Essentially to implement a lock in the user mode, you do:
while !compare_and_swap(&flag,0,1) // check if flag is free (0), lock it (1)
futex_wait(&flag,0) // sleep until flag changes
In the old days, it's just a busy loop running the CAS check continually which is expensive.
while !compare_and_swap(&flag, 0, 1)
// busy loop running
Yes, multiple threads can be wakened up, the nr_wake parameter in futex_wake [1] dictates how many threads to wake up. Yes, you can get a "thundering herd" problem. For fairness, see priority usage in my other comment https://news.ycombinator.com/item?id=17526849
[1] https://elixir.bootlin.com/linux/v4.18-rc4/source/kernel/fut...
Can you elaborate on this? That sentence isn't reading well for me.
>"Actually multiple threads sleeping on the futex flag can be awaken at the same time (depending on the FUTEX_WAKE command's parameter). All of them need to do compare-and-set to acquire the lock."
So whatever thread kernel schedules first will get the lock? I.e there is no fairness or weighting? Does this potentially cause a "thundering heard" when they're awakened at the same time?