Test-test-and-set Lock uses an atomic value for indicating that some thread has engaged the lock and is executing its critical section (CS).
Each thread that wants to enter CS waits until the atomic lock is disengaged. Then it tries to reengage it, but as several threads might be trying the same, it may or may not succeed. So it checks if it succeeded (with the same atomic operation), and if so its done. If not it retries all over again.
Once the thread is done with CS, it simply disengages the lock.
As all thread wait (spin) for the lock to disengage at the same time, and try reengaging it only when they see it as unlocked, unnecessary bus traffic (storm) is reduced. However, this does not provide first-come-first-served fairness, or efficient cache usage. Since it only uses a single atomic value per lock it is only suitable for low-contention memory-limited architectures.
The TTASLock is due to Clyde Kruskal, Larry Rudolph, and Marc Snir.
Course: Concurrent Data Structures, Monsoon 2020
Taught by: Prof. Govindarajulu Regeti
lock():
1. When thread wants to access critical
section, it checks to see if lock is already
engaged, and if so, waits (spins).
2. Once lock is disengaged it tries to reengage
it. So do all other threads wanting to enter
CS. Its a race between threads. So this
thread checks to see it was the one who was
successful, and if so its done.
3. If not, it retries again.unlock():
1. When a thread is done with its critical
section, it simply sets the "locked" state
to false.See TTASLock.java for code, Main.java for test, and repl.it for output.