A thread unsafe operation in Gauge::Change

[lbermes]

Hi,
getting the value and then exchanging it is not an atomic thread safe operation.
Between the load and the exchange another thread may also change the value.
Correct would be value += current;

void Gauge::Change(const double value) {
auto current = value_.load();
while (!value_.compare_exchange_weak(current, current + value))
;
}

Best regards

Lukas

[gjasny]

Hello,
according to https://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange it is working as expected:

Atomically compares the object representation (until C++20)value representation (since C++20) of *this with that of expected, and if those are bitwise-equal, replaces the former with desired (performs read-modify-write operation). Otherwise, loads the actual value stored in *this into expected (performs load operation).

If expected does not match, expected gets updated with observed value and desired is re-calculated.

[lbermes]

Hi,
thank you for your explanation.
I made a stupid assumption.
This will not happen again.