949 lines
30 KiB
ReStructuredText
949 lines
30 KiB
ReStructuredText
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======================
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Thread Safety Analysis
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======================
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Introduction
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============
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Clang Thread Safety Analysis is a C++ language extension which warns about
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potential race conditions in code. The analysis is completely static (i.e.
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compile-time); there is no run-time overhead. The analysis is still
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under active development, but it is mature enough to be deployed in an
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industrial setting. It is being developed by Google, in collaboration with
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CERT/SEI, and is used extensively in Google's internal code base.
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Thread safety analysis works very much like a type system for multi-threaded
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programs. In addition to declaring the *type* of data (e.g. ``int``, ``float``,
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etc.), the programmer can (optionally) declare how access to that data is
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controlled in a multi-threaded environment. For example, if ``foo`` is
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*guarded by* the mutex ``mu``, then the analysis will issue a warning whenever
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a piece of code reads or writes to ``foo`` without first locking ``mu``.
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Similarly, if there are particular routines that should only be called by
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the GUI thread, then the analysis will warn if other threads call those
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routines.
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Getting Started
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----------------
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.. code-block:: c++
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#include "mutex.h"
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class BankAccount {
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private:
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Mutex mu;
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int balance GUARDED_BY(mu);
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void depositImpl(int amount) {
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balance += amount; // WARNING! Cannot write balance without locking mu.
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}
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void withdrawImpl(int amount) REQUIRES(mu) {
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balance -= amount; // OK. Caller must have locked mu.
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}
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public:
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void withdraw(int amount) {
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mu.Lock();
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withdrawImpl(amount); // OK. We've locked mu.
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} // WARNING! Failed to unlock mu.
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void transferFrom(BankAccount& b, int amount) {
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mu.Lock();
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b.withdrawImpl(amount); // WARNING! Calling withdrawImpl() requires locking b.mu.
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depositImpl(amount); // OK. depositImpl() has no requirements.
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mu.Unlock();
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}
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};
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This example demonstrates the basic concepts behind the analysis. The
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``GUARDED_BY`` attribute declares that a thread must lock ``mu`` before it can
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read or write to ``balance``, thus ensuring that the increment and decrement
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operations are atomic. Similarly, ``REQUIRES`` declares that
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the calling thread must lock ``mu`` before calling ``withdrawImpl``.
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Because the caller is assumed to have locked ``mu``, it is safe to modify
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``balance`` within the body of the method.
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The ``depositImpl()`` method does not have ``REQUIRES``, so the
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analysis issues a warning. Thread safety analysis is not inter-procedural, so
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caller requirements must be explicitly declared.
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There is also a warning in ``transferFrom()``, because although the method
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locks ``this->mu``, it does not lock ``b.mu``. The analysis understands
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that these are two separate mutexes, in two different objects.
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Finally, there is a warning in the ``withdraw()`` method, because it fails to
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unlock ``mu``. Every lock must have a corresponding unlock, and the analysis
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will detect both double locks, and double unlocks. A function is allowed to
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acquire a lock without releasing it, (or vice versa), but it must be annotated
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as such (using ``ACQUIRE``/``RELEASE``).
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Running The Analysis
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--------------------
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To run the analysis, simply compile with the ``-Wthread-safety`` flag, e.g.
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.. code-block:: bash
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clang -c -Wthread-safety example.cpp
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Note that this example assumes the presence of a suitably annotated
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:ref:`mutexheader` that declares which methods perform locking,
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unlocking, and so on.
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Basic Concepts: Capabilities
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============================
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Thread safety analysis provides a way of protecting *resources* with
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*capabilities*. A resource is either a data member, or a function/method
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that provides access to some underlying resource. The analysis ensures that
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the calling thread cannot access the *resource* (i.e. call the function, or
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read/write the data) unless it has the *capability* to do so.
