clang/docs/ThreadSanitizer.rst - llvm-project - Git at Google

 ThreadSanitizer
 ===============

 Introduction
 ------------

 ThreadSanitizer is a tool that detects data races.  It consists of a compiler
 instrumentation module and a run-time library.  Typical slowdown introduced by
 ThreadSanitizer is about **5x-15x**.  Typical memory overhead introduced by
 ThreadSanitizer is about **5x-10x**.

 How to build
 ------------

 Build LLVM/Clang with `CMake <https://llvm.org/docs/CMake.html>`_.

 Supported Platforms
 -------------------

 ThreadSanitizer is supported on the following OS:

 * Android aarch64, x86_64
 * Darwin arm64, x86_64
 * FreeBSD
 * Linux aarch64, x86_64, powerpc64, powerpc64le
 * NetBSD

 Support for other 64-bit architectures is possible, contributions are welcome.
 Support for 32-bit platforms is problematic and is not planned.

 Usage
 -----

 Simply compile and link your program with ``-fsanitize=thread``.  To get a
 reasonable performance add ``-O1`` or higher.  Use ``-g`` to get file names
 and line numbers in the warning messages.

 Example:

 .. code-block:: console

   % cat projects/compiler-rt/lib/tsan/lit_tests/tiny_race.c
   #include <pthread.h>
   int Global;
   void *Thread1(void *x) {
     Global = 42;
     return x;
   }
   int main() {
     pthread_t t;
     pthread_create(&t, NULL, Thread1, NULL);
     Global = 43;
     pthread_join(t, NULL);
     return Global;
   }

   $ clang -fsanitize=thread -g -O1 tiny_race.c

 If a bug is detected, the program will print an error message to stderr.
 Currently, ThreadSanitizer symbolizes its output using an external
 ``addr2line`` process (this will be fixed in future).

 .. code-block:: bash

   % ./a.out
   WARNING: ThreadSanitizer: data race (pid=19219)
     Write of size 4 at 0x7fcf47b21bc0 by thread T1:
       #0 Thread1 tiny_race.c:4 (exe+0x00000000a360)

     Previous write of size 4 at 0x7fcf47b21bc0 by main thread:
       #0 main tiny_race.c:10 (exe+0x00000000a3b4)

     Thread T1 (running) created at:
       #0 pthread_create tsan_interceptors.cc:705 (exe+0x00000000c790)
       #1 main tiny_race.c:9 (exe+0x00000000a3a4)

 ``__has_feature(thread_sanitizer)``
 ------------------------------------

 In some cases one may need to execute different code depending on whether
 ThreadSanitizer is enabled.
 :ref:`\_\_has\_feature <langext-__has_feature-__has_extension>` can be used for
 this purpose.

 .. code-block:: c

     #if defined(__has_feature)
     #  if __has_feature(thread_sanitizer)
     // code that builds only under ThreadSanitizer
     #  endif
     #endif

 ``__attribute__((no_sanitize("thread")))``
 -----------------------------------------------

 Some code should not be instrumented by ThreadSanitizer.  One may use the
 function attribute ``no_sanitize("thread")`` to disable instrumentation of plain
 (non-atomic) loads/stores in a particular function.  ThreadSanitizer still
 instruments such functions to avoid false positives and provide meaningful stack
 traces.  This attribute may not be supported by other compilers, so we suggest
 to use it together with ``__has_feature(thread_sanitizer)``.

 ``__attribute__((disable_sanitizer_instrumentation))``
 --------------------------------------------------------

 The ``disable_sanitizer_instrumentation`` attribute can be applied to functions
 to prevent all kinds of instrumentation. As a result, it may introduce false
 positives and incorrect stack traces. Therefore, it should be used with care,
 and only if absolutely required; for example for certain code that cannot
 tolerate any instrumentation and resulting side-effects. This attribute
 overrides ``no_sanitize("thread")``.

