Michael S (already5chosen.delete@this.yahoo.com) on January 7, 2020 1:35 am wrote:
> Can you point me to explanation of how SRW locks are implemented under the hood?
> "No additional memory" is intriguing and right now (still sort of morning here, not
> the time of day when I am able to think) I can't figure out how they did it.
> Or, may be, you could give us a short explanation right here?
>

It is helpful to understand mcs locks (aka queued spinlocks). https://lwn.net/Articles/590243/

The key is that only the list of waiting threads needs to be tracked, which can be done as a list of linked "wait block" elements on the waiting threads' stacks. When the lock is held and there are waiters, the lock points at the last element added to the linked list. This list can be maintained using compare and swap operations with guaranteed forward progress for adding elements to the list.

The mcs lock is described as a spinning lock, but instead of the waiter spinning, the waiter can block and the lock releaser can wake up the waiter based on information stored in the wait block. Recent versions of NT use a wake threads by ID kernel primitive to wake up a thread. In practice on NT, we spin for a while before blocking, so it's really like an mcs lock with blocking added.

Compared to a critical section, an srw lock exclusive uncontended acquire should be about the same. The release should also be about the same.