On Mon, May 21, 2018 at 10:19:51PM -0400, Kent Overstreet wrote: > New lock for bcachefs, like read/write locks but with a third state, > intent. > > Intent locks conflict with each other, but not with read locks; taking a > write lock requires first holding an intent lock. Can you put something in the description that these are sleeping locks (like mutexes), not spinning locks (like spinlocks)? (Yeah, I know there's the opportunistic spin, but conceptually, they're sleeping locks). Some other things I'd like documented: - Any number of readers can hold the lock - Once one thread acquires the lock for intent, further intent acquisitions will block. May new readers acquire the lock? - You cannot acquire the lock for write directly, you must acquire it for intent first, then upgrade to write. - Can you downgrade to read from intent, or downgrade from write back to intent? - Once you are trying to upgrade from intent to write, are new read acquisitions blocked? (can readers starve writers?) - When you drop the lock as a writer, do we prefer reader acquisitions over intent acquisitions? That is, if we have a queue of RRIRIRIR, and we drop the lock, does the queue look like II or IRIR?