 Interaction of Inlining with Disabling Sanitizer Instrumentation
 -----------------------------------------------------------------

 * A `no_sanitize` function will not be inlined heuristically by the compiler into a sanitized function.
 * An `always_inline` function will adopt the instrumentation status of the function it is inlined into.
 * Forcibly combining `no_sanitize` and ``__attribute__((always_inline))`` is not supported, and will often lead to unexpected results. To avoid mixing these attributes, use:

 . code-block:: c

     // Note, __has_feature test for sanitizers is deprecated, and Clang will support __SANITIZE_<sanitizer>__ similar to GCC.
     #if __has_feature(thread_sanitizer) || defined(__SANITIZE_THREAD__) || ... <other sanitizers>
     #define ALWAYS_INLINE_IF_UNINSTRUMENTED
     #else
     #define ALWAYS_INLINE_IF_UNINSTRUMENTED __attribute__((always_inline))
     #endif

 Explicit Sanitizer Checks with ``__builtin_allow_sanitize_check``
 -----------------------------------------------------------------

 The ``__builtin_allow_sanitize_check("thread")`` builtin can be used to
 conditionally execute code depending on whether ThreadSanitizer checks are
 enabled and permitted by the current policy (after inlining). This is
 particularly useful for inserting explicit, sanitizer-specific checks around
 operations like syscalls or inline assembly, which might otherwise be unchecked
 by the sanitizer.

 Example:

 .. code-block:: c

     void __tsan_read8(void *);

     inline __attribute__((always_inline))
     void my_helper(void *addr) {
       if (__builtin_allow_sanitize_check("thread"))
         __tsan_read8(addr);
       // ... actual logic, e.g. inline assembly ...
       asm volatile ("..." : : "r" (addr) : "memory");
     }

     void instrumented_function() {
       ...
       my_helper(&shared_data); // checks are active
       ...
     }

     __attribute__((no_sanitize("thread")))
     void uninstrumented_function() {
       ...
       my_helper(&shared_data); // checks are skipped
       ...
     }

 Ignorelist
 ----------

 ThreadSanitizer supports ``src`` and ``fun`` entity types in
 :doc:`SanitizerSpecialCaseList`, that can be used to suppress data race reports
 in the specified source files or functions. Unlike functions marked with
 ``no_sanitize("thread")`` attribute, ignored functions are not instrumented
 at all. This can lead to false positives due to missed synchronization via
 atomic operations and missed stack frames in reports.

 Limitations
 -----------

 * ThreadSanitizer uses more real memory than a native run. At the default
   settings the memory overhead is 5x plus 1Mb per each thread. Settings with 3x
   (less accurate analysis) and 9x (more accurate analysis) overhead are also
   available.
 * ThreadSanitizer maps (but does not reserve) a lot of virtual address space.
   This means that tools like ``ulimit`` may not work as usually expected.
 * Libc/libstdc++ static linking is not supported.
 * Non-position-independent executables are not supported.  Therefore, the
   ``fsanitize=thread`` flag will cause Clang to act as though the ``-fPIE``
   flag had been supplied if compiling without ``-fPIC``, and as though the
   ``-pie`` flag had been supplied if linking an executable.

 Security Considerations
 -----------------------

 ThreadSanitizer is a bug detection tool and its runtime is not meant to be
 linked against production executables. While it may be useful for testing,
 ThreadSanitizer's runtime was not developed with security-sensitive
 constraints in mind and may compromise the security of the resulting executable.

 Current Status
 --------------

 ThreadSanitizer is in beta stage.  It is known to work on large C++ programs
 using pthreads, but we do not promise anything (yet).  C++11 threading is
 supported with llvm libc++.  The test suite is integrated into CMake build
 and can be run with ``make check-tsan`` command.

 We are actively working on enhancing the tool --- stay tuned.  Any help,
 especially in the form of minimized standalone tests is more than welcome.

