ClickHouse/src/Common/MultiVersion.h
Robert Schulze 5a4f21c50f
Support for Clang Thread Safety Analysis (TSA)
- 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
2022-06-20 16:13:25 +02:00

58 lines
1.6 KiB
C++

#pragma once
#include <mutex>
#include <memory>
#include <base/defines.h>
/** Allow to store and read-only usage of an object in several threads,
* and to atomically replace an object in another thread.
* The replacement is atomic and reading threads can work with different versions of an object.
*
* Usage:
* MultiVersion<T> x;
* - on data update:
* x.set(new value);
* - on read-only usage:
* {
* MultiVersion<T>::Version current_version = x.get();
* // use *current_version
* } // now we finish own current version; if the version is outdated and no one else is using it - it will be destroyed.
*
* All methods are thread-safe.
*/
template <typename T>
class MultiVersion
{
public:
/// Version of object for usage. shared_ptr manage lifetime of version.
using Version = std::shared_ptr<const T>;
/// Default initialization - by nullptr.
MultiVersion() = default;
explicit MultiVersion(std::unique_ptr<const T> && value)
{
set(std::move(value));
}
/// Obtain current version for read-only usage. Returns shared_ptr, that manages lifetime of version.
Version get() const
{
/// NOTE: is it possible to lock-free replace of shared_ptr?
std::lock_guard lock(mutex);
return current_version;
}
/// Update an object with new version.
void set(std::unique_ptr<const T> && value)
{
std::lock_guard lock(mutex);
current_version = std::move(value);
}
private:
Version current_version TSA_GUARDED_BY(mutex);
mutable std::mutex mutex;
};