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Capabilities are associated with named C++ objects which declare specific
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methods to acquire and release the capability. The name of the object serves
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to identify the capability. The most common example is a mutex. For example,
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if ``mu`` is a mutex, then calling ``mu.Lock()`` causes the calling thread
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to acquire the capability to access data that is protected by ``mu``. Similarly,
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calling ``mu.Unlock()`` releases that capability.
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A thread may hold a capability either *exclusively* or *shared*. An exclusive
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capability can be held by only one thread at a time, while a shared capability
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can be held by many threads at the same time. This mechanism enforces a
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multiple-reader, single-writer pattern. Write operations to protected data
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require exclusive access, while read operations require only shared access.
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At any given moment during program execution, a thread holds a specific set of
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capabilities (e.g. the set of mutexes that it has locked.) These act like keys
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or tokens that allow the thread to access a given resource. Just like physical
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security keys, a thread cannot make copy of a capability, nor can it destroy
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one. A thread can only release a capability to another thread, or acquire one
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from another thread. The annotations are deliberately agnostic about the
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exact mechanism used to acquire and release capabilities; it assumes that the
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underlying implementation (e.g. the Mutex implementation) does the handoff in
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an appropriate manner.
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The set of capabilities that are actually held by a given thread at a given
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point in program execution is a run-time concept. The static analysis works
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by calculating an approximation of that set, called the *capability
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environment*. The capability environment is calculated for every program point,
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and describes the set of capabilities that are statically known to be held, or
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not held, at that particular point. This environment is a conservative
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approximation of the full set of capabilities that will actually held by a
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thread at run-time.
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Reference Guide
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===============
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The thread safety analysis uses attributes to declare threading constraints.
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Attributes must be attached to named declarations, such as classes, methods,
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and data members. Users are *strongly advised* to define macros for the various
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attributes; example definitions can be found in :ref:`mutexheader`, below.
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The following documentation assumes the use of macros.
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For historical reasons, prior versions of thread safety used macro names that
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were very lock-centric. These macros have since been renamed to fit a more
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general capability model. The prior names are still in use, and will be
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mentioned under the tag *previously* where appropriate.
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GUARDED_BY(c) and PT_GUARDED_BY(c)
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----------------------------------
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``GUARDED_BY`` is an attribute on data members, which declares that the data
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member is protected by the given capability. Read operations on the data
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require shared access, while write operations require exclusive access.
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``PT_GUARDED_BY`` is similar, but is intended for use on pointers and smart
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pointers. There is no constraint on the data member itself, but the *data that
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it points to* is protected by the given capability.
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.. code-block:: c++
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Mutex mu;
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int *p1 GUARDED_BY(mu);
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int *p2 PT_GUARDED_BY(mu);
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unique_ptr<int> p3 PT_GUARDED_BY(mu);
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void test() {
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p1 = 0; // Warning!
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*p2 = 42; // Warning!
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p2 = new int; // OK.
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*p3 = 42; // Warning!
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p3.reset(new int); // OK.
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}
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REQUIRES(...), REQUIRES_SHARED(...)
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-----------------------------------
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*Previously*: ``EXCLUSIVE_LOCKS_REQUIRED``, ``SHARED_LOCKS_REQUIRED``
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``REQUIRES`` is an attribute on functions or methods, which
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declares that the calling thread must have exclusive access to the given
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capabilities. More than one capability may be specified. The capabilities
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must be held on entry to the function, *and must still be held on exit*.
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``REQUIRES_SHARED`` is similar, but requires only shared access.
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.. code-block:: c++
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Mutex mu1, mu2;
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int a GUARDED_BY(mu1);
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int b GUARDED_BY(mu2);
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void foo() REQUIRES(mu1, mu2) {
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a = 0;
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b = 0;
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}
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void test() {
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mu1.Lock();
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foo(); // Warning! Requires mu2.
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mu1.Unlock();
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}
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ACQUIRE(...), ACQUIRE_SHARED(...), RELEASE(...), RELEASE_SHARED(...)
