Test and set

Test-and-set works on a Boolean value: we read it, set it to true, and provide the value it held beforehand. C and C++ offer a type dedicated to this purpose, called atomic_flag. The initial value of an atomic_flag is indeterminate until initialized with the ATOMIC_FLAG_INIT macro.

Test-and-set operations are not limited to just RMW functions; they can also be utilized for constructing a simple spinlock. In this scenario, the flag acts as a shared resource for communication between threads. Thus, a spinlock implemented with Test-and-set operations ensures that each full RMW operation on shared resources is performed atomically, as shown in the figure.

atomic_flag af = ATOMIC_FLAG_INIT;

void lock()
{
    while (atomic_flag_test_and_set(&af)) { /* wait */ }
}

void unlock() { atomic_flag_clear(&af); }

If we call lock() and the previous value is false, we are the first to acquire the lock, and can proceed with exclusive access to whatever the lock protects. If the previous value is true, someone else has acquired the lock and we must wait until they release it by clearing the flag.