I don’t know the details of the Rust port but I don’t imagine the part that involves the two bits to require unsafe, other than in the ways that any locking algorithm dances with unsafety in Rust (ownership relies on locking algorithms being correct)
This is very similar to how Java's object monitors are implemented. In OpenJDK, the markWord uses two bits to describe the state of an Object's monitor (see markWord.hpp:55). On contention, the monitor is said to become inflated, which basically means revving up a heavier lock and knowing how to find it.
I'm a bit disappointed though, I assumed that you had a way of only using 2 bits of an object's memory somehow, but it seems like the lock takes a full byte?
It’s just that if you use the WTF::Lock class the. You get a full byte simply because the smallest possible size of a class instance in C++ is one byte.
But there’s a template mixing
thing you can use to get it to be two bits (you tell the mixin which byte to steal the two bits from and which two bits).
I suspend the same situation holds in the Rust port.
I am very familiar with how Java does locks. This is different. Look at the ParkingLot/parking_lot API. It lets you do much more than just locks, and there’s no direct equivalent of what Java VMs call the inflated or fat lock. The closest thing is the on demand created queue keyed by address.
>I am very familiar with how Java does locks. This is different. Look at the ParkingLot/parking_lot API. It lets you do much more than just locks, and there’s no direct equivalent of what Java VMs call the inflated or fat lock. The closest thing is the on demand created queue keyed by address.
Are you familiar with the new LightweightSynchronizer approach with an indirection via a table, instead of overwriting the markWord? I'd say that this has pushed the ParkingLot approach and Java's (Hotspot's, really) approach closer to each other than before. I think the table approach in Java could be encoded trivially into ParkingLot API, and the opposite maybe. Obviously the latter would be a lot more hamfisted.
The idea is that six bits in the byte are free to use as you wish. Of course you'll need to implement operations on those six bits as CAS loops (which nonetheless allow for any arbitrary RMW operation) to avoid interfering with the mutex state.
Post that mentions the two bit lock: https://webkit.org/blog/7122/introducing-riptide-webkits-ret...
I don’t know the details of the Rust port but I don’t imagine the part that involves the two bits to require unsafe, other than in the ways that any locking algorithm dances with unsafety in Rust (ownership relies on locking algorithms being correct)