- 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
Here is oneliner:
$ gg 'LOG_\(DEBUG\|TRACE\|INFO\|TEST\|WARNING\|ERROR\|FATAL\)([^,]*, [a-zA-Z]' -- :*.cpp :*.h | cut -d: -f1 | sort -u | xargs -r sed -E -i 's#(LOG_[A-Z]*)\(([^,]*), ([A-Za-z][^,)]*)#\1(\2, fmt::runtime(\3)#'
Note, that I tried to do this with coccinelle (tool for semantic
patchin), but it cannot parse C++:
$ cat fmt.cocci
@@
expression log;
expression var;
@@
-LOG_DEBUG(log, var)
+LOG_DEBUG(log, fmt::runtime(var))
I've also tried to use some macros/templates magic to do this implicitly
in logger_useful.h, but I failed to do so, and apparently it is not
possible for now.
Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>
v2: manual fixes
Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>
