Re: [PATCH] [RFC] bcachefs: SIX locks (shared/intent/exclusive)

[Matthew Wilcox]

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?