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--------------------------------------------------------------------
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*Previously*: ``EXCLUSIVE_LOCK_FUNCTION``, ``SHARED_LOCK_FUNCTION``,
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``UNLOCK_FUNCTION``
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``ACQUIRE`` is an attribute on functions or methods, which
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declares that the function acquires a capability, but does not release it. The
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caller must not hold the given capability on entry, and it will hold the
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capability on exit. ``ACQUIRE_SHARED`` is similar.
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``RELEASE`` and ``RELEASE_SHARED`` declare that the function releases the given
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capability. The caller must hold the capability on entry, and will no longer
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hold it on exit. It does not matter whether the given capability is shared or
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exclusive.
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.. code-block:: c++
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Mutex mu;
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MyClass myObject GUARDED_BY(mu);
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void lockAndInit() ACQUIRE(mu) {
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mu.Lock();
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myObject.init();
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}
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void cleanupAndUnlock() RELEASE(mu) {
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myObject.cleanup();
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} // Warning! Need to unlock mu.
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void test() {
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lockAndInit();
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myObject.doSomething();
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cleanupAndUnlock();
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myObject.doSomething(); // Warning, mu is not locked.
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}
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If no argument is passed to ``ACQUIRE`` or ``RELEASE``, then the argument is
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assumed to be ``this``, and the analysis will not check the body of the
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function. This pattern is intended for use by classes which hide locking
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details behind an abstract interface. For example:
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.. code-block:: c++
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template <class T>
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class CAPABILITY("mutex") Container {
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private:
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Mutex mu;
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T* data;
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public:
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// Hide mu from public interface.
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void Lock() ACQUIRE() { mu.Lock(); }
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void Unlock() RELEASE() { mu.Unlock(); }
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T& getElem(int i) { return data[i]; }
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};
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void test() {
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Container<int> c;
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c.Lock();
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int i = c.getElem(0);
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c.Unlock();
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}
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EXCLUDES(...)
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-------------
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*Previously*: ``LOCKS_EXCLUDED``
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``EXCLUDES`` is an attribute on functions or methods, which declares that
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the caller must *not* hold the given capabilities. This annotation is
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used to prevent deadlock. Many mutex implementations are not re-entrant, so
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deadlock can occur if the function acquires the mutex a second time.
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.. code-block:: c++
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Mutex mu;
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int a GUARDED_BY(mu);
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void clear() EXCLUDES(mu) {
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mu.Lock();
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a = 0;
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mu.Unlock();
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}
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void reset() {
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mu.Lock();
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clear(); // Warning! Caller cannot hold 'mu'.
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mu.Unlock();
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}
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Unlike ``REQUIRES``, ``EXCLUDES`` is optional. The analysis will not issue a
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warning if the attribute is missing, which can lead to false negatives in some
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cases. This issue is discussed further in :ref:`negative`.
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NO_THREAD_SAFETY_ANALYSIS
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-------------------------
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``NO_THREAD_SAFETY_ANALYSIS`` is an attribute on functions or methods, which
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turns off thread safety checking for that method. It provides an escape hatch
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for functions which are either (1) deliberately thread-unsafe, or (2) are
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thread-safe, but too complicated for the analysis to understand. Reasons for
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(2) will be described in the :ref:`limitations`, below.
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.. code-block:: c++
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class Counter {
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Mutex mu;
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int a GUARDED_BY(mu);
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void unsafeIncrement() NO_THREAD_SAFETY_ANALYSIS { a++; }
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};
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Unlike the other attributes, NO_THREAD_SAFETY_ANALYSIS is not part of the
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interface of a function, and should thus be placed on the function definition
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(in the ``.cc`` or ``.cpp`` file) rather than on the function declaration
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(in the header).
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RETURN_CAPABILITY(c)
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--------------------
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*Previously*: ``LOCK_RETURNED``
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``RETURN_CAPABILITY`` is an attribute on functions or methods, which declares
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that the function returns a reference to the given capability. It is used to
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annotate getter methods that return mutexes.
