135 lines
4.6 KiB
ReStructuredText
135 lines
4.6 KiB
ReStructuredText
ThreadSanitizer
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===============
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Introduction
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------------
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ThreadSanitizer is a tool that detects data races. It consists of a compiler
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instrumentation module and a run-time library. Typical slowdown introduced by
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ThreadSanitizer is about **5x-15x**. Typical memory overhead introduced by
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ThreadSanitizer is about **5x-10x**.
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How to build
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------------
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Build LLVM/Clang with `CMake <http://llvm.org/docs/CMake.html>`_.
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Supported Platforms
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-------------------
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ThreadSanitizer is supported on Linux x86_64 (tested on Ubuntu 12.04).
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Support for other 64-bit architectures is possible, contributions are welcome.
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Support for 32-bit platforms is problematic and is not planned.
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Usage
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-----
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Simply compile and link your program with ``-fsanitize=thread``. To get a
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reasonable performance add ``-O1`` or higher. Use ``-g`` to get file names
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and line numbers in the warning messages.
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Example:
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.. code-block:: console
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% cat projects/compiler-rt/lib/tsan/lit_tests/tiny_race.c
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#include <pthread.h>
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int Global;
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void *Thread1(void *x) {
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Global = 42;
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return x;
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}
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int main() {
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pthread_t t;
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pthread_create(&t, NULL, Thread1, NULL);
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Global = 43;
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pthread_join(t, NULL);
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return Global;
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}
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$ clang -fsanitize=thread -g -O1 tiny_race.c
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If a bug is detected, the program will print an error message to stderr.
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Currently, ThreadSanitizer symbolizes its output using an external
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``addr2line`` process (this will be fixed in future).
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.. code-block:: bash
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% ./a.out
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WARNING: ThreadSanitizer: data race (pid=19219)
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Write of size 4 at 0x7fcf47b21bc0 by thread T1:
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#0 Thread1 tiny_race.c:4 (exe+0x00000000a360)
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Previous write of size 4 at 0x7fcf47b21bc0 by main thread:
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#0 main tiny_race.c:10 (exe+0x00000000a3b4)
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Thread T1 (running) created at:
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#0 pthread_create tsan_interceptors.cc:705 (exe+0x00000000c790)
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#1 main tiny_race.c:9 (exe+0x00000000a3a4)
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``__has_feature(thread_sanitizer)``
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------------------------------------
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In some cases one may need to execute different code depending on whether
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ThreadSanitizer is enabled.
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:ref:`\_\_has\_feature <langext-__has_feature-__has_extension>` can be used for
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this purpose.
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.. code-block:: c
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#if defined(__has_feature)
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# if __has_feature(thread_sanitizer)
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// code that builds only under ThreadSanitizer
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# endif
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#endif
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``__attribute__((no_sanitize("thread")))``
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-----------------------------------------------
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Some code should not be instrumented by ThreadSanitizer. One may use the
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function attribute ``no_sanitize("thread")`` to disable instrumentation of plain
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(non-atomic) loads/stores in a particular function. ThreadSanitizer still
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instruments such functions to avoid false positives and provide meaningful stack
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traces. This attribute may not be supported by other compilers, so we suggest
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to use it together with ``__has_feature(thread_sanitizer)``.
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Blacklist
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---------
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ThreadSanitizer supports ``src`` and ``fun`` entity types in
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:doc:`SanitizerSpecialCaseList`, that can be used to suppress data race reports
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in the specified source files or functions. Unlike functions marked with
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``no_sanitize("thread")`` attribute, blacklisted functions are not instrumented
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at all. This can lead to false positives due to missed synchronization via
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atomic operations and missed stack frames in reports.
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Limitations
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-----------
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* ThreadSanitizer uses more real memory than a native run. At the default
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settings the memory overhead is 5x plus 1Mb per each thread. Settings with 3x
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(less accurate analysis) and 9x (more accurate analysis) overhead are also
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available.
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* ThreadSanitizer maps (but does not reserve) a lot of virtual address space.
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This means that tools like ``ulimit`` may not work as usually expected.
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* Libc/libstdc++ static linking is not supported.
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* Non-position-independent executables are not supported. Therefore, the
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``fsanitize=thread`` flag will cause Clang to act as though the ``-fPIE``
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flag had been supplied if compiling without ``-fPIC``, and as though the
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``-pie`` flag had been supplied if linking an executable.
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Current Status
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--------------
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ThreadSanitizer is in beta stage. It is known to work on large C++ programs
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using pthreads, but we do not promise anything (yet). C++11 threading is
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supported with llvm libc++. The test suite is integrated into CMake build
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and can be run with ``make check-tsan`` command.
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We are actively working on enhancing the tool --- stay tuned. Any help,
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especially in the form of minimized standalone tests is more than welcome.
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More Information
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----------------
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`<https://github.com/google/sanitizers/wiki/ThreadSanitizerCppManual>`_
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