clang thread sanitizer reports issues when using conditon variables - c++

Question

Here is a simple example for using std::condition_variable. When using clang+tsan for building the following code,
#include <cassert>
#include <chrono>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <thread>
class WaitSignalLock {
private:
std::condition_variable conditionVariable;
std::mutex mtx;
bool condition = false;
public:
bool wait(const std::chrono::milliseconds& timeout);
void signal();
private:
bool conditionMet() const;
bool waitForConditionVariableSignal(std::unique_lock<std::mutex>& lck, const std::chrono::milliseconds& timeout);
};
bool WaitSignalLock::wait(const std::chrono::milliseconds& timeout) {
std::unique_lock<std::mutex> lck(this->mtx);
const auto result = waitForConditionVariableSignal(lck, timeout);
condition = false;
return result;
}
void WaitSignalLock::signal() {
// the variable used for checking the condition has to be modified under a lock (even if the variable is atomic),
// and the condition variable should not be notified under a lock
{
std::lock_guard<std::mutex> lck(this->mtx);
condition = true;
}
conditionVariable.notify_one();
}
bool WaitSignalLock::conditionMet() const {
// do not lock, method will be called under the lock already
return condition;
}
bool WaitSignalLock::waitForConditionVariableSignal(std::unique_lock<std::mutex>& lck, const std::chrono::milliseconds& timeout) {
assert(lck.owns_lock());
if(this->conditionVariable.wait_for(lck, timeout, [this]() {
return this->conditionMet();
})) {
return true;
}
else {
return false;
}
}
int main(int, char**) {
WaitSignalLock waitSignalLock;
std::thread sendSignal([&waitSignalLock]() {
std::this_thread::sleep_for(std::chrono::milliseconds(500));
waitSignalLock.signal();
});
const auto result = waitSignalLock.wait(std::chrono::seconds(1));
sendSignal.join();
return !result;
}
tsan reports the following two issues:
WARNING: ThreadSanitizer: double lock of a mutex (pid=16421)
#0 pthread_mutex_lock <null> (a.out+0x7e358)
#1 __gthread_mutex_lock(pthread_mutex_t*) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/x86_64-pc-linux-gnu/bits/gthr-default.h:749:12 (a.out+0xd39a6)
#2 std::mutex::lock() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/std_mutex.h:100:17 (a.out+0xd45e9)
#3 std::lock_guard<std::mutex>::lock_guard(std::mutex&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/std_mutex.h:159:19 (a.out+0xd4148)
#4 WaitSignalLock::signal() /home/jae/projects/condition_variable/main.cpp:38:37 (a.out+0xd367e)
#5 main::$_1::operator()() const /home/jae/projects/condition_variable/main.cpp:67:24 (a.out+0xd3f2f)
#6 void std::__invoke_impl<void, main::$_1>(std::__invoke_other, main::$_1&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/invoke.h:60:14 (a.out+0xd3eb1)
#7 std::__invoke_result<main::$_1>::type std::__invoke<main::$_1>(main::$_1&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/invoke.h:95:14 (a.out+0xd3e01)
#8 void std::thread::_Invoker<std::tuple<main::$_1> >::_M_invoke<0ul>(std::_Index_tuple<0ul>) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/thread:264:13 (a.out+0xd3db9)
#9 std::thread::_Invoker<std::tuple<main::$_1> >::operator()() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/thread:271:11 (a.out+0xd3d69)
#10 std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::$_1> > >::_M_run() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/thread:215:13 (a.out+0xd3c9d)
#11 execute_native_thread_routine /build/gcc/src/gcc/libstdc++-v3/src/c++11/thread.cc:80:18 (libstdc++.so.6+0xcfb73)
Location is stack of main thread.
