Test-and-set
wikipedia Test-and-set
In computer science, the test-and-set instruction is an instruction used to write 1 (set) to a memory location and return its old value as a single atomic (i.e., non-interruptible) operation. If multiple processes may access the same memory location, and if a process is currently performing a test-and-set, no other process may begin another test-and-set until the first process's test-and-set is finished. A CPU may use a test-and-set instruction offered by another electronic component, such as dual-port RAM; a CPU itself may also offer a test-and-set instruction.
NOTE:
1、shared data会不断地被写入，会导致cache coherence flood、high interconnect contention
function Lock(boolean *lock) {
while (test_and_set(lock) == 1);
}
The calling process obtains the lock if the old value was 0, otherwise the while-loop spins waiting to acquire the lock. This is called a spinlock. "Test and Test-and-set" is another example.
Mutual exclusion using test-and-set
Pseudo-C implementation
volatile int lock = 0;
void critical() {
while (test_and_set(&lock) == 1);
critical section // only one process can be in this section at a time
lock = 0 // release lock when finished with the critical section
}