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.. code-block:: c++
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class MyClass {
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private:
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Mutex mu;
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int a GUARDED_BY(mu);
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public:
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Mutex* getMu() RETURN_CAPABILITY(mu) { return μ }
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// analysis knows that getMu() == mu
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void clear() REQUIRES(getMu()) { a = 0; }
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};
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ACQUIRED_BEFORE(...), ACQUIRED_AFTER(...)
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-----------------------------------------
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``ACQUIRED_BEFORE`` and ``ACQUIRED_AFTER`` are attributes on member
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declarations, specifically declarations of mutexes or other capabilities.
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These declarations enforce a particular order in which the mutexes must be
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acquired, in order to prevent deadlock.
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.. code-block:: c++
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Mutex m1;
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Mutex m2 ACQUIRED_AFTER(m1);
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// Alternative declaration
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// Mutex m2;
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// Mutex m1 ACQUIRED_BEFORE(m2);
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void foo() {
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m2.Lock();
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m1.Lock(); // Warning! m2 must be acquired after m1.
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m1.Unlock();
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m2.Unlock();
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}
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CAPABILITY(<string>)
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--------------------
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*Previously*: ``LOCKABLE``
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``CAPABILITY`` is an attribute on classes, which specifies that objects of the
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class can be used as a capability. The string argument specifies the kind of
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capability in error messages, e.g. ``"mutex"``. See the ``Container`` example
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given above, or the ``Mutex`` class in :ref:`mutexheader`.
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SCOPED_CAPABILITY
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-----------------
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*Previously*: ``SCOPED_LOCKABLE``
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``SCOPED_CAPABILITY`` is an attribute on classes that implement RAII-style
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locking, in which a capability is acquired in the constructor, and released in
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the destructor. Such classes require special handling because the constructor
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and destructor refer to the capability via different names; see the
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``MutexLocker`` class in :ref:`mutexheader`, below.
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TRY_ACQUIRE(<bool>, ...), TRY_ACQUIRE_SHARED(<bool>, ...)
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---------------------------------------------------------
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*Previously:* ``EXCLUSIVE_TRYLOCK_FUNCTION``, ``SHARED_TRYLOCK_FUNCTION``
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These are attributes on a function or method that tries to acquire the given
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capability, and returns a boolean value indicating success or failure.
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The first argument must be ``true`` or ``false``, to specify which return value
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indicates success, and the remaining arguments are interpreted in the same way
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as ``ACQUIRE``. See :ref:`mutexheader`, below, for example uses.
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ASSERT_CAPABILITY(...) and ASSERT_SHARED_CAPABILITY(...)
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--------------------------------------------------------
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*Previously:* ``ASSERT_EXCLUSIVE_LOCK``, ``ASSERT_SHARED_LOCK``
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These are attributes on a function or method that does a run-time test to see
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whether the calling thread holds the given capability. The function is assumed
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to fail (no return) if the capability is not held. See :ref:`mutexheader`,
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below, for example uses.
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GUARDED_VAR and PT_GUARDED_VAR
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------------------------------
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Use of these attributes has been deprecated.
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Warning flags
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-------------
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* ``-Wthread-safety``: Umbrella flag which turns on the following three:
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+ ``-Wthread-safety-attributes``: Sanity checks on attribute syntax.
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+ ``-Wthread-safety-analysis``: The core analysis.
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+ ``-Wthread-safety-precise``: Requires that mutex expressions match precisely.
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This warning can be disabled for code which has a lot of aliases.
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+ ``-Wthread-safety-reference``: Checks when guarded members are passed by reference.
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:ref:`negative` are an experimental feature, which are enabled with:
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* ``-Wthread-safety-negative``: Negative capabilities. Off by default.
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When new features and checks are added to the analysis, they can often introduce
|
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additional warnings. Those warnings are initially released as *beta* warnings
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for a period of time, after which they are migrated into the standard analysis.
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* ``-Wthread-safety-beta``: New features. Off by default.
