4 |
21 | |
22 | // NUMPROCS 3 : does not cause event loss because buffers are big enough. |
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23 | // #define NUMPROCS 3 |
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24 | // e.g. 3 events, 1 switch, read 1 subbuffer |
25 | |
26 | #define NUMSWITCH 1 |
27 | #define BUFSIZE 4 |
28 | #define NR_SUBBUFS 2 |
29 | #define SUBBUF_SIZE (BUFSIZE / NR_SUBBUFS) |
30 | |
31 | // Writer counters |
32 | byte write_off = 0; |
33 | byte commit_count[NR_SUBBUFS]; |
34 | |
35 | // Reader counters |
36 | byte read_off = 0; |
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37 | |
38 | byte events_lost = 0; |
39 | byte refcount = 0; |
40 | |
41 | bool deliver = 0; |
42 | |
43 | // buffer slot in-use bit. Detects racy use (more than a single process |
44 | // accessing a slot at any given step). |
45 | bool buffer_use[BUFSIZE]; |
46 | |
47 | // Proceed to a sub-subber switch is needed. |
48 | // Used in a periodical timer interrupt to fill and ship the current subbuffer |
49 | // to the reader so we can guarantee a steady flow. If a subbuffer is |
50 | // completely empty, don't switch. |
51 | // Also used as "finalize" operation to complete the last subbuffer after |
52 | // all writers have finished so the last subbuffer can be read by the reader. |
53 | proctype switcher() |
54 | { |
55 | byte prev_off, new_off, tmp_commit; |
56 | byte size; |
57 | |
58 | cmpxchg_loop: |
59 | atomic { |
60 | prev_off = write_off; |
61 | size = SUBBUF_SIZE - (prev_off % SUBBUF_SIZE); |
62 | new_off = prev_off + size; |
63 | if |
64 | :: (new_off - read_off > BUFSIZE && new_off - read_off < HALF_UCHAR) |
65 | || size == SUBBUF_SIZE -> |
66 | refcount = refcount - 1; |
67 | goto not_needed; |
68 | :: else -> skip |
69 | fi; |
70 | } |
71 | atomic { |
72 | if |
73 | :: prev_off != write_off -> goto cmpxchg_loop |
74 | :: else -> write_off = new_off; |
75 | fi; |
76 | } |
77 | |
78 | atomic { |
79 | tmp_commit = commit_count[(prev_off % BUFSIZE) / SUBBUF_SIZE] + size; |
80 | commit_count[(prev_off % BUFSIZE) / SUBBUF_SIZE] = tmp_commit; |
81 | if |
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82 | :: (((prev_off / BUFSIZE) * BUFSIZE) / NR_SUBBUFS) + SUBBUF_SIZE - |
83 | tmp_commit |
84 | -> deliver = 1 |
85 | :: else |
86 | -> skip |
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87 | fi; |
88 | refcount = refcount - 1; |
89 | } |
90 | not_needed: |
91 | skip; |
92 | } |
93 | |
94 | // tracer |
95 | // Writes 1 byte of information in the buffer at the current |
96 | // "write_off" position and then increment the commit_count of the sub-buffer |
97 | // the information has been written to. |
98 | proctype tracer() |
99 | { |
100 | byte size = 1; |
101 | byte prev_off, new_off, tmp_commit; |
102 | byte i, j; |
103 | |
104 | cmpxchg_loop: |
105 | atomic { |
106 | prev_off = write_off; |
107 | new_off = prev_off + size; |
108 | } |
109 | atomic { |
110 | if |
111 | :: new_off - read_off > BUFSIZE && new_off - read_off < HALF_UCHAR -> |
112 | goto lost |
113 | :: else -> skip |
114 | fi; |
115 | } |
116 | atomic { |
117 | if |
118 | :: prev_off != write_off -> goto cmpxchg_loop |
119 | :: else -> write_off = new_off; |
120 | fi; |
121 | i = 0; |
122 | do |
123 | :: i < size -> |
124 | assert(buffer_use[(prev_off + i) % BUFSIZE] == 0); |
125 | buffer_use[(prev_off + i) % BUFSIZE] = 1; |
126 | i++ |
127 | :: i >= size -> break |
128 | od; |
129 | } |
130 | |
131 | // writing to buffer... |
132 | |
133 | atomic { |
134 | i = 0; |
135 | do |
136 | :: i < size -> |
137 | buffer_use[(prev_off + i) % BUFSIZE] = 0; |
138 | i++ |
139 | :: i >= size -> break |
140 | od; |
141 | tmp_commit = commit_count[(prev_off % BUFSIZE) / SUBBUF_SIZE] + size; |
142 | commit_count[(prev_off % BUFSIZE) / SUBBUF_SIZE] = tmp_commit; |
143 | if |
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144 | :: (((prev_off / BUFSIZE) * BUFSIZE) / NR_SUBBUFS) + SUBBUF_SIZE - |
145 | tmp_commit |
146 | -> deliver = 1 |
147 | :: else |
148 | -> skip |
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149 | fi; |
150 | } |
151 | atomic { |
152 | goto end; |
153 | lost: |
154 | events_lost++; |
155 | end: |
156 | refcount = refcount - 1; |
157 | } |
158 | } |
159 | |
160 | // reader |
