1 CDSChecker: A Model Checker for C11 and C++11 Atomics
2 =====================================================
4 Copyright © 2013 Regents of the University of California. All rights reserved.
6 CDSChecker is distributed under the GPL v2. See the LICENSE file for details.
12 CDSChecker is a model checker for C11/C++11 which exhaustively explores the
13 behaviors of code under the C/C++ memory model. It uses partial order reduction
14 as well as a few other novel techniques to eliminate time spent on redundant
15 execution behaviors and to significantly shrink the state space. The model
16 checking algorithm is described in more detail in this paper (published in
19 > <http://demsky.eecs.uci.edu/publications/c11modelcheck.pdf>
21 It is designed to support unit tests on concurrent data structure written using
24 CDSChecker is constructed as a dynamically-linked shared library which
25 implements the C and C++ atomic types and portions of the other thread-support
26 libraries of C/C++ (e.g., std::atomic, std::mutex, etc.). Notably, we only
27 support the C version of threads (i.e., `thrd_t` and similar, from `<threads.h>`),
28 because C++ threads require features which are only available to a C++11
29 compiler (and we want to support others, at least for now).
31 CDSChecker should compile on Linux and Mac OSX with no dependencies and has been
32 tested with LLVM (clang/clang++) and GCC. It likely can be ported to other \*NIX
33 flavors. We have not attempted to port to Windows.
39 If you haven't done so already, you may download CDSChecker using
40 [git](http://git-scm.com/) (for those without git, snapshots can be found at the
43 git clone git://demsky.eecs.uci.edu/model-checker.git
45 Get the benchmarks (not required; distributed separately):
48 git clone git://demsky.eecs.uci.edu/model-checker-benchmarks.git benchmarks
50 Compile the model checker:
54 Compile the benchmarks:
58 Run a simple example (the `run.sh` script does some very minimal processing for
61 ./run.sh test/userprog.o
63 To see the help message on how to run CDSChecker, execute:
73 > Controls the liveness of the memory system. Note that multithreaded programs
74 > often rely on memory liveness for termination, so this parameter is
75 > necessary for such programs.
77 > Liveness is controlled by `num`: the number of times a load is allowed to
78 > see the same store when a newer store exists---one that is ordered later in
79 > the modification order.
83 > Turns on CHESS-like yield-based fairness support (requires `thrd_yield()`
84 > instrumentation in test program).
88 > Turns on alternative fairness support (less desirable than `-y`).
92 > Verbose: show all executions and not just buggy ones.
96 > Constrain how long we will run to wait for a future value past when it is
101 > Value to provide to atomics loads from uninitialized memory locations. The
102 > default is 0, but this may cause some programs to throw exceptions
103 > (segfault) before the model checker prints a trace.
117 Many simple tests are located in the `tests/` directory. You may also want to
118 try the larger benchmarks (distributed separately), which can be placed under
119 the `benchmarks/` directory. After building CDSChecker, you can build and run
120 the benchmarks as follows:
125 > # run barrier test with fairness/memory liveness
126 > ./run.sh barrier/barrier -y -m 2
128 > # Linux reader/write lock test with fairness/memory liveness
129 > ./run.sh linuxrwlocks/linuxrwlocks -y -m 2
131 > # run all benchmarks and provide timing results
135 Running your own code
136 ---------------------
138 You likely want to test your own code, not just our simple tests. To do so, you
139 need to perform a few steps.
141 First, because CDSChecker executes your program dozens (if not hundreds or
142 thousands) of times, you will have the most success if your code is written as a
143 unit test and not as a full-blown program.
145 Second, because CDSChecker must be able to manage your program for you, your
146 program should declare its main entry point as `user_main(int, char**)` rather
147 than `main(int, char**)`.
149 Third, test programs must use the standard C11/C++11 library headers (see below
150 for supported APIs). Notably, we only support C11 thread syntax (`thrd_t`, etc.
153 Test programs may also use our included happens-before race detector by
154 including <librace.h> and utilizing the appropriate functions
155 (`store_{8,16,32,64}()` and `load_{8,16,32,64}()`) for storing/loading data
156 to/from non-atomic shared memory.
158 CDSChecker can also check boolean assertions in your test programs. Just
159 include `<model-assert.h>` and use the `MODEL_ASSERT()` macro in your test program.
160 CDSChecker will report a bug in any possible execution in which the argument to
161 `MODEL_ASSERT()` evaluates to false (that is, 0).
163 Test programs should be compiled against our shared library (libmodel.so) using
164 the headers in the `include/` directory. Then the shared library must be made
165 available to the dynamic linker, using the `LD_LIBRARY_PATH` environment
166 variable, for instance.