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.. _negative:
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Negative Capabilities
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=====================
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Thread Safety Analysis is designed to prevent both race conditions and
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deadlock. The GUARDED_BY and REQUIRES attributes prevent race conditions, by
|
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ensuring that a capability is held before reading or writing to guarded data,
|
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and the EXCLUDES attribute prevents deadlock, by making sure that a mutex is
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*not* held.
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However, EXCLUDES is an optional attribute, and does not provide the same
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safety guarantee as REQUIRES. In particular:
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* A function which acquires a capability does not have to exclude it.
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* A function which calls a function that excludes a capability does not
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have transitively exclude that capability.
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As a result, EXCLUDES can easily produce false negatives:
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|
.. code-block:: c++
|
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class Foo {
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Mutex mu;
|
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|
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void foo() {
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mu.Lock();
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bar(); // No warning.
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baz(); // No warning.
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mu.Unlock();
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}
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void bar() { // No warning. (Should have EXCLUDES(mu)).
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mu.Lock();
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// ...
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mu.Unlock();
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}
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void baz() {
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bif(); // No warning. (Should have EXCLUDES(mu)).
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}
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void bif() EXCLUDES(mu);
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};
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Negative requirements are an alternative EXCLUDES that provide
|
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a stronger safety guarantee. A negative requirement uses the REQUIRES
|
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attribute, in conjunction with the ``!`` operator, to indicate that a capability
|
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should *not* be held.
|
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For example, using ``REQUIRES(!mu)`` instead of ``EXCLUDES(mu)`` will produce
|
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the appropriate warnings:
|
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|
|
.. code-block:: c++
|
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|
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class FooNeg {
|
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Mutex mu;
|
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void foo() REQUIRES(!mu) { // foo() now requires !mu.
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mu.Lock();
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bar();
|
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baz();
|
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mu.Unlock();
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}
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void bar() {
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mu.Lock(); // WARNING! Missing REQUIRES(!mu).
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// ...
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mu.Unlock();
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}
|
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void baz() {
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bif(); // WARNING! Missing REQUIRES(!mu).
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}
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void bif() REQUIRES(!mu);
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};
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|
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Negative requirements are an experimental feature which is off by default,
|
|
because it will produce many warnings in existing code. It can be enabled
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|
by passing ``-Wthread-safety-negative``.
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.. _faq:
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Frequently Asked Questions
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==========================
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(Q) Should I put attributes in the header file, or in the .cc/.cpp/.cxx file?
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(A) Attributes are part of the formal interface of a function, and should
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always go in the header, where they are visible to anything that includes
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the header. Attributes in the .cpp file are not visible outside of the
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immediate translation unit, which leads to false negatives and false positives.
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(Q) "*Mutex is not locked on every path through here?*" What does that mean?
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(A) See :ref:`conditional_locks`, below.
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.. _limitations:
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Known Limitations
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=================
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Lexical scope
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-------------
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Thread safety attributes contain ordinary C++ expressions, and thus follow
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ordinary C++ scoping rules. In particular, this means that mutexes and other
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capabilities must be declared before they can be used in an attribute.
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Use-before-declaration is okay within a single class, because attributes are
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parsed at the same time as method bodies. (C++ delays parsing of method bodies
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until the end of the class.) However, use-before-declaration is not allowed
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between classes, as illustrated below.
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.. code-block:: c++
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class Foo;
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class Bar {
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void bar(Foo* f) REQUIRES(f->mu); // Error: mu undeclared.
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};
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class Foo {
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Mutex mu;
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};
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Private Mutexes
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---------------
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Good software engineering practice dictates that mutexes should be private
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members, because the locking mechanism used by a thread-safe class is part of
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its internal implementation. However, private mutexes can sometimes leak into
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the public interface of a class.
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Thread safety attributes follow normal C++ access restrictions, so if ``mu``
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is a private member of ``c``, then it is an error to write ``c.mu`` in an
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attribute.
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One workaround is to (ab)use the ``RETURN_CAPABILITY`` attribute to provide a
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public *name* for a private mutex, without actually exposing the underlying
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mutex. For example:
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.. code-block:: c++
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class MyClass {
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private:
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Mutex mu;
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public:
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// For thread safety analysis only. Does not actually return mu.
