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- TSA is a static analyzer build by Google which finds race conditions
and deadlocks at compile time.
- It works by associating a shared member variable with a
synchronization primitive that protects it. The compiler can then
check at each access if proper locking happened before. A good
introduction are [0] and [1].
- TSA requires some help by the programmer via annotations. Luckily,
LLVM's libcxx already has annotations for std::mutex, std::lock_guard,
std::shared_mutex and std::scoped_lock. This commit enables them
(--> contrib/libcxx-cmake/CMakeLists.txt).
- Further, this commit adds convenience macros for the low-level
annotations for use in ClickHouse (--> base/defines.h). For
demonstration, they are leveraged in a few places.
- As we compile with "-Wall -Wextra -Weverything", the required compiler
flag "-Wthread-safety-analysis" was already enabled. Negative checks
are an experimental feature of TSA and disabled
(--> cmake/warnings.cmake). Compile times did not increase noticeably.
- TSA is used in a few places with simple locking. I tried TSA also
where locking is more complex. The problem was usually that it is
unclear which data is protected by which lock :-(. But there was
definitely some weird code where locking looked broken. So there is
some potential to find bugs.
*** Limitations of TSA besides the ones listed in [1]:
- The programmer needs to know which lock protects which piece of shared
data. This is not always easy for large classes.
- Two synchronization primitives used in ClickHouse are not annotated in
libcxx:
(1) std::unique_lock: A releaseable lock handle often together with
std::condition_variable, e.g. in solve producer-consumer problems.
(2) std::recursive_mutex: A re-entrant mutex variant. Its usage can be
considered a design flaw + typically it is slower than a standard
mutex. In this commit, one std::recursive_mutex was converted to
std::mutex and annotated with TSA.
- For free-standing functions (e.g. helper functions) which are passed
shared data members, it can be tricky to specify the associated lock.
This is because the annotations use the normal C++ rules for symbol
resolution.
[0] https://clang.llvm.org/docs/ThreadSafetyAnalysis.html
[1] https://static.googleusercontent.com/media/research.google.com/en//pubs/archive/42958.pdf
58 lines
1.6 KiB
C++
58 lines
1.6 KiB
C++
#pragma once
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#include <mutex>
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#include <memory>
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#include <base/defines.h>
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/** Allow to store and read-only usage of an object in several threads,
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* and to atomically replace an object in another thread.
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* The replacement is atomic and reading threads can work with different versions of an object.
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*
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* Usage:
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* MultiVersion<T> x;
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* - on data update:
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* x.set(new value);
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* - on read-only usage:
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* {
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* MultiVersion<T>::Version current_version = x.get();
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* // use *current_version
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* } // now we finish own current version; if the version is outdated and no one else is using it - it will be destroyed.
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*
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* All methods are thread-safe.
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*/
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template <typename T>
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class MultiVersion
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{
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public:
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/// Version of object for usage. shared_ptr manage lifetime of version.
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using Version = std::shared_ptr<const T>;
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/// Default initialization - by nullptr.
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MultiVersion() = default;
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explicit MultiVersion(std::unique_ptr<const T> && value)
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{
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set(std::move(value));
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}
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/// Obtain current version for read-only usage. Returns shared_ptr, that manages lifetime of version.
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Version get() const
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{
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/// NOTE: is it possible to lock-free replace of shared_ptr?
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std::lock_guard lock(mutex);
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return current_version;
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}
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/// Update an object with new version.
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void set(std::unique_ptr<const T> && value)
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{
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std::lock_guard lock(mutex);
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current_version = std::move(value);
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}
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private:
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Version current_version TSA_GUARDED_BY(mutex);
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mutable std::mutex mutex;
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};
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