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README
**************************************** CDSChecker Readme **************************************** Copyright (c) 2013 Regents of the University of California. All rights reserved. CDSChecker is distributed under the GPL v2. See the LICENSE file for details. This README is divided into sections as follows: I. Overview II. Basic build and run III. Running your own code IV. Reading an execution trace Appendix A. References ---------------------------------------- I. Overview ---------------------------------------- CDSChecker is a model checker for C11/C++11 exhaustively explores the behaviors of code under the C11/C++11 memory model. It uses partial order reduction to eliminate redundant executions to significantly shrink the state space. The model checking algorithm is described in more detail in this paper (currently under review): http://demsky.eecs.uci.edu/publications/c11modelcheck.pdf It is designed to support unit tests on concurrent data structure written using C11/C++11 atomics. CDSChecker is constructed as a dynamically-linked shared library which implements the C and C++ atomic types and portions of the other thread-support libraries of C/C++ (e.g., std::atomic, std::mutex, etc.). Notably, we only support the C version of threads (i.e., thrd_t and similar, from <threads.h>), because C++ threads require features which are only available to a C++11 compiler (and we want to support others, at least for now). CDSChecker should compile on Linux and Mac OSX with no dependencies and has been tested with LLVM (clang/clang++) and GCC. It likely can be ported to other *NIX flavors. We have not attempted to port to Windows. Other references can be found at the main project page: http://demsky.eecs.uci.edu/c11modelchecker.php ---------------------------------------- II. Basic build and run ---------------------------------------- Sample run instructions: $ make $ export LD_LIBRARY_PATH=. $ ./test/userprog.o # Runs simple test program $ ./test/userprog.o -h # Prints help information Copyright (c) 2013 Regents of the University of California. All rights reserved. Distributed under the GPLv2 Written by Brian Norris and Brian Demsky Usage: ./test/userprog.o [MODEL-CHECKER OPTIONS] -- [PROGRAM ARGS] MODEL-CHECKER OPTIONS can be any of the model-checker options listed below. Arguments provided after the `--' (the PROGRAM ARGS) are passed to the user program. Model-checker options: -h, --help Display this help message and exit -m, --liveness=NUM Maximum times a thread can read from the same write while other writes exist. Default: 0 -M, --maxfv=NUM Maximum number of future values that can be sent to the same read. Default: 0 -s, --maxfvdelay=NUM Maximum actions that the model checker will wait for a write from the future past the expected number of actions. Default: 6 -S, --fvslop=NUM Future value expiration sloppiness. Default: 4 -y, --yield Enable CHESS-like yield-based fairness support. Default: disabled -Y, --yieldblock Prohibit an execution from running a yield. Default: disabled -f, --fairness=WINDOW Specify a fairness window in which actions that are enabled sufficiently many times should receive priority for execution (not recommended). Default: 0 -e, --enabled=COUNT Enabled count. Default: 1 -b, --bound=MAX Upper length bound. Default: 0 -v[NUM], --verbose[=NUM] Print verbose execution information. NUM is optional: 0 is quiet; 1 is noisy; 2 is noisier. Default: 0 -u, --uninitialized=VALUE Return VALUE any load which may read from an uninitialized atomic. Default: 0 -t, --analysis=NAME Use Analysis Plugin. -o, --options=NAME Option for previous analysis plugin. -o help for a list of options -- Program arguments follow. Analysis plugins: SC Note that we also provide a series of benchmarks (distributed separately), which can be placed under the benchmarks/ directory. After building CDSChecker, you can build and run the benchmarks as follows: cd benchmarks make ./run.sh barrier/barrier -y -m 2 # runs barrier test with fairness/memory liveness ./bench.sh # run all benchmarks twice, with timing results ---------------------------------------- III. Running your own code ---------------------------------------- We provide several test and sample programs under the test/ directory, which should compile and run with no trouble. Of course, you likely want to test your own code. To do so, you need to perform a few steps. First, because CDSChecker executes your program dozens (if not hundreds or thousands) of times, you will have the most success if your code is written as a unit test and not as a full-blown program. Next, test programs should use the standard C11/C++11 library headers (<atomic>/<stdatomic.h>, <mutex>, <condition_variable>, <thread.h>) and must name their main routine as user_main(int, char**) rather than main(int, char**). We only support C11 thread syntax (thrd_t, etc. from <thread.h>). Test programs may also use our included happens-before race detector by including <librace.h> and