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Mutex* getMu() RETURN_CAPABILITY(mu) { return 0; }
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void doSomething() REQUIRES(mu);
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};
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void doSomethingTwice(MyClass& c) REQUIRES(c.getMu()) {
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// The analysis thinks that c.getMu() == c.mu
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c.doSomething();
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c.doSomething();
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}
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In the above example, ``doSomethingTwice()`` is an external routine that
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requires ``c.mu`` to be locked, which cannot be declared directly because ``mu``
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is private. This pattern is discouraged because it
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violates encapsulation, but it is sometimes necessary, especially when adding
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annotations to an existing code base. The workaround is to define ``getMu()``
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as a fake getter method, which is provided only for the benefit of thread
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safety analysis.
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.. _conditional_locks:
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No conditionally held locks.
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----------------------------
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The analysis must be able to determine whether a lock is held, or not held, at
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every program point. Thus, sections of code where a lock *might be held* will
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generate spurious warnings (false positives). For example:
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.. code-block:: c++
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void foo() {
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bool b = needsToLock();
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if (b) mu.Lock();
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... // Warning! Mutex 'mu' is not held on every path through here.
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if (b) mu.Unlock();
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}
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No checking inside constructors and destructors.
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------------------------------------------------
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The analysis currently does not do any checking inside constructors or
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destructors. In other words, every constructor and destructor is treated as
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if it was annotated with ``NO_THREAD_SAFETY_ANALYSIS``.
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The reason for this is that during initialization, only one thread typically
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has access to the object which is being initialized, and it is thus safe (and
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common practice) to initialize guarded members without acquiring any locks.
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The same is true of destructors.
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Ideally, the analysis would allow initialization of guarded members inside the
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object being initialized or destroyed, while still enforcing the usual access
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restrictions on everything else. However, this is difficult to enforce in
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practice, because in complex pointer-based data structures, it is hard to
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determine what data is owned by the enclosing object.
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No inlining.
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------------
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Thread safety analysis is strictly intra-procedural, just like ordinary type
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checking. It relies only on the declared attributes of a function, and will
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not attempt to inline any method calls. As a result, code such as the
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following will not work:
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.. code-block:: c++
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template<class T>
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class AutoCleanup {
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T* object;
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void (T::*mp)();
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public:
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AutoCleanup(T* obj, void (T::*imp)()) : object(obj), mp(imp) { }
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~AutoCleanup() { (object->*mp)(); }
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};
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Mutex mu;
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void foo() {
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mu.Lock();
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AutoCleanup<Mutex>(&mu, &Mutex::Unlock);
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// ...
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} // Warning, mu is not unlocked.
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In this case, the destructor of ``Autocleanup`` calls ``mu.Unlock()``, so
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the warning is bogus. However,
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thread safety analysis cannot see the unlock, because it does not attempt to
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inline the destructor. Moreover, there is no way to annotate the destructor,
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because the destructor is calling a function that is not statically known.
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This pattern is simply not supported.
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No alias analysis.
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------------------
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The analysis currently does not track pointer aliases. Thus, there can be
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false positives if two pointers both point to the same mutex.
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.. code-block:: c++
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class MutexUnlocker {
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Mutex* mu;
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public:
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MutexUnlocker(Mutex* m) RELEASE(m) : mu(m) { mu->Unlock(); }
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~MutexUnlocker() ACQUIRE(mu) { mu->Lock(); }
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};
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Mutex mutex;
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void test() REQUIRES(mutex) {
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{
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MutexUnlocker munl(&mutex); // unlocks mutex
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doSomeIO();
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} // Warning: locks munl.mu
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}
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The MutexUnlocker class is intended to be the dual of the MutexLocker class,
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defined in :ref:`mutexheader`. However, it doesn't work because the analysis
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doesn't know that munl.mu == mutex. The SCOPED_CAPABILITY attribute handles
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aliasing for MutexLocker, but does so only for that particular pattern.