169 ### Supported C11/C++11 APIs ###
171 To model-check multithreaded code properly, CDSChecker needs to instrument any
172 concurrency-related API calls made in your code. Currently, we support parts of
173 the following thread-support libraries. The C versions can be used in either C
176 * `<atomic>`, `<cstdatomic>`, `<stdatomic.h>`
177 * `<condition_variable>`
181 Because we want to extend support to legacy (i.e., non-C++11) compilers, we do
182 not support some new C++11 features that can't be implemented in C++03 (e.g.,
185 Reading an execution trace
186 --------------------------
188 When CDSChecker detects a bug in your program (or when run with the `--verbose`
189 flag), it prints the output of the program run (STDOUT) along with some summary
190 trace information for the execution in question. The trace is given as a
191 sequence of lines, where each line represents an operation in the execution
192 trace. These lines are ordered by the order in which they were run by CDSChecker
193 (i.e., the "execution order"), which does not necessarily align with the "order"
194 of the values observed (i.e., the modification order or the reads-from
197 The following list describes each of the columns in the execution trace output:
199 * \#: The sequence number within the execution. That is, sequence number "9"
200 means the operation was the 9th operation executed by CDSChecker. Note that
201 this represents the execution order, not necessarily any other order (e.g.,
202 modification order or reads-from).
204 * t: The thread number
206 * Action type: The type of operation performed
208 * MO: The memory-order for this operation (i.e., `memory_order_XXX`, where `XXX` is
209 `relaxed`, `release`, `acquire`, `rel_acq`, or `seq_cst`)
211 * Location: The memory location on which this operation is operating. This is
212 well-defined for atomic write/read/RMW, but other operations are subject to
213 CDSChecker implementation details.
215 * Value: For reads/writes/RMW, the value returned by the operation. Note that
216 for RMW, this is the value that is *read*, not the value that was *written*.
217 For other operations, 'value' may have some CDSChecker-internal meaning, or
218 it may simply be a don't-care (such as `0xdeadbeef`).
220 * Rf: For reads, the sequence number of the operation from which it reads.
221 [Note: If the execution is a partial, infeasible trace (labeled INFEASIBLE),
222 as printed during `--verbose` execution, reads may not be resolved and so may
223 have Rf=? or Rf=Px, where x is a promised future value.]
225 * CV: The clock vector, encapsulating the happens-before relation (see our
226 paper, or the C/C++ memory model itself). We use a Lamport-style clock vector
227 similar to [1]. The "clock" is just the sequence number (#). The clock vector
228 can be read as follows:
230 Each entry is indexed as CV[i], where
232 i = 0, 1, 2, ..., <number of threads>
234 So for any thread i, we say CV[i] is the sequence number of the most recent
235 operation in thread i such that operation i happens-before this operation.
236 Notably, thread 0 is reserved as a dummy thread for certain CDSChecker
239 See the following example trace:
242 ------------------------------------------------------------------------------------
243 # t Action type MO Location Value Rf CV
244 ------------------------------------------------------------------------------------
245 1 1 thread start seq_cst 0x7f68ff11e7c0 0xdeadbeef ( 0, 1)
246 2 1 init atomic relaxed 0x601068 0 ( 0, 2)
247 3 1 init atomic relaxed 0x60106c 0 ( 0, 3)
248 4 1 thread create seq_cst 0x7f68fe51c710 0x7f68fe51c6e0 ( 0, 4)
249 5 2 thread start seq_cst 0x7f68ff11ebc0 0xdeadbeef ( 0, 4, 5)
250 6 2 atomic read relaxed 0x60106c 0 3 ( 0, 4, 6)
251 7 1 thread create seq_cst 0x7f68fe51c720 0x7f68fe51c6e0 ( 0, 7)
252 8 3 thread start seq_cst 0x7f68ff11efc0 0xdeadbeef ( 0, 7, 0, 8)
253 9 2 atomic write relaxed 0x601068 0 ( 0, 4, 9)
254 10 3 atomic read relaxed 0x601068 0 2 ( 0, 7, 0, 10)
255 11 2 thread finish seq_cst 0x7f68ff11ebc0 0xdeadbeef ( 0, 4, 11)
256 12 3 atomic write relaxed 0x60106c 0x2a ( 0, 7, 0, 12)
257 13 1 thread join seq_cst 0x7f68ff11ebc0 0x2 ( 0, 13, 11)
258 14 3 thread finish seq_cst 0x7f68ff11efc0 0xdeadbeef ( 0, 7, 0, 14)
259 15 1 thread join seq_cst 0x7f68ff11efc0 0x3 ( 0, 15, 11, 14)
260 16 1 thread finish seq_cst 0x7f68ff11e7c0 0xdeadbeef ( 0, 16, 11, 14)
262 ------------------------------------------------------------------------------------
265 Now consider, for example, operation 10:
267 This is the 10th operation in the execution order. It is an atomic read-relaxed
268 operation performed by thread 3 at memory address `0x601068`. It reads the value
269 "0", which was written by the 2nd operation in the execution order. Its clock
270 vector consists of the following values:
272 CV[0] = 0, CV[1] = 7, CV[2] = 0, CV[3] = 10
274 End of Execution Summary
275 ------------------------
277 CDSChecker prints summary statistics at the end of each execution. These
278 summaries are based off of a few different properties of an execution, which we
279 will break down here:
281 * An _infeasible_ execution is an execution which is not consistent with the
282 memory model. Such an execution can be considered overhead for the
283 model-checker, since it should never appear in practice.