utilizing the appropriate functions (store_{8,16,32,64}() and load_{8,16,32,64}()) for loading/storing data from/to non-atomic shared memory. CDSChecker can also check boolean assertions in your test programs. Just include <model-assert.h> and use the MODEL_ASSERT() macro in your test program. CDSChecker will report a bug in any possible execution in which the argument to MODEL_ASSERT() evaluates to false (that is, 0). Test programs should be compiled against our shared library (libmodel.so) using the headers in the include/ directory. Then the shared library must be made available to the dynamic linker, using the LD_LIBRARY_PATH environment variable, for instance. ---------------------------------------- IV. Reading an execution trace ---------------------------------------- When CDSChecker detects a bug in your program (or when run with the --verbose flag), it prints the output of the program run (STDOUT) along with some summary trace information for the execution in question. The trace is given as a sequence of lines, where each line represents an operation in the execution trace. These lines are ordered by the order in which they were run by CDSChecker (i.e., the "execution order"), which does not necessarily align with the "order" of the values observed (i.e., the modification order or the reads-from relation). The following list describes each of the columns in the execution trace output: o #: The sequence number within the execution. That is, sequence number "9" means the operation was the 9th operation executed by CDSChecker. Note that this represents the execution order, not necessarily any other order (e.g., modification order or reads-from). o t: The thread number o Action type: The type of operation performed o MO: The memory-order for this operation (i.e., memory_order_XXX, where XXX is relaxed, release, acquire, rel_acq, or seq_cst) o Location: The memory location on which this operation is operating. This is well-defined for atomic write/read/RMW, but other operations are subject to CDSChecker implementation details. o Value: For reads/writes/RMW, the value returned by the operation. Note that for RMW, this is the value that is *read*, not the value that was *written*. For other operations, 'value' may have some CDSChecker-internal meaning, or it may simply be a don't-care (such as 0xdeadbeef). o Rf: For reads, the sequence number of the operation from which it reads. [Note: If the execution is a partial, infeasible trace (labeled INFEASIBLE), as printed during --verbose execution, reads may not be resolved and so may have Rf=? or Rf=Px, where x is a promised future value.] o CV: The clock vector, encapsulating the happens-before relation (see our paper, or the C/C++ memory model itself). We use a Lamport-style clock vector similar to [1]. The "clock" is just the sequence number (#). The clock vector can be read as follows: Each entry is indexed as CV[i], where i = 0, 1, 2, ..., <number of threads> So for any thread i, we say CV[i] is the sequence number of the most recent operation in thread i such that operation i happens-before this operation. Notably, thread 0 is reserved as a dummy thread for certain CDSChecker operations. See the following example trace: ------------------------------------------------------------------------------------ # t Action type MO Location Value Rf CV ------------------------------------------------------------------------------------ 1 1 thread start seq_cst 0x7f68ff11e7c0 0xdeadbeef ( 0, 1) 2 1 init atomic relaxed 0x601068 0 ( 0, 2) 3 1 init atomic relaxed 0x60106c 0 ( 0, 3) 4 1 thread create seq_cst 0x7f68fe51c710 0x7f68fe51c6e0 ( 0, 4) 5 2 thread start seq_cst 0x7f68ff11ebc0 0xdeadbeef ( 0, 4, 5) 6 2 atomic read relaxed 0x60106c 0 3 ( 0, 4, 6) 7 1 thread create seq_cst 0x7f68fe51c720 0x7f68fe51c6e0 ( 0, 7) 8 3 thread start seq_cst 0x7f68ff11efc0 0xdeadbeef ( 0, 7, 0, 8) 9 2 atomic write relaxed 0x601068 0 ( 0, 4, 9) 10 3 atomic read relaxed 0x601068 0 2 ( 0, 7, 0, 10) 11 2 thread finish seq_cst 0x7f68ff11ebc0 0xdeadbeef ( 0, 4, 11) 12 3 atomic write relaxed 0x60106c 0x2a ( 0, 7, 0, 12) 13 1 thread join seq_cst 0x7f68ff11ebc0 0x2 ( 0, 13, 11) 14 3 thread finish seq_cst 0x7f68ff11efc0 0xdeadbeef ( 0, 7, 0, 14) 15 1 thread join seq_cst 0x7f68ff11efc0 0x3 ( 0, 15, 11, 14) 16 1 thread finish seq_cst 0x7f68ff11e7c0 0xdeadbeef ( 0, 16, 11, 14) HASH 4073708854 ------------------------------------------------------------------------------------ Now consider, for example, operation 10: This is the 10th operation in the execution order. It is an atomic read-relaxed operation performed by thread 3 at memory address 0x601068. It reads the value "0", which was written by the 2nd operation in the execution order. Its clock vector consists of the following values: CV[0] = 0, CV[1] = 7, CV[2] = 0, CV[3] = 10 ---------------------------------------- A. References ---------------------------------------- [1] L. Lamport. Time, clocks, and the ordering of events in a distributed system. CACM, 21(7):558–565, July 1978.