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ACQUIRED_BEFORE(...) and ACQUIRED_AFTER(...) are currently unimplemented.
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-------------------------------------------------------------------------
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To be fixed in a future update.
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.. _mutexheader:
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mutex.h
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=======
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Thread safety analysis can be used with any threading library, but it does
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require that the threading API be wrapped in classes and methods which have the
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appropriate annotations. The following code provides ``mutex.h`` as an example;
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these methods should be filled in to call the appropriate underlying
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implementation.
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.. code-block:: c++
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#ifndef THREAD_SAFETY_ANALYSIS_MUTEX_H
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#define THREAD_SAFETY_ANALYSIS_MUTEX_H
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// Enable thread safety attributes only with clang.
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// The attributes can be safely erased when compiling with other compilers.
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#if defined(__clang__) && (!defined(SWIG))
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#define THREAD_ANNOTATION_ATTRIBUTE__(x) __attribute__((x))
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#else
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#define THREAD_ANNOTATION_ATTRIBUTE__(x) // no-op
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#endif
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#define CAPABILITY(x) \
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THREAD_ANNOTATION_ATTRIBUTE__(capability(x))
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#define SCOPED_CAPABILITY \
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THREAD_ANNOTATION_ATTRIBUTE__(scoped_lockable)
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#define GUARDED_BY(x) \
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THREAD_ANNOTATION_ATTRIBUTE__(guarded_by(x))
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#define PT_GUARDED_BY(x) \
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THREAD_ANNOTATION_ATTRIBUTE__(pt_guarded_by(x))
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#define ACQUIRED_BEFORE(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(acquired_before(__VA_ARGS__))
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#define ACQUIRED_AFTER(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(acquired_after(__VA_ARGS__))
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#define REQUIRES(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(requires_capability(__VA_ARGS__))
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#define REQUIRES_SHARED(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(requires_shared_capability(__VA_ARGS__))
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#define ACQUIRE(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(acquire_capability(__VA_ARGS__))
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#define ACQUIRE_SHARED(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(acquire_shared_capability(__VA_ARGS__))
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#define RELEASE(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(release_capability(__VA_ARGS__))
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#define RELEASE_SHARED(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(release_shared_capability(__VA_ARGS__))
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#define TRY_ACQUIRE(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(try_acquire_capability(__VA_ARGS__))
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#define TRY_ACQUIRE_SHARED(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(try_acquire_shared_capability(__VA_ARGS__))
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#define EXCLUDES(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(locks_excluded(__VA_ARGS__))
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#define ASSERT_CAPABILITY(x) \
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THREAD_ANNOTATION_ATTRIBUTE__(assert_capability(x))
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#define ASSERT_SHARED_CAPABILITY(x) \
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THREAD_ANNOTATION_ATTRIBUTE__(assert_shared_capability(x))
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#define RETURN_CAPABILITY(x) \
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THREAD_ANNOTATION_ATTRIBUTE__(lock_returned(x))
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#define NO_THREAD_SAFETY_ANALYSIS \
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THREAD_ANNOTATION_ATTRIBUTE__(no_thread_safety_analysis)
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// Defines an annotated interface for mutexes.
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// These methods can be implemented to use any internal mutex implementation.
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class CAPABILITY("mutex") Mutex {
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public:
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// Acquire/lock this mutex exclusively. Only one thread can have exclusive
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// access at any one time. Write operations to guarded data require an
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// exclusive lock.
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void Lock() ACQUIRE();
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// Acquire/lock this mutex for read operations, which require only a shared
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// lock. This assumes a multiple-reader, single writer semantics. Multiple
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// threads may acquire the mutex simultaneously as readers, but a writer
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// must wait for all of them to release the mutex before it can acquire it
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// exclusively.
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void ReaderLock() ACQUIRE_SHARED();
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// Release/unlock an exclusive mutex.
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void Unlock() RELEASE();
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// Release/unlock a shared mutex.