 More Information
 ----------------
 `<https://github.com/google/sanitizers/wiki/ThreadSanitizerCppManual>`_
	ThreadSanitizer
	===============

	Introduction
	------------

	ThreadSanitizer is a tool that detects data races. It consists of a compiler
	instrumentation module and a run-time library. Typical slowdown introduced by
	ThreadSanitizer is about 5x-15x. Typical memory overhead introduced by
	ThreadSanitizer is about 5x-10x.

	How to build
	------------

	Build LLVM/Clang with `CMake <https://llvm.org/docs/CMake.html>`_.

	Supported Platforms
	-------------------

	ThreadSanitizer is supported on the following OS:

	* Android aarch64, x86_64
	* Darwin arm64, x86_64
	* FreeBSD
	* Linux aarch64, x86_64, powerpc64, powerpc64le
	* NetBSD

	Support for other 64-bit architectures is possible, contributions are welcome.
	Support for 32-bit platforms is problematic and is not planned.

	Usage
	-----

	Simply compile and link your program with ``-fsanitize=thread``. To get a
	reasonable performance add ``-O1`` or higher. Use ``-g`` to get file names
	and line numbers in the warning messages.

	Example:

	.. code-block:: console

	% cat projects/compiler-rt/lib/tsan/lit_tests/tiny_race.c
	#include <pthread.h>
	int Global;
	void Thread1(void x) {
	Global = 42;
	return x;
	}
	int main() {
	pthread_t t;
	pthread_create(&t, NULL, Thread1, NULL);
	Global = 43;
	pthread_join(t, NULL);
	return Global;
	}

	$ clang -fsanitize=thread -g -O1 tiny_race.c

	If a bug is detected, the program will print an error message to stderr.
	Currently, ThreadSanitizer symbolizes its output using an external
	``addr2line`` process (this will be fixed in future).

	.. code-block:: bash

	% ./a.out
	WARNING: ThreadSanitizer: data race (pid=19219)
	Write of size 4 at 0x7fcf47b21bc0 by thread T1:
	#0 Thread1 tiny_race.c:4 (exe+0x00000000a360)

	Previous write of size 4 at 0x7fcf47b21bc0 by main thread:
	#0 main tiny_race.c:10 (exe+0x00000000a3b4)

	Thread T1 (running) created at:
	#0 pthread_create tsan_interceptors.cc:705 (exe+0x00000000c790)
	#1 main tiny_race.c:9 (exe+0x00000000a3a4)

	``__has_feature(thread_sanitizer)``
	------------------------------------

	In some cases one may need to execute different code depending on whether
	ThreadSanitizer is enabled.
	:ref:`\_\_has\_feature <langext-__has_feature-__has_extension>` can be used for
	this purpose.

	.. code-block:: c

	#if defined(__has_feature)
	# if __has_feature(thread_sanitizer)
	// code that builds only under ThreadSanitizer
	# endif
	#endif

	``__attribute__((no_sanitize("thread")))``
	-----------------------------------------------

	Some code should not be instrumented by ThreadSanitizer. One may use the
	function attribute ``no_sanitize("thread")`` to disable instrumentation of plain
	(non-atomic) loads/stores in a particular function. ThreadSanitizer still
	instruments such functions to avoid false positives and provide meaningful stack
	traces. This attribute may not be supported by other compilers, so we suggest
	to use it together with ``__has_feature(thread_sanitizer)``.

	``__attribute__((disable_sanitizer_instrumentation))``
	--------------------------------------------------------

	The ``disable_sanitizer_instrumentation`` attribute can be applied to functions
	to prevent all kinds of instrumentation. As a result, it may introduce false
	positives and incorrect stack traces. Therefore, it should be used with care,
	and only if absolutely required; for example for certain code that cannot
	tolerate any instrumentation and resulting side-effects. This attribute
	overrides ``no_sanitize("thread")``.