285 * A _buggy_ execution is an execution in which CDSChecker has found a real
286 bug: a data race, a deadlock, failure of a user-provided assertion, or an
287 uninitialized load, for instance. CDSChecker will only report bugs in feasible
290 * A _redundant_ execution is a feasible execution that is exploring the same
291 state space explored by a previous feasible execution. Such exploration is
292 another instance of overhead, so CDSChecker terminates these executions as
293 soon as they are detected. CDSChecker is mostly able to avoid such executions
294 but may encounter them if a fairness option is enabled.
296 Now, we can examine the end-of-execution summary of one test program:
298 $ ./run.sh test/rmwprog.o
300 ******* Model-checking complete: *******
301 Number of complete, bug-free executions: 6
302 Number of redundant executions: 0
303 Number of buggy executions: 0
304 Number of infeasible executions: 29
307 * _Number of complete, bug-free executions:_ these are feasible, non-buggy, and
308 non-redundant executions. They each represent different, legal behaviors you
309 can expect to see in practice.
311 * _Number of redundant executions:_ these are feasible but redundant executions
312 that were terminated as soon as CDSChecker noticed the redundancy.
314 * _Number of buggy executions:_ these are feasible, buggy executions. These are
315 the trouble spots where your program is triggering a bug or assertion.
316 Ideally, this number should be 0.
318 * _Number of infeasible executions:_ these are infeasible executions,
319 representing some of the overhead of model-checking.
321 * _Total executions:_ the total number of executions explored by CDSChecker.
322 Should be the sum of the above categories, since they are mutually exclusive.
325 Other Notes and Pitfalls
326 ------------------------
328 * Deadlock detection: CDSChecker can detect deadlocks. For instance, try the
329 following test program.
331 > ./run.sh test/deadlock.o
333 Deadlock detection currently detects when a thread is about to step into a
334 deadlock, without actually including the final step in the trace. But you can
335 examine the program to see the next step.
337 * CDSChecker has to speculatively explore many execution behaviors due to the
338 relaxed memory model, and many of these turn out to be infeasible (that is,
339 they cannot be legally produced by the memory model). CDSChecker discards
340 these executions as soon as it identifies them (see the "Number of infeasible
341 executions" statistic); however, the speculation can occasionally cause
342 CDSChecker to hit unexpected parts of the unit test program (causing a
343 division by 0, for instance). In such programs, you might consider running
344 CDSChecker with the `-u num` option.
346 * Related to the previous point, CDSChecker may report more than one bug for a
347 particular candidate execution. This is because some bugs may not be
348 reportable until CDSChecker has explored more of the program, and in the
349 time between initial discovery and final assessment of the bug, CDSChecker may
350 discover another bug.
352 * Data races may be reported as multiple bugs, one for each byte-address of the
353 data race in question. See, for example, this run:
355 $ ./run.sh test/releaseseq.o
357 Bug report: 4 bugs detected
358 [BUG] Data race detected @ address 0x601078:
359 Access 1: write in thread 2 @ clock 4
360 Access 2: read in thread 3 @ clock 9
361 [BUG] Data race detected @ address 0x601079:
362 Access 1: write in thread 2 @ clock 4
363 Access 2: read in thread 3 @ clock 9
364 [BUG] Data race detected @ address 0x60107a:
365 Access 1: write in thread 2 @ clock 4
366 Access 2: read in thread 3 @ clock 9
367 [BUG] Data race detected @ address 0x60107b:
368 Access 1: write in thread 2 @ clock 4
369 Access 2: read in thread 3 @ clock 9
375 The CDSChecker project page:
377 > <http://demsky.eecs.uci.edu/c11modelchecker.php>
379 The CDSChecker source and accompanying benchmarks on Gitweb:
381 > <http://demsky.eecs.uci.edu/git/?p=model-checker.git>
383 > <http://demsky.eecs.uci.edu/git/?p=model-checker-benchmarks.git>
389 Please feel free to contact us for more information. Bug reports are welcome,
390 and we are happy to hear from our users. We are also very interested to know if
391 CDSChecker catches bugs in your programs.
393 Contact Brian Norris at <banorris@uci.edu> or Brian Demsky at <bdemsky@uci.edu>.
399 [1] L. Lamport. Time, clocks, and the ordering of events in a distributed
400 system. CACM, 21(7):558-565, July 1978.