161 | // Read the information sub-buffer per sub-buffer when available. |
162 | // |
163 | // Reads the information as soon as it is ready, or may be delayed by |
164 | // an asynchronous delivery. Being modeled as a process insures all cases |
165 | // (scheduled very quickly or very late, causing event loss) are covered. |
166 | // Only one reader per buffer (normally ensured by a mutex). This is modeled |
167 | // by using a single reader process. |
168 | proctype reader() |
169 | { |
170 | byte i, j; |
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171 | |
172 | do |
173 | :: (write_off / SUBBUF_SIZE) - (read_off / SUBBUF_SIZE) > 0 |
174 | && (write_off / SUBBUF_SIZE) - (read_off / SUBBUF_SIZE) < HALF_UCHAR |
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175 | && (commit_count[(read_off % BUFSIZE) / SUBBUF_SIZE] |
176 | - SUBBUF_SIZE - (((read_off / BUFSIZE) * BUFSIZE) / NR_SUBBUFS) |
177 | == 0) -> |
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178 | atomic { |
179 | i = 0; |
180 | do |
181 | :: i < SUBBUF_SIZE -> |
182 | assert(buffer_use[(read_off + i) % BUFSIZE] == 0); |
183 | buffer_use[(read_off + i) % BUFSIZE] = 1; |
184 | i++ |
185 | :: i >= SUBBUF_SIZE -> break |
186 | od; |
187 | } |
188 | // reading from buffer... |
189 | |
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190 | atomic { |
191 | i = 0; |
192 | do |
193 | :: i < SUBBUF_SIZE -> |
194 | buffer_use[(read_off + i) % BUFSIZE] = 0; |
195 | i++ |
196 | :: i >= SUBBUF_SIZE -> break |
197 | od; |
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198 | read_off = read_off + SUBBUF_SIZE; |
199 | } |
200 | :: read_off >= (NUMPROCS - events_lost) -> break; |
201 | od; |
202 | } |
203 | |
204 | // Waits for all tracer and switcher processes to finish before finalizing |
205 | // the buffer. Only after that will the reader be allowed to read the |
206 | // last subbuffer. |
207 | proctype cleaner() |
208 | { |
209 | atomic { |
210 | do |
211 | :: refcount == 0 -> |
212 | refcount = refcount + 1; |
213 | run switcher(); // Finalize the last sub-buffer so it can be read. |
214 | break; |
215 | od; |
216 | } |
217 | } |
218 | |
219 | init { |
220 | byte i = 0; |
221 | byte j = 0; |
222 | byte sum = 0; |
223 | byte commit_sum = 0; |
224 | |
225 | atomic { |
226 | i = 0; |
227 | do |
228 | :: i < NR_SUBBUFS -> |
229 | commit_count[i] = 0; |
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230 | i++ |
231 | :: i >= NR_SUBBUFS -> break |
232 | od; |
233 | i = 0; |
234 | do |
235 | :: i < BUFSIZE -> |
236 | buffer_use[i] = 0; |
237 | i++ |
238 | :: i >= BUFSIZE -> break |
239 | od; |
240 | run reader(); |
241 | run cleaner(); |
242 | i = 0; |
243 | do |
244 | :: i < NUMPROCS -> |
245 | refcount = refcount + 1; |
246 | run tracer(); |
247 | i++ |
248 | :: i >= NUMPROCS -> break |
249 | od; |
250 | i = 0; |
251 | do |
252 | :: i < NUMSWITCH -> |
253 | refcount = refcount + 1; |
254 | run switcher(); |
255 | i++ |
256 | :: i >= NUMSWITCH -> break |
257 | od; |
258 | } |
259 | // Assertions. |
260 | atomic { |
261 | // The writer head must always be superior or equal to the reader head. |
262 | assert(write_off - read_off >= 0 && write_off - read_off < HALF_UCHAR); |
263 | j = 0; |
264 | commit_sum = 0; |
265 | do |
266 | :: j < NR_SUBBUFS -> |
267 | commit_sum = commit_sum + commit_count[j]; |
268 | // The commit count of a particular subbuffer must always be higher |
269 | // or equal to the retrieve_count of this subbuffer. |
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270 | // assert(commit_count[j] - retrieve_count[j] >= 0 && |
271 | // commit_count[j] - retrieve_count[j] < HALF_UCHAR); |
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272 | j++ |
273 | :: j >= NR_SUBBUFS -> break |
274 | od; |
275 | // The sum of all subbuffer commit counts must always be lower or equal |
276 | // to the writer head, because space must be reserved before it is |
277 | // written to and then committed. |
278 | assert(write_off - commit_sum >= 0 && write_off - commit_sum < HALF_UCHAR); |
279 | |
280 | // If we have less writers than the buffer space available, we should |
281 | // not lose events |
282 | assert(NUMPROCS + NUMSWITCH > BUFSIZE || events_lost == 0); |
283 | } |
284 | } |