Some reddit comments mention it reproduces back to Vista (2008). I am kind of shocked no one has noticed this bug in that time. I guess under typical rwlock usage you just get random instances of shared lockers unable to acquire the lock and no deadlock, but still.
I think part of the reason is that there's a very similar code pattern which is user error, and avoiding that pattern tends to avoid this pattern as well.
The similar case occurs when you have:
- 1+ threads holding a shared lock (Readers)
- 1+ threads waiting to acquire an exclusive lock (Pending Writers)
- 1+ threads trying to acquire the shared lock (Pending Readers)
- 1+ Reader is waiting on a Pending Reader
In this case the Pending Readers will be unable to acquire the shared lock even though it is still in "read mode" because in a fair RW lock Pending Writers are prioritised above Pending Readers so as not to starve the writer side of the lock.
It's doing a pretty weird thing with the locks, I wonder what the actual use case was. Readers should almost never care about other readers. Typically you just grab the lock, read the thing, and release it. You always have to be super careful about deadlocks if you're holding a lock and also waiting around for something else to happen.
