On Tue, Jan 10, 2017 at 09:08:50PM +0100, Peter Zijlstra wrote: > But I still feel this document is very hard to read and presents things > backwards. I admit it. I think I need to modify the document more.. I will try it. > > > +Let's take a look at more complicated example. > > + > > + TASK X TASK Y > > + ------ ------ > > + acquire B > > + > > + release B > > + > > + acquire C > > + > > + release C > > + (1) > > + fork Y > > + acquire AX > > + acquire D > > + /* A dependency 'AX -> D' exists */ > > + acquire F > > + release D > > + acquire G > > + /* A dependency 'F -> G' exists */ > > + acquire E > > + /* A dependency 'AX -> E' exists */ > > + acquire H > > + /* A dependency 'G -> H' exists */ > > + release E > > + release H > > + release AX held by Y > > + release G > > + > > + release F > > + > > + where AX, B, C,..., H are different lock classes, and a suffix 'X' is > > + added on crosslocks. > > + > > +Does a dependency 'AX -> B' exist? Nope. > > I think the above without the "fork Y" line is a much more interesting > example, because then the answer becomes: maybe. Sure. The dependency 'AX -> B' might exist in that case. Then we can add the dependency once we detect it, in other words, once we prove it's a true dependency. But we cannot add it before we prove it, though it might be a true one, because it might not be a true one. > This all boils down to the asynchonous nature of the primitive. There is > no well defined point other than what is observed (as I think you tried > to point out in our earlier exchanges). Exactly. > The "acquire AX" point is entirely random wrt any action in other > threads, _however_ the time between "acquire" and "release" of any > 'lock' is the only time we can be certain of things. > > > +============== > > +Implementation > > +============== > > + > > +Data structures > > +--------------- > > + > > +Crossrelease feature introduces two main data structures. > > + > > +1. pend_lock > > I'm not sure 'pending' is the right name here, but I'll consider that > more when I review the code patches. Thank you. > > + > > + This is an array embedded in task_struct, for keeping locks queued so > > + that real dependencies can be added using them at commit step. Since > > + it's local data, it can be accessed locklessly in the owner context. > > + The array is filled at acquire step and consumed at commit step. And > > + it's managed in circular manner. > > + > > +2. cross_lock > > + > > + This is a global linked list, for keeping all crosslocks in progress. > > + The list grows at acquire step and is shrunk at release step. > > FWIW, this is a perfect example of why I say the document is written > backwards. At this point there is no demonstrated need or use for this > list. I will consider that more. > OK, so commit adds multiple dependencies, that makes more sense. > Previously I understood commit to only add a single dependency, which > does not make sense (except in the special case where there is but one). > > I dislike how I have to reconstruct this from an example instead of > first having had the rules stated though. So do I. > > > + * > > + * In pend_lock: D, E > > + * In graph: 'B -> C', 'C -> D', > > + * 'AX -> D', 'AX -> E' > > + */ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>