Location is global '??' at 0x7ffd19818000 ([stack]+0x00000001ec48)
Mutex M12 (0x7ffd19836c48) created at:
#0 pthread_mutex_lock <null> (a.out+0x7e358)
#1 __gthread_mutex_lock(pthread_mutex_t*) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/x86_64-pc-linux-gnu/bits/gthr-default.h:749:12 (a.out+0xd39a6)
#2 std::mutex::lock() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/std_mutex.h:100:17 (a.out+0xd45e9)
#3 std::unique_lock<std::mutex>::lock() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/unique_lock.h:138:17 (a.out+0xd46df)
#4 std::unique_lock<std::mutex>::unique_lock(std::mutex&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/unique_lock.h:68:2 (a.out+0xd40b3)
#5 WaitSignalLock::wait(std::chrono::duration<long, std::ratio<1l, 1000l> > const&) /home/jae/projects/condition_variable/main.cpp:28:34 (a.out+0xd3563)
#6 main /home/jae/projects/condition_variable/main.cpp:70:40 (a.out+0xd3860)
SUMMARY: ThreadSanitizer: double lock of a mutex (/home/jae/projects/condition_variable/a.out+0x7e358) in pthread_mutex_lock
==================
==================
WARNING: ThreadSanitizer: data race (pid=16421)
Write of size 1 at 0x7ffd19836c70 by thread T1 (mutexes: write M12):
#0 WaitSignalLock::signal() /home/jae/projects/condition_variable/main.cpp:39:19 (a.out+0xd3687)
#1 main::$_1::operator()() const /home/jae/projects/condition_variable/main.cpp:67:24 (a.out+0xd3f2f)
#2 void std::__invoke_impl<void, main::$_1>(std::__invoke_other, main::$_1&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/invoke.h:60:14 (a.out+0xd3eb1)
#3 std::__invoke_result<main::$_1>::type std::__invoke<main::$_1>(main::$_1&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/invoke.h:95:14 (a.out+0xd3e01)
#4 void std::thread::_Invoker<std::tuple<main::$_1> >::_M_invoke<0ul>(std::_Index_tuple<0ul>) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/thread:264:13 (a.out+0xd3db9)
#5 std::thread::_Invoker<std::tuple<main::$_1> >::operator()() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/thread:271:11 (a.out+0xd3d69)
#6 std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::$_1> > >::_M_run() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/thread:215:13 (a.out+0xd3c9d)
#7 execute_native_thread_routine /build/gcc/src/gcc/libstdc++-v3/src/c++11/thread.cc:80:18 (libstdc++.so.6+0xcfb73)
Previous read of size 1 at 0x7ffd19836c70 by main thread (mutexes: write M12):
#0 WaitSignalLock::conditionMet() const /home/jae/projects/condition_variable/main.cpp:46:12 (a.out+0xd36ea)
#1 WaitSignalLock::waitForConditionVariableSignal(std::unique_lock<std::mutex>&, std::chrono::duration<long, std::ratio<1l, 1000l> > const&)::$_0::operator()() const /home/jae/projects/condition_variable/main.cpp:52:18 (a.out+0xd3af1)
#2 bool std::condition_variable::wait_until<std::chrono::_V2::steady_clock, std::chrono::duration<long, std::ratio<1l, 1000000000l> >, WaitSignalLock::waitForConditionVariableSignal(std::unique_lock<std::mutex>&, std::chrono::duration<long, std::ratio<1l, 1000l> > const&)::$_0>(std::unique_lock<std::mutex>&, std::chrono::time_point<std::chrono::_V2::steady_clock, std::chrono::duration<long, std::ratio<1l, 1000000000l> > > const&, WaitSignalLock::waitForConditionVariableSignal(std::unique_lock<std::mutex>&, std::chrono::duration<long, std::ratio<1l, 1000l> > const&)::$_0) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/condition_variable:157:10 (a.out+0xd3a37)
#3 bool std::condition_variable::wait_for<long, std::ratio<1l, 1000l>, WaitSignalLock::waitForConditionVariableSignal(std::unique_lock<std::mutex>&, std::chrono::duration<long, std::ratio<1l, 1000l> > const&)::$_0>(std::unique_lock<std::mutex>&, std::chrono::duration<long, std::ratio<1l, 1000l> > const&, WaitSignalLock::waitForConditionVariableSignal(std::unique_lock<std::mutex>&, std::chrono::duration<long, std::ratio<1l, 1000l> > const&)::$_0) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/condition_variable:185:9 (a.out+0xd378e)
#4 WaitSignalLock::waitForConditionVariableSignal(std::unique_lock<std::mutex>&, std::chrono::duration<long, std::ratio<1l, 1000l> > const&) /home/jae/projects/condition_variable/main.cpp:51:32 (a.out+0xd3608)