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void ReaderUnlock() RELEASE_SHARED();
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// Try to acquire the mutex. Returns true on success, and false on failure.
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bool TryLock() TRY_ACQUIRE(true);
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// Try to acquire the mutex for read operations.
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bool ReaderTryLock() TRY_ACQUIRE_SHARED(true);
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// Assert that this mutex is currently held by the calling thread.
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void AssertHeld() ASSERT_CAPABILITY(this);
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// Assert that is mutex is currently held for read operations.
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void AssertReaderHeld() ASSERT_SHARED_CAPABILITY(this);
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// For negative capabilities.
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const Mutex& operator!() const { return *this; }
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};
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// MutexLocker is an RAII class that acquires a mutex in its constructor, and
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// releases it in its destructor.
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class SCOPED_CAPABILITY MutexLocker {
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private:
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Mutex* mut;
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public:
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MutexLocker(Mutex *mu) ACQUIRE(mu) : mut(mu) {
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mu->Lock();
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}
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~MutexLocker() RELEASE() {
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mut->Unlock();
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}
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};
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#ifdef USE_LOCK_STYLE_THREAD_SAFETY_ATTRIBUTES
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// The original version of thread safety analysis the following attribute
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// definitions. These use a lock-based terminology. They are still in use
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// by existing thread safety code, and will continue to be supported.
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// Deprecated.
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#define PT_GUARDED_VAR \
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THREAD_ANNOTATION_ATTRIBUTE__(pt_guarded_var)
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// Deprecated.
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#define GUARDED_VAR \
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THREAD_ANNOTATION_ATTRIBUTE__(guarded_var)
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// Replaced by REQUIRES
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#define EXCLUSIVE_LOCKS_REQUIRED(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(exclusive_locks_required(__VA_ARGS__))
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// Replaced by REQUIRES_SHARED
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#define SHARED_LOCKS_REQUIRED(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(shared_locks_required(__VA_ARGS__))
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// Replaced by CAPABILITY
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#define LOCKABLE \
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THREAD_ANNOTATION_ATTRIBUTE__(lockable)
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// Replaced by SCOPED_CAPABILITY
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#define SCOPED_LOCKABLE \
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THREAD_ANNOTATION_ATTRIBUTE__(scoped_lockable)
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// Replaced by ACQUIRE
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#define EXCLUSIVE_LOCK_FUNCTION(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(exclusive_lock_function(__VA_ARGS__))
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// Replaced by ACQUIRE_SHARED
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#define SHARED_LOCK_FUNCTION(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(shared_lock_function(__VA_ARGS__))
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// Replaced by RELEASE and RELEASE_SHARED
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#define UNLOCK_FUNCTION(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(unlock_function(__VA_ARGS__))
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// Replaced by TRY_ACQUIRE
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#define EXCLUSIVE_TRYLOCK_FUNCTION(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(exclusive_trylock_function(__VA_ARGS__))
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// Replaced by TRY_ACQUIRE_SHARED
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#define SHARED_TRYLOCK_FUNCTION(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(shared_trylock_function(__VA_ARGS__))
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// Replaced by ASSERT_CAPABILITY
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#define ASSERT_EXCLUSIVE_LOCK(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(assert_exclusive_lock(__VA_ARGS__))
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// Replaced by ASSERT_SHARED_CAPABILITY
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#define ASSERT_SHARED_LOCK(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(assert_shared_lock(__VA_ARGS__))
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// Replaced by EXCLUDE_CAPABILITY.
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#define LOCKS_EXCLUDED(...) \
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THREAD_ANNOTATION_ATTRIBUTE__(locks_excluded(__VA_ARGS__))
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// Replaced by RETURN_CAPABILITY
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#define LOCK_RETURNED(x) \
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THREAD_ANNOTATION_ATTRIBUTE__(lock_returned(x))
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#endif // USE_LOCK_STYLE_THREAD_SAFETY_ATTRIBUTES
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#endif // THREAD_SAFETY_ANALYSIS_MUTEX_H
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