	Interaction of Inlining with Disabling Sanitizer Instrumentation
	-----------------------------------------------------------------

	* A `no_sanitize` function will not be inlined heuristically by the compiler into a sanitized function.
	* An `always_inline` function will adopt the instrumentation status of the function it is inlined into.
	* Forcibly combining `no_sanitize` and ``__attribute__((always_inline))`` is not supported, and will often lead to unexpected results. To avoid mixing these attributes, use:

	. code-block:: c

	// Note, __has_feature test for sanitizers is deprecated, and Clang will support __SANITIZE_<sanitizer>__ similar to GCC.
	#if __has_feature(thread_sanitizer) \|\| defined(__SANITIZE_THREAD__) \|\| ... <other sanitizers>
	#define ALWAYS_INLINE_IF_UNINSTRUMENTED
	#else
	#define ALWAYS_INLINE_IF_UNINSTRUMENTED __attribute__((always_inline))
	#endif

	Explicit Sanitizer Checks with ``__builtin_allow_sanitize_check``
	-----------------------------------------------------------------

	The ``__builtin_allow_sanitize_check("thread")`` builtin can be used to
	conditionally execute code depending on whether ThreadSanitizer checks are
	enabled and permitted by the current policy (after inlining). This is
	particularly useful for inserting explicit, sanitizer-specific checks around
	operations like syscalls or inline assembly, which might otherwise be unchecked
	by the sanitizer.

	Example:

	.. code-block:: c

	void __tsan_read8(void *);

	inline __attribute__((always_inline))
	void my_helper(void *addr) {
	if (__builtin_allow_sanitize_check("thread"))
	__tsan_read8(addr);
	// ... actual logic, e.g. inline assembly ...
	asm volatile ("..." : : "r" (addr) : "memory");
	}

	void instrumented_function() {
	...
	my_helper(&shared_data); // checks are active
	...
	}

	__attribute__((no_sanitize("thread")))
	void uninstrumented_function() {
	...
	my_helper(&shared_data); // checks are skipped
	...
	}

	Ignorelist
	----------

	ThreadSanitizer supports ``src`` and ``fun`` entity types in
	:doc:`SanitizerSpecialCaseList`, that can be used to suppress data race reports
	in the specified source files or functions. Unlike functions marked with
	``no_sanitize("thread")`` attribute, ignored functions are not instrumented
	at all. This can lead to false positives due to missed synchronization via
	atomic operations and missed stack frames in reports.

	Limitations
	-----------

	* ThreadSanitizer uses more real memory than a native run. At the default
	settings the memory overhead is 5x plus 1Mb per each thread. Settings with 3x
	(less accurate analysis) and 9x (more accurate analysis) overhead are also
	available.
	* ThreadSanitizer maps (but does not reserve) a lot of virtual address space.
	This means that tools like ``ulimit`` may not work as usually expected.
	* Libc/libstdc++ static linking is not supported.
	* Non-position-independent executables are not supported. Therefore, the
	``fsanitize=thread`` flag will cause Clang to act as though the ``-fPIE``
	flag had been supplied if compiling without ``-fPIC``, and as though the
	``-pie`` flag had been supplied if linking an executable.

	Security Considerations
	-----------------------

	ThreadSanitizer is a bug detection tool and its runtime is not meant to be
	linked against production executables. While it may be useful for testing,
	ThreadSanitizer's runtime was not developed with security-sensitive
	constraints in mind and may compromise the security of the resulting executable.

	Current Status
	--------------

	ThreadSanitizer is in beta stage. It is known to work on large C++ programs
	using pthreads, but we do not promise anything (yet). C++11 threading is
	supported with llvm libc++. The test suite is integrated into CMake build
	and can be run with ``make check-tsan`` command.

	We are actively working on enhancing the tool --- stay tuned. Any help,
	especially in the form of minimized standalone tests is more than welcome.

	More Information
	----------------
	`<https://github.com/google/sanitizers/wiki/ThreadSanitizerCppManual>`_