#5 WaitSignalLock::wait(std::chrono::duration<long, std::ratio<1l, 1000l> > const&) /home/jae/projects/condition_variable/main.cpp:29:25 (a.out+0xd3572)
#6 main /home/jae/projects/condition_variable/main.cpp:70:40 (a.out+0xd3860)
As if synchronized via sleep:
#0 nanosleep <null> (a.out+0x6aadc)
#1 void std::this_thread::sleep_for<long, std::ratio<1l, 1000l> >(std::chrono::duration<long, std::ratio<1l, 1000l> > const&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/thread:405:9 (a.out+0xd556a)
#2 main::$_1::operator()() const /home/jae/projects/condition_variable/main.cpp:66:9 (a.out+0xd3f1f)
#3 void std::__invoke_impl<void, main::$_1>(std::__invoke_other, main::$_1&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/invoke.h:60:14 (a.out+0xd3eb1)
#4 std::__invoke_result<main::$_1>::type std::__invoke<main::$_1>(main::$_1&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/invoke.h:95:14 (a.out+0xd3e01)
#5 void std::thread::_Invoker<std::tuple<main::$_1> >::_M_invoke<0ul>(std::_Index_tuple<0ul>) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/thread:264:13 (a.out+0xd3db9)
#6 std::thread::_Invoker<std::tuple<main::$_1> >::operator()() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/thread:271:11 (a.out+0xd3d69)
#7 std::thread::_State_impl<std::thread::_Invoker<std::tuple<main::$_1> > >::_M_run() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/thread:215:13 (a.out+0xd3c9d)
#8 execute_native_thread_routine /build/gcc/src/gcc/libstdc++-v3/src/c++11/thread.cc:80:18 (libstdc++.so.6+0xcfb73)
Location is stack of main thread.
Location is global '??' at 0x7ffd19818000 ([stack]+0x00000001ec70)
Mutex M12 (0x7ffd19836c48) created at:
#0 pthread_mutex_lock <null> (a.out+0x7e358)
#1 __gthread_mutex_lock(pthread_mutex_t*) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/x86_64-pc-linux-gnu/bits/gthr-default.h:749:12 (a.out+0xd39a6)
#2 std::mutex::lock() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/std_mutex.h:100:17 (a.out+0xd45e9)
#3 std::unique_lock<std::mutex>::lock() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/unique_lock.h:138:17 (a.out+0xd46df)
#4 std::unique_lock<std::mutex>::unique_lock(std::mutex&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/bits/unique_lock.h:68:2 (a.out+0xd40b3)
#5 WaitSignalLock::wait(std::chrono::duration<long, std::ratio<1l, 1000l> > const&) /home/jae/projects/condition_variable/main.cpp:28:34 (a.out+0xd3563)
#6 main /home/jae/projects/condition_variable/main.cpp:70:40 (a.out+0xd3860)
Thread T1 (tid=16423, running) created by main thread at:
#0 pthread_create <null> (a.out+0x8dbce)
#1 __gthread_create /build/gcc/src/gcc-build/x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/gthr-default.h:663:35 (libstdc++.so.6+0xcfe49)
#2 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) /build/gcc/src/gcc/libstdc++-v3/src/c++11/thread.cc:135:37 (libstdc++.so.6+0xcfe49)
#3 main /home/jae/projects/condition_variable/main.cpp:65:17 (a.out+0xd3816)
SUMMARY: ThreadSanitizer: data race /home/jae/projects/condition_variable/main.cpp:39:19 in WaitSignalLock::signal()
==================
ThreadSanitizer: reported 2 warnings
I neither understand why the first nor the second warning is reported.
The first warning claims that the mutex is locked twice, but how is this even possible? The mutex is not locked by the same thread while holding it, which is, as far as I understand, what tsan is claiming.
The second warning indicates that the variable 'condition' is not read/written in a thread-safe manner. However, it is only read and written when the mutex is locked. Or is the lock not locked after condition_variable::wait_for returns? According to https://en.cppreference.com/w/cpp/thread/condition_variable/wait_for, the lock should be locked.
Is it possible that this is a bug in clang/tsan? I couldn't find a bug report for this issue, and have a hard time believing that this is the case. But I simply don't see what's wrong with the code above.
I used the following command for compiling the source:
clang++ -O1 -g -fsanitize=thread -fno-omit-frame-pointer -lpthread main.cpp
The clang version is
$ clang --version
clang version 10.0.0